Physical Sciences

▪ 2009

Introduction

Scientists discovered a new family of superconducting materials and obtained unique images of individual hydrogen atoms and of a multiple-exoplanet system. Europe completed the Large Hadron Collider, and China and India took new steps in space exploration.

Chemistry

Solid-State Chemistry.

      The discovery in early 2008 of superconductivity in a rare-earth iron-arsenide compound (an iron pnictide) touched off a wave of intense research that quickly produced a large new chemical family of superconductors based on iron. More than 20 years had passed since researchers last discovered a new collection of chemically related superconducting materials—the ceramic copper-oxide superconductors.

      A superconductor loses all electrical resistance when cooled below a characteristic temperature called its critical temperature (T_c). Most superconductors, such as those used in the powerful electromagnets of magnetic-resonance devices, have a T_c close to absolute zero (0 K, −273.15 °C, or −459.67 °F). The identification of a new family of superconductors reawakened researchers' long-standing hope of finding materials with a critical temperature of room temperature (about 300 K) or above, which would open the door to many applications. (Among copper-oxide superconductors, the highest T_c that had been obtained was 138 K.)

      Hideo Hosono and co-workers at the Tokyo Institute of Technology in February created the first-known iron-arsenide superconductor, lanthanum iron arsenide (LaOFeAs) doped with fluoride ions. The material, created through a combination of high-temperature and high-pressure methods, contained alternating layers of iron arsenide and lanthanum oxides and became superconducting at 26 K. Other laboratories soon began experimenting with similar compounds that used other rare-earth elements in place of lanthanum, and by late April researchers at laboratories in China had reported raising the T_c to 43 K by using samarium and to 52 K by using praseodymium. By late in the year, the highest T_c that had been established for the iron-arsenide family of superconductors was 56 K. It was reported by scientists at Zhejiang University, Hangzhou, China, in a material that contained gadolinium and thorium (Gd_0.8Th_0.2FeAsO).

Physical Chemistry.

      Individual heavy atoms—and single molecules made from those atoms—could be routinely examined in exquisite detail by means of a transmission electron microscope (TEM). The imaging of lower-mass atoms such as carbon by TEM continued to present a challenge, however, because they yielded extremely weak signals that were difficult to distinguish from instrument noise. For this reason the TEM imaging of atoms of hydrogen, the lightest element, had long been considered to be all but impossible. Nevertheless, in 2008 Alex Zettl and co-workers at the University of California, Berkeley, reported a technique for producing such images. They succeeded in part by developing methods for supporting hydrogen atoms on pristine films of graphene—one-atom-thick sheets of carbon—that were transparent under the team's imaging conditions. The researchers also utilized data-averaging techniques to boost their ability to extract high-resolution spatial data from directly imaged individual hydrogen and carbon atoms as well as carbon chains adsorbed on graphene.

      The capabilities of TEMs were also broadened through the development of methods for capturing images and videos of single organic molecules in motion. The strategy used by Eiichi Nakamura and co-workers at the University of Tokyo was to trap molecules either inside or on the exterior of carbon nanotubes. For example, the group recorded rotational and bond-flexing changes in single molecules of aminopyrene derivatives that were fixed inside a nanotube. They also imaged a biotinylated triamide as it underwent a variety of shape-altering conformational changes while attached to a nanotube exterior. These studies captured the evolution of organic molecules in real time and might lead to new ways of directly observing reaction dynamics of complex molecules.

Organic Chemistry.

      Unexpected details of a classic reaction mechanism in organic chemistry were revealed by Roland Wester at the University of Freiburg, Ger., and colleagues. By using high-vacuum techniques to react beams of methyl iodide molecules and chloride ions, the team recorded direct evidence of a reaction mechanism called bimolecular nucleophilic substitution (S_N2), but they found that the reaction did not always proceed in the way that had been taught for decades in introductory organic-chemistry courses.

      According to the classic mechanism that depicted the interplay between reactants in this ubiquitous type of substitution reaction, the chloride ions should approach methyl iodide from the opposite side of the molecule's carbon-iodine bond. The “attacking” ion would cause the molecule to invert its tripod-shaped methyl radical (CH₃) and eject an iodide ion along the original carbon-iodine axis. The team found direct evidence of that mechanism when they collided molecular beams at low energy. In higher-energy collisions, however, they found that chloride slammed into the methyl iodide and imparted rotational energy that caused the molecule to tumble. After one rotation the chloride then displaced the iodide ion to complete the reaction.

Applied Chemistry.

      Solid catalysts lay at the heart of most of the chemical industry's production processes. Many of the factors critical to catalyst performance remained poorly understood, however, because of the difficulties in scrutinizing the inside of a working chemical reactor. For example, little was known about the way catalysts changed chemically and physically during the course of a reaction and the locations where reactions took place.

      To learn about the way in which reactant gases flowed across a bed of powdered catalyst and where products were formed within the bed, Alexander Pines of the University of California, Berkeley, and co-workers developed a nuclear magnetic resonance (NMR) imaging method that enabled them to “see” inside a reactor. By reacting para-hydrogen (hydrogen molecules in which the two nuclei have opposite spin) with propylene in a microreactor, the team was able to monitor the highly enhanced NMR signals of the labeled propane-product molecules and thereby map their distribution throughout the reactor.

      Another approach, used by a research team led by Bert M. Weckhuysen and Frank M.F. de Groot of the University of Utrecht, Neth., was based on scanning transmission X-ray spectroscopy. They showed that the X-ray method was well suited to probing the changing nature of solid catalysts during reaction. Demonstrating the method's strengths, the group mapped—with about 15-nm (nanometre; 1 nm = 10⁻⁹ m) spatial resolution—the locations of chemical species that formed on the surface of an iron catalyst while the solid was mediating Fischer-Tropsch synthesis. That carbon-coupling process was used commercially for making liquid (transportation) fuels from carbon sources such as natural gas and coal. A key feature of the customized microreactor used in the study was the device's ability to tolerate reaction conditions (atmospheric pressure and temperatures up to 350 °C [662 °F]) that were typical of industrial processes. The team found that as the reaction proceeded, the initial form of the catalyst, alpha ferric oxide (α-Fe₂O₃), changed to metallic iron and ferrosoferric oxide (Fe₃O₄). They also observed formation of an iron silicate (Fe₂SiO₄), a buildup of hydrocarbon products, and formation of other chemical species. This type of information could be used to design more effective and longer-lasting catalysts and more efficient chemical reactors. Further improvements to the system's X-ray optics were expected to increase the method's spatial resolution, with the goal of obtaining atomic-scale information.

Biochemistry.

      Changes in glycan (polysaccharide) structures in cell membranes accompanied the progression of disease and other key physiological cellular processes. As a result, glycans were attractive targets for biochemical imaging. Carolyn R. Bertozzi and co-workers at the University of California, Berkeley, described a method that made it possible to image carbohydrates as they were produced on the cell surfaces of living organisms. The researchers introduced an azide-tagged sugar (azide-derivatized N-acetylgalactosamine) into developing zebra-fish embryos in order to label their cell-surface glycans with azides. The group treated the embryos with a difluorinated cyclooctyne reagent to cause the labeled glycans to fluoresce. Then, by using a fluorescence microscopy method, the group imaged an increase in glycan biosynthesis in the jaw region, pectoral fins, and other organs of the living embryos. The researchers proposed that the technique could be generalized to other types of biomolecules.

Industrial Chemistry.

      DEET (N,N-diethyl-meta-toluamide) had been widely used around the world for decades as a potent repellent of blood-feeding insects. As the active component in commercial mosquito repellents, the compound had a reputation for effectively warding off mosquitoes and other annoying and disease-carrying pests. The molecular basis of DEET's effects, however, had not been clear. In experiments conducted with fruit flies and the mosquito that transmits malaria, Leslie B. Vosshall and co-workers at Rockefeller University, New York City, found that DEET blocked the electrophysiological responses of the insects' olfactory sensory neurons to attractive odour compounds, including lactic acid, a component of human sweat. Specifically, the repellent impeded the insects' ability to sniff out humans by inhibiting olfactory receptors that formed a complex with a coreceptor called OR83b. Knowing the way DEET worked and its molecular target, scientists could begin to use high-throughput screening methods to search for new insect repellents that would be even more effective and safer than DEET.

Mitch Jacoby

Physics

Particle Physics.

      In 2008 the European Organization for Nuclear Research ( CERN) near Geneva completed the construction of and inaugurated its new particle accelerator, the Large Hadron Collider, but full-scale operation was postponed until past the end of the year. (See Sidebar (Large Hadron Collider-The World's Most Powerful Particle Accelerator ).) Meanwhile, experiments at two other research facilities produced surprising results.

      Physicists at the Belle Collaboration, which was based at the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan, reported an unexpected asymmetry in the decay rates of exotic particles known as B mesons. The discovery suggested a possible solution to a major problem in particle physics: only tiny amounts of antimatter existed in the universe, but according to theoretical models, equal amounts of matter and antimatter would have been produced at the beginning of the universe in the big bang.

      Yuri M. Litvinov and co-workers at Germany's Society for Heavy Ion Research in Darmstadt observed periodic oscillations in what should have been simple exponential decay curves of two radioactive isotopes (praesodymium-140 and promethium-142). The researchers concluded that this was caused by the oscillation between two different types of neutrinos emitted in the decay. Such oscillations had previously been observed only in solar neutrinos with experiments that had required the use of huge underground detection systems. If the findings were confirmed, it might be possible to examine the properties of neutrinos through the decay characteristics of heavy ions and would therefore be relatively easy to investigate.

Quantum Information Storage and Retrieval.

      Many research groups were studying structures called quantum dots, which might form the next generation of computers. Quantum dots could be made either from tiny groups of atoms (usually of semiconductor materials) that acted together as a single atom or from Bose-Einstein condensates (BECs), tiny clouds of atoms that shared the same quantum state. Information was transmitted in such structures as qubits—bits of information carried by individual quanta.

      A major problem in developing quantum computers was the retention and storage of information over a long period of time. Brian D. Gerardot of Heriot-Watt University, Edinburgh, and colleagues demonstrated the storage of information via the two spin states of a valence hole in a semiconductor quantum dot that remained stable for about one millisecond. Using a different approach, Sylvain Bertaina and co-workers at the National Centre for Scientific Research at Grenoble, France, used a molecular magnet that consisted of a vanadium V^IV₁₅ molecule about one nanometre in diameter. The molecule contained magnetic ions whose coupled spins were able to form collective-spin qubits. The researchers suggested that such systems might have a stability of about 100 microseconds.

      One of the amazing features of quantum-dot systems was that they might be able to teleport information from one quantum dot to another instantaneously by a phenomenon called quantum entanglement. Kwang Seong Choi and colleagues at the California Institute of Technology succeeded in storing two entangled photon states in separate atomic clouds and then retrieving the states after a short delay. Yu-Ao Chen and colleagues at the University of Heidelberg, Ger., went one step farther and demonstrated teleportation between photonic (light-based) and atomic qubits. The polarization state of a single photon was teleported over a distance of 7 m (23 ft) onto a remote atomic qubit that served as a quantum memory. The state was stored for up to eight microseconds. The researchers also produced a type of “quantum repeater” in which “entanglement swapping” with the storage and retrieval of light between two atomic ensembles was possible. This approach addressed the degradation of signals over long distances, which was a major problem in working with quantum-dot systems.

Condensed-Matter Physics.

      Physicists had begun to use Bose-Einstein condensates (BECs) to produce bright coherent matter waves, called atom lasers, which held great promise for precision measurements and for fundamental tests of quantum mechanics. In 2008 Nicholas P. Robins and colleagues at the Australian National University in Acton claimed to be the first to have generated a continuous atom-laser beam from a rubidium BEC cloud that was continuously supplied with new atoms pumped in from a physically separate cloud.

      Thorsten Schumm and associates at the Vienna University of Technology constructed so-called atom chips—blocks of material with microscopic wire structures to manipulate ultracold gases—that were able to perform BEC operations such as splitting one condensate into two parts that could then be held in place.

Superconductivity.

      Hideo Hosono and co-workers at the Tokyo Institute of Technology discovered an entirely new class of superconductor (a material that loses all electrical resistance when cooled below a characteristic temperature). The new material consisted of a layered iron-based compound and became superconducting at 26 K (−247 °C [−413 °F]). (See Chemistry (Physical Sciences ).)

      At the Argonne National Laboratory near Chicago, Valerii M. Vinokur and colleagues devised the inverse of a superconductor—a “superinsulator,” which had zero electrical conductance. They used a film of titanium nitride, which was usually superconducting. It became a super insulator, however, when cooled below a certain critical temperature in the presence of a magnetic field. The conductive state of the material depended on the strength of the applied magnetic field and the thickness of the sample.

Solid-State Physics.

      In a move toward realizable technological devices, Alberto Politi and co-workers at the Centre for Quantum Photonics, University of Bristol, Eng., produced high-fidelity silica-on- silicon integrated optical realizations of key quantum-photonic circuits, including a two-photon quantum interference, a controlled-NOT gate, and a path-entangled state of two photons. These results showed that it was possible to form sophisticated photonic quantum circuits directly onto a silicon chip.

      Helena Alves and co-workers at Delft (Neth.) University of Technology investigated interfaces between crystals of organic molecules. Transfer of charge on a molecular scale produced a highly conducting metal-like interface, and the results could point to a new class of electronic material.

      As integrated circuits with ever-smaller components were developed, there would come a time when quantum-physical phenomena would prevent further size reduction. K. Nishiguchi and colleagues of the NTT Corp., Kanagawa, Japan, demonstrated a method of potentially circumventing this limitation by using the quantum-mechanical tunneling of single electrons in a transistor to carry out pattern-matching operations.

Lasers and Optics.

      The search continued for laser systems that generated radiation at new wavelengths. Harumasa Yoshida and colleagues at Hamamatsu (Japan) Photonics K.K. reported an aluminum-gallium-nitride laser diode that emitted ultraviolet light at 342 nanometres, the shortest wavelength reported for an electrically driven laser diode. Ying Yang and co-workers at the University of St. Andrews, Scot., described a laser that used an inorganic light-emitting diode (LED) to activate a polymer (organic) lasing material. Such a device could provide a cheap and compact source of radiation across the visible spectrum.

      In other laser systems, Jan Schäfer and colleagues at the University of Erlangen-Nürnberg (Ger.) observed multimode laser action in the red region of the spectrum from isolated spherical liquid microcavities that contained cadmium-selenide/zinc-sulfide nanocrystal quantum dots. S.I. Tsintzos and fellow workers at the University of Crete, Heraklion, Greece, produced a gallium-arsenide LED that involved quasiparticles called polaritons (a hybrid of light and matter). They were produced by the strong coupling between photons and excitons (another type of quasiparticle, formed by an electron and a positive hole) in semiconductor microcavities. The unique properties of polaritons might provide the basis for a new generation of polariton emitters and semiconductor lasers.

      In the field of general optics, physicists continued to work on negative-index metamaterials—artificially engineered structures with negative refractive indexes. Jason Valentine and co-workers at the University of California, Berkeley, produced a three-dimensional metamaterial with low energy loss and a negative refractive index in the optical region of the spectrum. Such materials opened up a vast field for new optical devices, which might possibly include “invisibility cloaks.”

Fundamental Physics.

      Two research groups added to the knowledge of the reality underlying modern physics. A major feature of quantum mechanics was the property of entanglement, by which information appeared to be transported instantaneously between two quantum devices. In terms of classical physics, this would imply that the information traveled faster than the speed of light, which was explicitly disallowed by relativity theory. Daniel Salart and co-workers at the University of Geneva carried out an experiment to determine the lowest speed at which such a transfer of information, if it existed, would take place. Taking measurements of two-photon interference between detectors that were 18 km (11 mi) apart, the researchers concluded that any interaction would have to travel at a speed greater than 10,000 times the speed of light. A second problem in modern physics was the apparent theoretical incompatibility of quantum mechanics with general relativity across very small distances. It had been suggested that this might be an indication that at such distances Newton's law of gravitational attraction broke down. Andrew Geraci and colleagues at Stanford University, however, showed that the law continued to hold down to a distance of 10 micrometres.

David G.C. Jones

Astronomy

      For information on Eclipses, Equinoxes, and Solstices, and Earth Perihelion and Aphelion in 2009, see Table (Earth Perihelion and Aphelion, 2009).

Solar System.

      A trio of spacecraft made a multitude of new discoveries about the planets Mercury, Mars, and Saturn in 2008. On January 14 and again on October 6, the NASA Messenger spacecraft flew within 200 km (125 mi) of the surface of Mercury, the solar system's innermost planet. This was the first mission to the planet since the Mariner 10 spacecraft made three flybys of Mercury in 1974–75. By the end of its October flyby, Messenger had photographed more than 90% of the planet, including most of the regions that had not been seen by Mariner 10. That mission had revealed that flat plains cover much of the planet, and a detailed analysis of the Messenger images showed that the plains were formed from lava flows rather than impact debris. Among new surface features that were detected was one, called “the spider,” formed by more than 100 trenches that radiate outward from a central mass complex. Multicolour images of some of the craters on the planet suggested that they are no more than a few hundred million years old. Messenger data showed that Mercury's magnetic field is highly symmetrical, which supported the idea that the field is being generated by an active dynamo in a hot molten iron core. Messenger was to make another flyby of Mercury in 2009 before it settled into orbit around the planet in 2011.

       NASA's Phoenix Mars Lander touched down on the surface of Mars on May 25. It was the first spacecraft to land on the northern polar regions of Mars. The main goal of the mission was for the lander to dig into the Martian surface and look for the presence of chemicals that could play a role in living organisms. Even before the analysis of the soil began, images of the Martian surface taken by cameras on the lander had revealed the presence of water ice. Analysis of scoops of Martian soil by the lander's miniature onboard laboratory—which included wet-chemistry labs and optical and atomic-force microscopes—revealed that the soil contained inorganic salts of chlorine, magnesium, sodium, and potassium. The soil was found to be slightly alkaline, with a pH of between 8 and 9. Although the lander was not designed to determine whether life had existed on Mars, its instruments could determine the presence or absence of organic molecules in the soil. The cold of the Martian winter brought an end to the mission in November. Also during the year observations by NASA's Mars Reconnaissance Orbiter revealed the presence of hydrated silica over large regions of the surface of Mars. These observations suggested that there had been liquid water on the surface of Mars as recently as two billion years ago.

      The Cassini spacecraft in orbit around Saturn continued to report new discoveries about the large gaseous planet and its many satellites. Saturn's tiny moons Atlas and Pan, which lie just inside and outside Saturn's A ring, have the general appearance of fat pancakes. They, together with the moons Prometheus, Pandora, and Daphnis appear to have a very low density—between 0.38 and 0.45 g per cu cm, or less than one-half the density of water. The observations suggested that these moons accreted material from the nearby rings of Saturn. The Cassini spacecraft came within 500 km (310 mi) of Rhea, Saturn's second largest moon, in 2005, and it unexpectedly detected the presence of rocky debris in orbit around the moon. After scientific analysis the first reported discovery of rings around a moon of any planet in the solar system was announced in March 2008.

Stars.

      Through the year new discoveries of planets in orbit around stars other than the Sun continued to excite scientists. Since their initial detection in 1995, more than 300 extrasolar planets had been found, and they ranged in mass from about four Earth masses to about 20 times the mass of Jupiter. Thirty of the more than 200 stars known to have an extrasolar planet had been found to have more than one planet.

      Although astronomers had found extrasolar planets mainly by indirect methods, such as by detecting tiny periodic motion in the stars they orbited, in 2008 two groups of astronomers succeeded in directly imaging extrasolar planets. Using a camera on the Hubble Space Telescope, a team of astronomers led by Paul Kalas of the University of California, Berkeley, took visible-light photographs of a planet in orbit around Fomalhaut, a relatively nearby star. Designated as Fomalhaut b, the planet was calculated to have a mass more than three times the mass of Jupiter and to orbit the star at a distance 10 times the distance between the Sun and Saturn. Because the planet appeared brighter than would be expected for an object of its size, however, some astronomers suggested that the body might be a clump of gas and dust in orbit around the planet. The second group of astronomers. led by Christian Marois of the Herzberg Institute of Astrophysics in Victoria, B.C., acquired infrared images for the first time of an extrasolar planet system with multiple planets. Using the Earth-based Gemini North and Keck telescopes in Hawaii, they found three planets orbiting star HR 8799, in the constellation Pegasus. Their respective distances from the star were about 25, 40, and 70 times that between the Earth and Sun.

      A team of astronomers led by Michel Mayor of Geneva Observatory detected a planetary system around the star HD 40307 that resembles the solar system. The star is about 42 light years from Earth and has a mass of about eight-tenths that of the Sun. The star's three planets have masses of 4.2, 6.8, and 9.4 Earth masses and move in circular orbits around the star with periods of 4.3, 9.6, and 20.5 days, respectively. Since they are so close to the central star, they would have to be rocky objects like Mercury, Venus, and the Earth.

      In March 2008 scientists announced the first discovery of an organic molecule in an extrasolar planet. Using the Hubble Space Telescope, they detected methane in the atmosphere of a hot Jupiter-sized planet that orbits the star HD 189733b. The methane was detected with Hubble's Near Infrared Camera and Multi-Object Spectrometer. Their observations also confirmed the existence of water in the atmosphere of the planet, which had been reported in 2007 from observations made with NASA's Spitzer Space Telescope. Together, all of these discoveries continued to reinforce the idea that the conditions for life might well exist on planets around many neighbouring stars.

      On Jan. 9, 2008, scientists for the first time witnessed the earliest stages of the death of a massive star. Astronomers Alicia Soderberg and Edo Berger of Princeton University were using NASA's Swift X-Ray Observatory to study the X-ray emission from supernova 2007uy, which had exploded 10 days earlier in the galaxy NGC 2770. By happenstance, they witnessed a burst of X-rays that lasted seven minutes. They realized that the burst was being produced by an exploding supernova in the outer reaches of NGC 2770. According to well-established theory, supernova explosions occur when a massive star (5–10 times the mass of the Sun) depletes its nuclear fuel. The star's core then collapses rapidly and releases as much as 50% of the rest-mass energy of the core in a matter of seconds. This leads to the formation of a shock wave that propagates through the outer layers of the star and produces a burst of X-rays. Subsequently, the stellar remnant expands and cools and produces an optical-light emission, which is ordinarily detected from supernovae days and weeks after the initial core collapse. The new supernova, subsequently named SN 2008D, was the first ever observed during the X-ray-burst stage. Immediately following the report of the explosion, dozens of astronomical telescopes, including the Hubble Space Telescope, the Chandra X-ray Observatory, the 508-cm (200-in) telescope at Mt. Palomar (California), the Gemini North telescope, and the Very Large Array radio telescope (New Mexico), detected the supernova at radio and optical wavelengths. They confirmed that the observed phenomena represented the death of a massive star.

Galaxies and Cosmology.

      Some star deaths lead to another class of phenomena— gamma-ray bursts. Such bursts, which last from seconds to minutes, had been detected over the course of more than four decades. Many such bursts were thought to be produced in supernova explosions in which a part of the emitted energy is beamed into relativistic jets of particles and radiation. On March 19, 2008, NASA's Swift spacecraft alerted astronomers to the brightest gamma-ray burst observed to date. Named GRB080319B, the gamma-ray burst came from a galaxy 7.5 billion light years from the Milky Way Galaxy in the direction of the constellation of Boötes. For about a minute the object emitted as much radiation as 10 million galaxies. Such gamma-ray bursts are typically followed by an afterglow of visible light, and the brightness of the afterglow that followed this event reached about the fifth magnitude. Consequently, it was the most distant object ever recorded that was bright enough to be directly observable with the unaided eye.

      In March scientists published a detailed analysis of the past five years of observations by NASA's Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001. According to the cosmological view supported by these observations, the universe began with a hot explosive event (the big bang), and as the universe expanded and cooled, it left behind radiation detectable at microwave wavelengths. The very small point-to-point fluctuations in the background radiation that remained amounted to only a few parts per million. The new analysis of the WMAP fluctuation data indicated that the universe is 13.73 billion years old with a precision of better than 1% and that the first stars formed only about 430 million years after the big bang. The data also implied that the universe is made up of only 4.5% ordinary matter (of the kind found in stars) and that the rest of the universe appears to be made up of 23.4% dark matter and 72.1% dark energy.

Kenneth Brecher

Space Exploration

      For Launches in Support of Human Space Flight in 2008, see Table (Human Spaceflight Launches and Returns, 2008).

Manned Spaceflight.

      A highlight of space exploration in 2008 was China's third manned space mission, on September 25–28. The Shenzhou 7 spacecraft carried three taikonauts (astronauts) into Earth orbit, and while in orbit taikonaut Zhai Zhigang conducted a 25-minute space walk—the program's first—to test a Chinese-built spacesuit. China said that a mission planned for 2010 would be the first step toward constructing a basic space station that would be composed of modules from two unmanned and one manned spacecraft.

      The political turmoil triggered by Russia's invasion of Georgia in August called into question the planned retirement of the U.S. space shuttle in 2010. The U.S. was to rely on Russian Soyuz space launches for manned spaceflight capability for several years between the final mission of the shuttle and the first mission of its replacement, Orion. Although many space shuttle contracts were already being closed, some U.S. officials started to examine the possibility of continuing support of the shuttle until Orion was ready in about 2014.

      In 2008 the space shuttle completed four flights to the International Space Station (ISS). The first, STS-122, delivered the European Space Agency's Columbus laboratory module. With a length of 7 m (23 ft) and diameter of 4.6 m (15 ft), it was larger than the American-built Destiny laboratory module, which was delivered to the ISS in 2001. Columbus could accommodate 10 laboratory racks for various types of gear for experiments. The third flight, STS-124, also delivered a new laboratory module—the Japanese Aerospace and Exploration Agency's Kibo (Hope). Kibo was Japan's first-ever component built for a manned space vehicle. About 11 m (36 ft) long, it barely fit inside the space shuttle's payload bay. Kibo could also hold up to 10 experiment racks, and it was equipped with two robotic arms that would be used with an external platform—to be delivered in 2009—for conducting experiments in the vacuum of space. Between the Columbus and Kibo missions, the STS-123 flight delivered the Canadian-built robot known as Dextre. The robot was designed to be attached to Canadarm 2 (a previously installed external manipulator arm), and it was to perform difficult tasks that would otherwise require a human to make a space walk. In addition, STS-123 carried a small Experiment Logistics Module that was stored on one of the station's nodes and later mounted atop Kibo. The STS-126 flight delivered equipment that included additional sleeping quarters, a new bathroom, and a water-recovery system to increase the crew capacity of the ISS to six persons. Members of the crew performed four space walks, including one to repair the jammed solar-array rotary joint that had severely restricted the power available on the station since September 2007.

      STS-125, the final shuttle mission for servicing the Hubble Space Telescope (HST), was to have been the fourth shuttle flight in 2008. A few days before its scheduled launch in October, however, a device to format data on the HST failed. Within a short time the HST was switched over to a backup data formatter, but the mission was postponed until spring 2009 to allow NASA to ready a spare that would be carried aboard the flight.

      The Soyuz TMA-12 mission took two new cosmonauts and a South Korean spaceflight participant to the ISS. The previous crew and the spaceflight participant returned on the Soyuz TMA-11 craft, which experienced a steep descent and rough landing about 400 km (250 mi) off target because the service module failed to separate from the descent module before their entry into the Earth's atmosphere. The previous Soyuz landing had experienced a similar incident, and Russia conducted a rigorous examination of the explosive bolts used to separate the Soyuz modules for entry into the atmosphere. Engineers determined that an electrical grounding problem was causing one of the bolts to malfunction, and in July cosmonauts removed the suspect bolt from the TMA-12 Soyuz while it was docked to the ISS. In October the Soyuz craft returned to Earth and landed normally.

      The European Space Agency launched its Autonomous Transfer Vehicle (ATV), an automatically piloted supply ship for the ISS. The first unit, dubbed Jules Verne, was launched on March 9. It made two test approaches by using the Global Positioning System and a laser tracking system, and then it performed an automated docking on April 3. After supplies were loaded onto the ISS and replaced with ISS waste, Jules Verne was undocked and sent into the atmosphere, where it burned up. Another three ATV missions were planned, and Japan was to introduce a similar transfer vehicle in 2009.

Space Probes.

       India joined the ranks of countries that had sent a spacecraft to the Moon when its Chandrayaan-1, launched on October 22, reached the Moon on November 8. (Chandrayaan is Hindi for “moon craft.”) From an orbit only 100 km (60 mi) above the lunar surface, the spacecraft was to map the lunar terrain at high spectral and spatial resolution and with stereo images. A miniature synthetic aperture radar was designed to search for indications of any water hidden in the soil in the Moon's north and south polar regions.

      NASA's Phoenix Mars Lander touched down in the north polar region of Mars on May 25. (Phoenix was constructed from a partially built spacecraft from the canceled Mars Surveyor 2001 program.) Its high-altitude landing site, on the plain Vastitas Borealis, permitted the lander's solar arrays to receive continuous summer daylight for its planned 90-day mission. Using a robotic arm, the spacecraft uncovered traces of water ice, and its equipment for chemical analysis showed that the surface-soil chemistry was highly alkaline. Phoenix continued to operate until early November.

      More than four years after having landed on Mars and fulfilled their planned 90-day mission, the Opportunity and Spirit rovers continued exploring the planet. After completing a 24-month exploration of Victoria crater, Opportunity headed toward a 22-km (13.7-mi)-wide crater about 12 km (7.5 mi) away on a two-year trip that was to be made with the aid of imagery from the Mars Reconnaissance Orbiter. Spirit, in Gusev crater, was parked for the Martian winter and survived a dust storm that coated its solar panels with dust.

      In 2008 the Messenger probe flew past Mercury on January 14 and October 6, and it was to make one additional flyby on Sept. 29, 2009. Each encounter reshaped the U.S. probe's solar orbit to target it for entry into Mercury orbit on March 18, 2011. Images obtained during the flybys revealed that Mercury's craters were only half as deep, proportionally, as those of the Moon.

      NASA's New Horizons probe crossed the orbit of Saturn (though the planet was on the other side of the solar system) as it continued on its way to a flyby of Pluto in 2015. The Ulysses solar polar mission formally ended on June 30, a few months after having completed its third pass over the northern hemisphere of the Sun. Ulysses had studied the solar wind at higher solar latitudes than had previously been possible.

      The Lunar Reconnaissance Orbiter was to be launched by NASA in early 2009 to scout potential landing sites for robotic and manned missions and for possible resources, including water. One instrument would measure the ambient radiation, data that were crucial for the safety of future crews. In an effort to determine the rate at which craters were being formed, cameras and other instruments would remap areas that had been studied during Project Apollo. The orbiter would also release a small probe that would impact the Moon.

Unmanned Satellites.

      Several space-science satellites were launched during the year. NASA's Gamma-ray Large Area Space Telescope, launched June 11, carried a large-area telescope for high-resolution studies of gamma-ray bursts. A burst monitor would immediately alert the spacecraft to any new gamma-ray bursts so that it could point the telescope at them within minutes and identify their source. On August 26, after completing a checkout of the onboard instruments, NASA renamed the spacecraft the Fermi Gamma Ray Space Telescope.

      The Interstellar Boundary Explorer (IBEX) was carried into space on October 19 aboard an aircraft-launched Pegasus rocket. IBEX's propulsion motor then was then used to form a high-apogee orbit from which the satellite was to map where the solar wind formed a shock wave as it collided with the interstellar medium at the far reaches of the solar system.

Launch Vehicles.

      The Sea Launch Zenit launch vehicles returned to service in January 2008 following repairs to fix damage caused to its floating ocean launch platform by the explosion of a Zenit rocket about one year earlier. Development of the Ares launch vehicle, derived from the space shuttle solid-rocket booster, encountered problems with unexpected vibrations that could affect crew performance during the first-stage burn. A shock-absorbing system was to be added to alleviate the problem. The first unmanned Ares I-X test launch was scheduled for mid-2009.

      Success finally came to the SpaceX venture of hotel magnate Elon Musk. SpaceX had experienced three failures of its Falcon 1 launch vehicle in as many tries—the latest on Aug. 2, 2008, when the first and second stages failed to separate. On September 28 a fourth Falcon 1 was launched from the Kwajalein Atoll in the central Pacific Ocean. Although the satellite it carried failed to separate from the second stage, the rocket launch was rated as a success. SpaceX was planning on providing unmanned and manned missions to the ISS in the period between the discontinuance of the space shuttle and the start of Orion operations.

Dave Dooling

▪ 2008

Introduction

Scientists improved catalysts and worked with synthetic molecule self-assembly, techniques for electron acceleration, and hyperlenses. Three space shuttle missions were flown, and Chinese and Japanese probes reached the Moon. Astronomers mapped dark matter and reported the brightest supernova, the most massive star, and the most Earth-like extrasolar planet.

Chemistry

Applied Chemistry.

 Platinum catalysts, because of their high chemical activity, were good candidates for making hydrogen fuel cells more efficient and cost-effective for use in cars, but they still needed much development. For example, the oxygen reduction that takes place on platinum catalysts in a fuel cell can form side products such as hydroxide ions (OH⁻), which can then react with platinum and render the catalytic surface unreactive. Two studies published in early 2007 looked at strategies that could increase the activity and overall efficiency of catalytic platinum surfaces. In one study Vojislav Stamenkovic and Nenad Markovic of Argonne (Ill.) National Laboratory and their colleagues described improved oxygen-reduction reactions with a surface that contained a 3:1 ratio of platinum to nickel. The atoms were packed as tightly as possible, an arrangement called a 111 surface. The surface alloy was 90 times more reactive than a traditional platinum-on-carbon catalyst and was 10 times more reactive than a pure platinum surface. In the second study Radoslav Adzic and colleagues at Brookhaven National Laboratory, Upton, N.Y., introduced gold nanoclusters to a platinum-carbon cathode. The modified cathode was equally effective in reducing oxygen, but the gold slowed the degradation of the cathode.

      Other researchers investigated molecular engineering through the chemistry of self-assembled molecules. Such synthetic systems were modeled after biological systems whose structure included all the necessary information to specify how a complex of different kinds of molecules would assemble and organize without external direction. The basic model for such systems was to build a “seed molecule” and add molecules to the initial nucleating structure. Ideally, researchers wanted to use these strategies to specify how molecules came together on the basis of external conditions so that the researchers could easily construct precise reproducible systems that assembled predictably on a molecular scale. Rebecca Shulman and Erik Winfree of the California Institute of Technology described conditions in which they were able to coax tiles made from DNA molecules to associate in a desired pattern to form ribbonlike structures. The researchers studied the thermodynamics of these structures—both the formation of new structures (nucleation) and the addition of tiles to the ends of the structures (elongation). Although both processes were energetically comparable, the wider ribbons had a slower rate of nucleation, which made it possible to specify the elongation of the structures. This type of control gave materials researchers another tool for fabricating materials at the micrometre scale.

Environmental Chemistry.

      As more consumer products included nanoscale materials—materials manufactured from particles 1 to 100 billionths of a metre in size—researchers worked to understand their possible effects on environment and health. In some cases the chemical properties of nanoscale particles differed from those of macroscopic particles of the same chemical composition. The distinctive or enhanced chemical activity of nanoscale particles provided opportunities for medical applications, such as for delivering drugs more effectively into living cells. The differences in chemical properties between macroscopic particles and nanoscale particles meant that their relative safety might also vary, however. In April, Ludwig Limbach of the Swiss Federal Institute of Technology, Zürich, and his colleagues examined how metal-oxide nanoparticles within a cell affected the production of reactive oxygen species (chemicals that contain oxygen atoms with unpaired electrons that can react with molecules such as DNA). Nanoparticles of oxides of iron, titanium, cobalt, or manganese oxide were found to elevate the production of reactive oxygen species in cultures of cells that line the human respiratory tract. Cell membranes were capable of blocking ions dissolved in solution from entering a cell, but the nanoparticles acted as a carrier to take the metal oxides inside the cell.

      Salts of chromium(VI), or hexavalent chromium, were usually considered to be industrial pollutants, but researchers explained how these toxic compounds could form naturally and build to unsafe levels in certain regions with chromium ores, such as California, Italy, Mexico, and New Caledonia. Chromium in chromite and other chromium ores typically exist in a nontoxic form called chromium(III). Scott Fendorf and colleagues of Stanford University used laboratory experiments to show that birnessite, a manganese-oxide mineral found in these regions, could oxidize the chromium(III) in chromite into chromium(VI). The World Health Organization's standard for maximum allowable chromium(VI) levels in drinking water was 50 micrograms per litre. Under neutral pH conditions, the experiments showed that chromium(VI) levels in such natural environments could exceed that value within a period of 100 days. Understanding these processes was expected to help scientists predict where natural chromium(VI) levels might exceed health standards.

Organic Chemistry.

      The synthesis of carbohydrate structures presented particularly difficult challenges in organic chemistry. It was notoriously difficult to maintain the stereochemistry (three-dimensional arrangement) of the glycosidic bond in carbohydrates that links one sugar molecule to another. In addition, the backbone of carbohydrate molecules is covered by many copies of the same functional group, a hydroxyl (OH) group, which made it difficult to attach different groups at specific positions along the ring. Synthesis of polysaccharides usually involved the tedious steps of adding and removing protecting groups to differentiate the alcohols and purification to remove unwanted side products. Hung Shang-cheng of National Tsing Hua University, Hsinchu, Taiwan, and his colleagues, however, demonstrated a method for producing multiple derivatives of glucose in “one pot”—that is, without successive isolation and purification steps. The one-pot technique relied on the use of catalytic trimethylsilyltriflate and benzyl ether and substituted protecting groups of benzyl ether. Subtle changes in the reaction conditions led to a variety of products, and the researchers demonstrated how these methods could be used to synthesize a number of polysaccharides, including the trisaccharide that binds to the H5N1 avian influenza virus. Such methods might be used to speed the synthesis of polysaccharides in chemical and biological studies.

      Organic chemists continued to develop new methods for synthesizing chiral molecules—molecules with two forms (enantiomers) that are mirror images of each other but are not identical. The manufacture of medications, pesticides, and other important compounds often required one enantiomer and not the other, and—for this purpose—organic chemists traditionally used metal catalysts with bound chiral ligands. Such molecules typically contained a central metal ion bound to a chiral organic complex that introduced overall right- or left-handedness into the product. F. Dean Toste and his colleagues at the University of California, Berkeley, demonstrated that the chiral portion of a molecule did not have to be directly attached to the metal ion in order to produce a chiral product. They used a gold-ion catalyst bound to a chiral binaphthol-derived counterion (an ion whose charge was opposite that of the gold ion). In solution the catalyst produced a high yield that had a 90% excess of one enantiomer by selectively cyclizing an allenic alcohol to produce a cyclic ether product.

Industrial Chemistry.

      Biaryl molecules (molecules that contain two aromatic rings, or groups, linked by a carbon-carbon bond) were important for a variety of industrial applications, including light-emitting diodes, electron-transport devices, liquid crystals, and medicines. Their synthesis was not straightforward, however, because the molecules could react with each other at a variety of positions along the aromatic rings. Previously, the synthesis of biaryl molecules generally required specific preactivation of each of the aromatic precursors to achieve the desired products. In May, David R. Stuart and Keith Fagnou of the University of Ottawa reported a catalytic method for cleanly and efficiently linking the aromatic compounds indole and benzene. The method required acetylation of the nitrogen on the indole ring and used a palladium catalyst with copper(II) acetate, 3-nitropyridine, and cesium pivalate. The reactions were carried out with thermal or microwave heating and showed cross-coupling and good regioselectivity for the carbon atom at position 2 of the indole group.

Physical Chemistry.

      Measuring the flow of heat energy on a large-scale surface could be as simple as using a thermometer. It was far more complicated, however, to measure heat flow at the microscopic scale of nanocircuits and molecular-scale electronic devices. Such measurements had to gauge both short time intervals and small space intervals accurately, and they had to be able to distinguish heat-energy transfer from other forms of energy transfer within the system. Dana Dlott and colleagues at the University of Illinois at Urbana-Champaign used a two-dimensional system of hydrocarbons that contained 6 to 24 carbon atoms attached to a gold surface to examine their vibrational movements while heated. The researchers used a laser to heat a gold surface to 800 °C (1,470 °F), and they measured how quickly the heat energy reached the methyl ends of the hydrocarbon chains. The experimenters found two time values that were proportional to the length of the carbon chain. One time value measured the time that it took for the end of the chain to become vibrationally disordered, and the other value tracked the movement of disorder through the hydrocarbon chain. The researchers' findings illustrated the similarities between heat-energy transport and electronic conduction. This research added to a growing body of knowledge that suggested that molecular-scale electronics systems would need to account for heat conduction in addition to electronic factors.

Sarah Webb

Physics

Particle Physics.

      Fundamental particle theory encompassed three of the forces of nature (the electromagnetic force and the strong and weak nuclear forces), but it had not been able to encompass the gravitational force. One attempt to do so required that the inverse square law of gravitational attraction for massive particles break down at very small separations. In 2007 a torsion-balance experiment by Dan J. Kapner and co-workers at the Center for Experimental Nuclear Physics and Astrophysics, University of Washington at Seattle, appeared to invalidate this attempt to unify the four forces. The experiment provided the most precise direct verification to date of the inverse square law and showed, to a confidence limit of 95%, that the inverse square law was obeyed down to a distance of 55 micrometres (0.002 in).

      The neutrino, one of the most common fundamental particles, was very difficult to study because it interacts only very weakly with other particles. Three types of neutrino exist, and in 1998 it was established that they oscillate (change from one type to another). This phenomenon was an indication that neutrinos have mass, which is an important parameter for the standard model of fundamental particle theory. Experimenters at the Los Alamos (N.M.) Meson Physics Facility (LAMPF), however, found evidence for mass differences between neutrino types so great that it was proposed that yet another type of neutrino, named the sterile neutrino, might exist. In 2007 scientists at the MiniBooNE neutrino detector at Fermilab, Batavia, Ill., reported that they could not reproduce the LAMPF results, which was seen as strong confirmation of the simpler picture. Some new puzzling results, however, suggested that the problem had not yet been completely solved.

      Each type of fundamental particle has its equivalent antiparticle, and a particle and its antiparticle annihilate on meeting. The production of atoms of antihydrogen, which consists of an antielectron bound to an antiproton, provided an important tool for looking for any differences between particles and their antiparticles. In 2007 researchers in the Antihydrogen Laser Physics Apparatus collaboration at the European Organization for Nuclear Research ( CERN) near Geneva managed to trap and store antihydrogen atoms for an interval of time that would be long enough to permit their detailed study for the first time.

      A major constraint on the investigation of the fundamental forces of nature was the requirement for ever-larger and more-expensive particle accelerators such as CERN's multibillion-dollar Large Hadron Collider, which was nearing completion for a 2008 startup. Meanwhile, Ian Blumenfeld and co-workers at the Stanford (Calif.) Linear Accelerator Center described a technique for accelerating electrons in the wake of an electron beam moving at an extremely high speed through an ionized gas. The new approach had the potential to produce beams of ultrahigh-energy electrons at much lower cost than established techniques.

Optics.

      The production of tailor-made materials made possible a new class of optical instruments. Researchers had produced materials with negative refractive indexes, which bend light in the opposite direction from that of conventional materials and therefore might be used for new kinds of lenses or, possibly, for so-called invisibility cloaks. Previously available materials with a negative refractive index worked only in the infrared region of the spectrum, but Gunner Dolling and colleagues of the University of Karlsruhe, Ger., built a metamaterial (a composite material that does not exist in nature) that had a negative refractive index at the red end of the visible spectrum. The new material consisted of etched layers of silver and magnesium fluoride on a glass substrate.

 Zhaowei Liu and co-workers at the University of California, Berkeley, and Igor Smolyaninov and colleagues of the University of Maryland published details of magnifying “hyperlenses.” These devices used the properties of evanescent waves (waves such as internally reflected waves that rapidly diminish over distance) to produce magnified images of structures with dimensions that were small compared with the wavelength of the illuminating light. Both teams used nanostructured metamaterials that had dielectric constants of opposite sign in perpendicular directions.

      Using similar techniques, René de Waele and colleagues of the FOM Institute for Atomic and Molecular Physics, Amsterdam, used a chain of tiny silver particles to function like a television antenna to direct light waves. The technique pointed the way to new types of devices for controlling light.

      Jun Ren and colleagues at Princeton University demonstrated a new method of amplifying and compressing a laser pulse through scattering in a millimetre-scale plasma, a technique that could make possible a new generation of compact low-cost ultrahigh-intensity laser systems.

Condensed-Matter Physics.

      Phase transitions, such as the condensation of water vapour on a cold surface, are common in nature. Exotic cases of phase transition, such as the formation of a Bose-Einstein condensate (BEC), were of great interest, and M. Hugbart and co-workers of the Institute of Optics, Orsay, France, and Stephan Ritter and collaborators of the Institute for Quantum Electronics, Zürich, were able to observe the formation of a BEC droplet. (A BEC is a clump of atoms that are all in the same quantum state and hence act as a single “super atom.”)

      A demonstration of the way in which BECs show quantum-mechanical effects on a macroscopic scale was given by Naomi S. Ginsberg and colleagues of Harvard University. Two independently prepared BECs of about 1.8 million sodium atoms each and separated by more than 100 micrometres (0.004 in) were coupled via a laser beam. A light pulse from a second (probe) laser was then imprinted on one of the condensates. In quantum-mechanical terms, the two clumps of atoms were indistinguishable objects, so the probe pulse imprinted on one condensate would theoretically be retrievable from the other. The researchers confirmed the phenomenon, and the experiment pointed to a whole new field of quantum information processing in which information stored in one condensate could be retrieved from one or many other condensates.

      The nature of high-temperature superconductors (materials with zero electrical resistance at or near room temperature) had been an enigma to researchers. Kenjiro K. Gomes and colleagues of Princeton University and, separately, Nicolas Doiron-Leyraud and colleagues at the University of Sherbrooke, Que., advanced the understanding of these materials by making progress in observing the phase transition of metallic oxides of copper to the superconducting state.

      In more-conventional solid-state physics, researchers were tackling the problem of increasing the speed and performance of computer systems via spintronics—the use of the spin of electrons to transport and store information. Xiaohua Lou and fellow workers at the University of Minnesota demonstrated a fully electrical scheme for achieving spin injection, transport, and detection in a single device that used ferromagnetic contacts on a gallium arsenate substrate. Ian Appelbaum and colleagues of the University of Delaware produced a similar device based on silicon, the most common material used in semiconductor electronics. Although this feat might provide a breakthrough, the device worked at 85 K (–188 °C, or –307 °F) rather than at room temperature, and considerable development would be needed before a commercial product emerged.

      Advancing in a different direction, Darrick E. Chang and co-workers from Harvard University developed a technique that allowed one light signal to control another and could serve as the basis for a single- photon transistor. The presence or absence of a single incident photon could permit or block the passage of signal photons along a microscopic wire.

Fundamental Physics.

      The Casimir Effect—first postulated in 1948 by Dutch physicist Hendrik Casimir—was a theoretical curiosity that had become important in the physics of nanostructures. This strange effect arises from the quantum theory of electromagnetic radiation, which predicts that the whole of space is permeated by random tiny amounts of energy, called zero-point energy, even when no fields are present. Casimir suggested that this energy might produce a tiny attractive force between two parallel metallic discs. This force was studied directly by Jeremy N. Munday and Federico Capasso of Harvard University, who carried out experiments at nanometre dimensions to make precision measurements of the force between two metals immersed in a fluid. They found that the results were compatible with the predictions of Casimir's theory. Capasso and co-workers proposed to use this effect to make microscopic motion-and-position sensors. Meanwhile, John Obrecht and colleagues at JILA (formerly Joint Institute for Laboratory Astrophysics), Boulder, Colo., measured the force between a glass plate and a cloud of rubidium atoms. As the plate was heated, the force increased in accordance with Casimir's theory.

      Most physicists accepted that an external reality exists, independent of observation. This belief, however, ran counter to some of the predictions of quantum mechanics. The famous Einstein-Podolsky-Rosen (EPR) “thought experiment” sought to demonstrate that if the predictions of quantum mechanics were correct, it was necessary for all real objects to be connected by some type of instantaneous action at a distance (nonlocal action)—which suggested to Einstein that quantum mechanics was incomplete. In 1972, however, John Clauser carried out an experiment that was equivalent to the EPR thought experiment and that vindicated the quantum-mechanical result; that is, the world could not be both “real” and “local.” Simon Gröblacher and colleagues from the University of Vienna investigated the issue and in 2007 reported on experiments that ruled out a whole class of real nonlocal theories. The result made the discussion of what physicists meant by “reality” yet more complex.

David G.C. Jones

Astronomy

Solar System.

      For information on Eclipses, Equinoxes and Solstices, and Earth Perihelion and Aphelion in 2008, see Table (Earth Perihelion and Aphelion, 2008).

      A host of new findings about the solar system's planets were made in 2007, including a confirmation that the innermost planet, Mercury, has a liquid core. Before 1974, when the Mariner 10 spacecraft detected a weak magnetic field around Mercury, geophysicists had thought that the planet was a completely solid body. Although the strength of the magnetic field was only about 1% that of Earth's, its presence suggested that the core might not be solid, because the convective motion of molten core material was a possible source of the field. One way to test for the presence of a fluid interior was to look for small variations in the planet's rate of spin. During 2002–06 a team of researchers led by Jean-Luc Margot of Cornell University, Ithaca, N.Y., directed high-power radar beams toward Mercury and analyzed the reflected signals. In 2007 the team announced that the radar signals revealed a wobble in Mercury's spin. Though the wobble was a mere 420 m (1,380 ft), it was greater than what it would be if Mercury's interior was completely solid. One possible explanation for the persistence of a liquid core was that the planet's metallic core might contain sulfur, which would reduce the core's melting point.

      The New Horizons spacecraft, which was to rendezvous with the dwarf planet Pluto in the year 2015, flew past Jupiter on Feb. 28, 2007, for a gravitational boost on its long journey. During the flyby the spacecraft made observations of Jupiter and its moons and ring system. Detailed images of the ring system did not reveal any embedded moonlets larger than about 1 km (0.6 mi). Astronomers expected to see such objects if the ring system had been built from the debris of shattered moons. The spacecraft's route took it along the tail of Jupiter's magnetosphere, and New Horizons found pulses of energetic particles flowing along the tail modulated by Jupiter's 10-hour rotation rate. The spacecraft also studied a major volcanic eruption on the moon Io, found global changes in Jupiter's weather, observed the formation of ammonia clouds in the atmosphere, and—for the first time—detected lightning in the planet's polar regions.

      In orbit around Saturn, the Cassini spacecraft continued its study of the planet and its satellites. Cassini's visual and infrared mapping spectrometer provided the first complete image of a cloud feature that appeared as a hexagonal pattern around Saturn's north pole. The 25,000-km (15,500-mi) wide feature was believed to extend about 100 km (60 mi) below the tops of the clouds that bordered it. On the basis of a Cassini flyby of the spongy-looking moon Hyperion, scientists computed that the moon's density was only about one-half that of water. Cassini data confirmed that the surface had frozen water and indicated that there were deposits of hydrocarbon substances, which suggested that Hyperion had all of the chemical ingredients, if not the physical conditions, for life.

      In late October, Comet 17P/Holmes—a normally dim periodic comet that orbits the Sun between Jupiter and Mars—suddenly brightened by a factor of up to one million to become an object visible to the unaided eye. Within a day its outer layers had expanded to give it the appearance through binoculars of a circular disk about the angular size of the Moon. The comet had had two similar outbursts 115 years earlier, when English amateur astronomer Edwin Holmes discovered it. The most likely explanation for the outbursts was that a layer of nonvolatile material that coated the surface fractured suddenly, releasing underlying volatile material.

Stars.

 In 2007 discoveries of planets again dominated the news of extrasolar system astronomy. Most of the roughly 250 extrasolar planets discovered to date had been found by detecting and measuring minute changes in the motion of stars that were orbited by a planet. About 20 extrasolar planets had been found by detecting changes in the brightness of a star as the orbiting planet passed in front of, or transited, the star. One such notable discovery was HAT-P-2b, an extrasolar planet that had both a large mass—about eight times that of Jupiter—and a density greater than that of Earth. The combination was puzzling, since giant planets were thought to be gaseous like Jupiter and therefore of relatively low density. Another notable discovery was Gliese 581c, which orbited the red dwarf star Gliese 581, about 20 light-years from Earth. The planet was of particular interest because, with a diameter about 1.5 times that of Earth, Gliese 581c was the smallest extrasolar planet yet discovered and the most Earth-like. The initial reports from the planet's discoverers, a team led by Stéphane Udry of the Geneva Observatory, suggested that the planet lay in the star's “habitable zone,” where conditions would permit the existence of liquid water on the planet's surface. Late in the year a team of astronomers led by Debra Fischer of San Francisco State University and Geoffrey Marcy of the University of California, Berkeley, announced the discovery of another planet in orbit around 55 Cancri—a relatively nearby star that had already been found to have four planets. All of these discoveries suggested that the solar system was far from unique in the galaxy.

      The year also brought reports of the some of biggest and brightest stars that had ever been observed. Anthony Moffat of the University of Montreal and his collaborators reported that they had found very high masses for two stars that revolved around one another in a binary star system, called A1, that lay within the star cluster NGC 3603 in the Milky Way Galaxy. The astronomers determined that one of the stars was 84 times as massive—and its companion 114 times as massive—as the Sun. The mass of the heavier star was believed close to the maximum that was possible for a stable nuclear-burning star. Such massive stars can eject their outer layers and therefore typically lose mass as they age. In view of this mass-loss effect, a discovery reported by Andrea Prestwich of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., and collaborators was surprising. Using NASA's Chandra X-ray Observatory, the researchers found a 24- to 33-solar-mass black hole in a binary star system in the nearby dwarf galaxy IC 10. It had been thought that the late evolution of the most massive stars would lead to stellar black holes of no more than 10–15 solar masses.

      Before becoming black holes, stars with a mass more than 5–10 times that of the Sun were believed to collapse and then explode as a Type II supernova, one of the most violent events in the universe. In April a team of astronomers led by Nathan Smith of the University of California, Berkeley, and Eran Ofek of the California Institute of Technology (Caltech) announced that supernova SN 2006gy reached a peak luminosity (intrinsic brightness) about 100 billion times that of the Sun and was the most luminous supernova then known. In the first two months of the outburst, the star emitted more energy than the Sun had released during its lifetime. The astronomers proposed that the event represented the death of a star that initially had a mass greater than 100 solar masses. Not to be outdone, the discoverer of supernova SN 2006gy, Robert Quimby of Caltech, announced in October that the luminosity of another supernova that he had discovered, SN 2005ap, was twice that of SN 2006gy.

Galaxies and Cosmology.

      Since the mid-1990s astronomers had shown that the universe consists of about 4% ordinary matter (such as stars and gases in galaxies), 22% dark matter, and 74% dark energy. In 2007 an international team of astronomers led by Nick Scoville of Caltech created a three-dimensional map of dark matter as part of the Cosmic Evolution Survey. The survey made use of nearly 1,000 hours of observing time by the Hubble Space Telescope and included observations made with the European Space Agency's XMM-Newton X-ray satellite and a variety of ground-based observatories. The astronomers mapped the dark matter by measuring the way it distorted light from galaxies beyond it. They found that the largest identifiable structures in the universe are filaments of dark matter 60 million light-years long that contain two trillion times the mass of the Sun.

      Other major astronomical surveys revealed the distribution of active galaxies called quasars throughout the universe. (A quasar was thought to be a galaxy that contained a supermassive black hole at its centre.) A map of more than 4,000 quasars compiled as part of the Sloan Digital Sky Survey, for example, revealed that quasars in the early universe were strongly clumped. A survey of a patch of the sky about the size of the full moon that was conducted with the Chandra X-ray Observatory, Spitzer Space Telescope, and two ground-based telescopes found evidence for more than 1,000 supermassive black holes. The intense radiation emitted from the vicinity of supermassive black holes was thought to be emitted from the accretion of mass around them, but the survey observations called into question exactly how this accretion took place. Most quasars were solitary objects, but a few had been found to form pairs and orbit each other. An American-Swiss team of astronomers led by George Djorgovski of Caltech discovered for the first time a triple quasar system, which was named QQQ 1432. The three quasars in the system were separated from each other by a distance less than the diameter of the Milky Way Galaxy.

Kenneth Brecher

Space Exploration

Manned Spaceflight.

      For Launches in Support of Human Space Flight in 2006, see Table (Human Spaceflight Launches and Returns, 2007).

  In 2007 three Space Shuttle missions—STS-117, 118, and 120—were flown to the International Space Station (ISS). The first mission installed the S3/S4 (starboard) truss and its pair of solar arrays. The additional solar-power capability was needed to power new modules that were to be delivered later. The STS-118 mission added the S5 truss (in preparation for the S6 truss and its solar arrays in 2008), a new control gyroscope to help the ISS maintain its orientation (the gyroscope replaced one that failed in 2006), and an external equipment-storage platform. During STS-120 the P6 solar array was relocated from top centre of the station (where it had been installed in 2000) to the end of the port truss, and the Harmony node module was berthed at a temporary location. Part of the array became torn as it was redeployed, however, and the shuttle crew made repairs during a risky spacewalk. Metal shavings were found in the rotary joint of another solar array, and it was to be locked in place until the problem could be addressed on a subsequent mission. After the shuttle's departure, the station crew used robot arms to relocate the Harmony node to the front of the Destiny laboratory module. With the new solar arrays providing more electrical power and the Harmony node allowing extra berthing ports, ISS expansion was expected to continue at a rapid pace. NASA still planned on completion of construction in 2010 so that it could retire the space shuttle and shift resources to the Orion spacecraft and Ares launcher.

      The space shuttle flights went smoothly for the most part. The STS-117 crew had to repair damaged insulation on a maneuvering-engine pod on Atlantis. Tiles on the lower surface of Endeavour were gouged when insulation broke loose during the STS-118 mission. A special space walk was planned to repair the tiles but was canceled when NASA Mission Control decided that the damage was not so deep that it would endanger the shuttle and its crew. STS-118 carried NASA mission specialist Barbara Morgan, who conducted several televised classroom presentations from space. A former schoolteacher, Morgan had been the backup for Christa McAuliffe, the schoolteacher-astronaut who perished in 1986 in the accident that destroyed the space shuttle Challenger. The launch of STS-120 was almost delayed because of erosion to tiles on the leading edge of one wing, but NASA decided that the damage would not endanger the mission.

      The future of the ISS as a research facility became brighter during the year. On August 14 NASA formally announced that it planned “to operate a share of U.S. accommodations on the International Space Station as a national laboratory … for research and development, and industrial processing purposes.” On September 12 NASA and the U.S. National Institutes of Health signed an agreement for the NIH to use the station for research that included basic biological and behavioral mechanisms in the absence of gravity, human physiology and metabolism, spatial orientation and cognition, cell-repair processes and tissue regeneration, pathogen infectivity and host immunity, health care delivery, health monitoring technologies, and medical countermeasures against enemy attack.

      Regarding private manned space flight, Bigelow Aerospace proceeded with plans to develop a space motel. Russia launched Bigelow's Genesis 2 satellite on June 28. The module, which was inflated in orbit from 1.9 to 3.8 m (6.2 to 12.5 ft) in diameter, incorporated better communications equipment and other technological improvements made since the launch of Genesis 1 in 2006.

Space Probes.

      NASA's Phoenix Mars Lander headed for the Red Planet on August 4 for a touchdown scheduled for May 25, 2008. Phoenix more closely resembled the Viking landers of the 1970s than the twin rovers that were still roaming the planet. Phoenix was designed to stay at a single location in the Martian arctic and drill for rock samples with a 2.35-m (7.7-ft) robotic arm. The samples would be analyzed in a small self-contained chemistry laboratory. Other instruments included a small weather station and a camera. Phoenix's main objective was to provide answers to the questions of whether the Martian arctic could support life, what the history of water was at the landing site, and how Martian climate was affected by polar dynamics. Meanwhile, the Mars rovers Spirit and Opportunity continued to work even after a significant Martian dust storm that for a time coated their solar cells. Opportunity entered Victoria crater on September 11 on the riskiest trek yet for either of the rovers.

      The first of the new wave of lunar exploration started on September 13 with the Japanese Aerospace and Exploration Agency's launching SELENE, the Selenological and Engineering Explorer (also known as Kaguya). It arrived in lunar orbit on October 4 after a series of gravity-assist maneuvers. Kaguya carried a variety of instruments, including X-ray, gamma-ray, and charged-particle spectrometers to measure radiation scattered back into space by subsurface minerals, a laser altimeter to measure surface elevations with an accuracy of up to 5 m (16 ft), and a radar that used long radio waves to probe soil structure to a depth of several kilometres. It also had a camera and multiband imager to provide stereo images in visible light and infrared radiation. Kaguya was to deploy two subsatellites—RSAT for ensuring near-continuous communications between Kaguya and Earth and VRAD for use as a “radiostar” for precise mapping of the lunar gravity field. It was joined November 5 by Chang'e-1, launched October 24 by China in its first venture beyond Earth orbit. Named for the Chinese goddess of the Moon, Chang'e-1 carried cameras, X-ray and gamma-ray spectrometers, and a laser altimeter to assay the lunar surface during its one-year mission.

      NASA launched its Dawn mission to explore asteroid Vesta and dwarf planet Ceres on September 27. It carried a visual and infrared spectrometer and a gamma-ray and neutron detector to map and assay the two bodies. Dawn was to make a gravity-assist flyby of Mars in February 2009 and go into orbit around Vesta in August 2011. The probe would then leave Vesta in May 2012 and arrive at Ceres in February 2015. Vesta was believed to be an entirely rocky body, but Ceres was believed to contain large amounts of frozen water. Europe's Rosetta craft (launched March 2, 2004) made successful gravity-assist flybys of Mars and Earth in 2007 on its way to flybys of the asteroids Steins and Lutetia and an eventual orbit of the comet 67P/Churyumov-Gerasimenko.

      The U.S. New Horizons probe, launched on a mission to Pluto on Jan. 19, 2006, zipped past Jupiter for a gravity assist on Feb. 28, 2007. In its observations of Jupiter, the probe recorded lightning near Jupiter's poles, boulder-size objects in the tenuous ring system, and charged particles far along the planet's magnetic tail. Arrival at Pluto was set for 2015. NASA's Messenger probe, launched Aug. 3, 2004, made its second Venus flyby on June 5, 2007, and would make its first Mercury flyby on Jan. 14, 2008. Two more flybys were to follow as part of a gradual reshaping of the probe's solar orbit until insertion into Mercury orbit on March 18, 2011. Europe's Venus Express, orbiting Venus since April 11, 2006, completed its originally planned mission on July 24, but the mission was extended for its atmospheric and imaging instruments through May 2009.

Unmanned Satellites.

 Five spacecraft that made up the mission named Time History of Events and Macroscale Interactions During Substorms were launched by NASA on February 17. The spacecraft were to follow elliptical orbits whose orientation would sift relative to the Earth, the Sun, and radiation belts to help unravel where and when substorm disturbances in Earth's magnetosphere began. The mission also involved an array of ground stations. NASA's Aeronomy of Ice in Mesosphere mission was launched April 25 to study noctilucent clouds, faint ice-bearing clouds that form at a height of about 82 km (50 mi) in the atmosphere. On April 23 India launched Italy's Agile high-energy astrophysics satellite, which carried X-ray and gamma-ray detectors to study astronomical objects in the Milky Way Galaxy. NASA shut down its Far Ultraviolet Spectroscopic Explorer satellite on October 18, after eight years of operation, because it was running out of fuel for accurate pointing.

Launch Vehicles.

      The year was marred by a handful of launch-vehicle failures. A Sea Launch Zenit 3SL rocket, used to launch satellites from an ocean platform, blew up on January 30, severely damaging the platform. The second launch of a Falcon 1 rocket failed during its second-stage burn on March 20, but private backer Elon Musk pledged to press forward (the first launch failed in 2006). The usually reliable Russian Proton failed during its boost phase on September 5. In commercial development, Rocketplane Kistler fell behind schedule and lost its backing from NASA. An explosion on July 26 during a propulsion system test at Scaled Composites, builder of Virgin Galactic's StarShipTwo space tourism vehicle, killed three people at its facility in Mojave, Calif. Although development of a spaceport for Galactic StarShip near Upham, N.M., had already begun, Virgin Galactic admitted that the mishap might delay initial flights.

Dave Dooling

▪ 2007

Introduction

Scientists developed nanoparticle catalysts, graphene composites, ultraviolet LEDs, and optical tweezers. Construction of the International Space Station resumed with three space shuttle missions. Jupiter gained a red spot, and NASA launched the first probe to Pluto, which astronomers decided to call a dwarf planet.

Chemistry

Nuclear Chemistry.

      In October 2006 a team of scientists from Lawrence Livermore National Laboratory, Livermore, Calif., and the Joint Institute for Nuclear Research, Dubna, Russia, announced it had created element 118. The Livermore-Dubna team bombarded californium with calcium ions to produce the element, which quickly decayed. The announcement came seven years after a team of researchers at Lawrence Berkeley National Laboratory, Berkeley, Calif., first announced the discovery of element 118. The team retracted its findings in 2001 after an investigation showed that a scientist on the team had fabricated data.

Industrial Chemistry.

      Many of the chemicals used in making medicines, plastics, and weed killers are made from anilines, molecules with an aromatic ring and amino group. One way to make these compounds is to reduce a nitro (NO₂) group to an amino (NH₂) group. This process typically required relatively large amounts of reducing agents or the use of metals dissolved in solution and was therefore relatively expensive. In addition, these reactions often created unwanted side products, such as hydroxylamine, which is toxic and unstable. Avelino Corma and Pedro Serna of the Institute of Chemical Technology, Polytechnic University, of Valencia, Spain, reported that catalysts of gold nanoparticles supported on either titanium dioxide (TiO₂) or iron (III) oxide (Fe₂O₃) provided a way to get around these problems. By using hydrogen with these catalysts instead of with traditional palladium-on-carbon or platinum-on-carbon catalysts, they were able to reduce nitro groups selectively in the presence of other potentially reactive groups and also avoid hydroxylamine by-products.

Applied Chemistry.

      Long linear carbon polymers with alternating, repeating triple bonds are attractive to chemists for their ability to form mechanically stiff structures and for their potential to conduct electricity. The simplest such molecule is polyacetylene, but it is both difficult to work with and explosive. One modification of polyacetylene that scientists had experimented with in order to build a similar but more stable molecule was the placement of functional groups such as aromatic rings on every other triple bond. By themselves, however, these long chains could still form kinked rather than long, straight structures. To avoid these limitations, Aiwu Sun and colleagues at the State University of New York at Stony Brook developed a solid-state method of polymerizing diiododiacetylene (C₄I₂) that could keep the compound stable and create long, ordered chains. The key was to form crystals of C₄I₂ with an oxalamide as a cocrystallizing compound. The oxalamide, a Lewis base, associated with the C-I bonds that were weakly Lewis acidic in the C₄I₂ molecule. That bonding pattern helped to create a scaffold that allowed the formation of poly(diiododiacetylene) within fibrous deep blue cocrystals that were up to 2 cm (0.8 in) long. These molecules were expected to provide new electronic materials for study and could potentially be used for creating stabilized linear carbon.

       Carbon nanotubes—minute stringlike structures of carbon atoms bonded together in a hexagonal framework—are mechanically strong and have interesting electrical properties. In 2006 nanotubes were the hot new material for a great variety of studies, but they were relatively expensive to produce. A cost-saving alternative to nanotubes that was explored by Sasha Stankovich of Northwestern University, Evanston, Ill., and colleagues was the synthesis of one-atom-thick sheets of carbon, which are known as graphene. The starting material for the investigators was graphite, an economical form of carbon with a layered structure that can be separated through oxidation. The oxygen groups can then be removed to leave graphene sheets, but without some kind of molecular spacer, the sheets simply form useless clumps. The researchers added hydrophobic groups to the graphene so that the sheets would maintain their form and their separation from each other. The sheets could then be incorporated into polymers such as polystyrene. The researchers examined the properties of the graphene-polymers and found that with only 0.1% by volume of graphene the composites could conduct electricity.

Organic Chemistry.

      For organic chemists one critical challenge is the synthesis of molecules that have chirality—that is, molecules that can exist in two structural forms (enantiomers) that, like right and left hands, are mirror images of each other. Many types of molecules in living organisms, such as proteins and carbohydrates, are chiral, and medications and other important compounds often need to consist of one enantiomer and not its mirror image. To produce specific enantiomers, organic chemists typically used chiral catalysts that contained metals or enzymes to achieve this goal. New research in the field was showing how organic molecules without metals or enzymes could serve as chiral catalysts. Two groups in Japan independently demonstrated that binaphthol phosphoric acid molecules could produce chiral products in different reactions. Masahiro Terada and co-workers at Tohoku University, Sendai, Japan, used low catalyst concentrations to combine N-benzoylimines with enamides to form ß-aminoimines with high yields and high selectivity for one enantiomer. Examining Diels-Alder reactions, Junji Itoh and co-workers at Gakushuin University, Tokyo, showed that similar catalysts in low concentrations produced enantioselective reactions between aldimines and 1,3-dimethoxy-1-(trimethylsiloxy)butadiene (Brassard's diene) to give dihydropyridones.

      Because of the difficulty in forming specific enantiomers of chiral molecules in organic chemistry, scientists often wondered how biological systems developed a preference for right- or left-handedness in molecules. Experiments by Martin Klussmann and co-workers at the Imperial College, London, presented one possibility. Many amino acids, the building blocks of proteins, are chiral but can exist as equal mixtures of their two enantiomers. The researchers discovered that in concentrated mixtures the amino acids often consisted of uneven ratios of the two enantiomers. They also observed that when these mixtures served as catalysts for an aldol reaction, the resulting products had an enhanced ratio of one enantiomer over the other that varied with the chiral ratios of the amino-acid mixtures. Such an enhancement might explain how chiral molecules initially developed in nature without enzymes or other complex catalysts.

Environmental Chemistry.

      Some scientists were investigating alternatives to petroleum as source materials for producing the polymers found in everyday products. Such alternatives typically required manufacturing processes that were too expensive to be practical. One potential renewable starting material was fructose (the sugar in fruit) to produce 5-hydroxymethylfurfural (HMF), which in turn could be used for making many kinds of plastics. The major problem in isolating HMF from fructose, however, was that it could form a variety of side products by reacting with other molecules in the reaction mixture. It also could be difficult to isolate from the solvent. Yuriy Román-Leshkov and co-workers at the University of Wisconsin at Madison reported a way to convert HMF in a way that allowed the product to be cleanly isolated from other products. The researchers optimized the reaction and obtained an 85% yield of the product by using a biphasic mixture in which the aqueous phase included dimethylsulfoxide and poly(1-vinyl-2-pyrrolidinone) and the organic layer was methylisobutylketone (MIBK) with a small amount of 2-butanol. The 2-butanol helped make the HMF more soluble in the MIBK and kept it from reacting with the remaining fructose.

      Chemists continued to work out methods for “ green” chemistry—chemical processes that did not require the use of toxic reagents and that did not produce toxic by-products. One method demonstrated by Marcel Veerman and co-workers at the University of California, Los Angeles, increased the efficiency of chemical reactions of solid materials by using nanocrystals of the material. The researchers studied a photochemical reaction in which dicumyl ketone (DCK) formed dicumene. They were able to perform the reaction on a quantity of several grams of finely ground DCK that was suspended in water that contained sodium dodecylsulfate to reduce surface tension. By filtering the product through cellulose, they were able to obtain yields of up to 98%.

Physical Chemistry.

      Chemists sought ways to increase the reactivity of certain chemical bonds over others. Chemical bonds vibrate selectively with different frequencies of infrared radiation, but chemists had generally not been able to harness those vibrations for selective reactions. Zhiheng Liu of the University of Minnesota and colleagues showed that infrared signals could selectively remove hydrogen (H₂) from a hydrogen-coated silicon surface. The researchers used infrared radiation at the vibration frequency of the Si-H bond and showed that the vibration excitation and not heat energy was responsible for releasing H₂ from the surface. To test for selectivity, they mixed hydrogen and deuterium (a heavier isomer of hydrogen) and showed that when the surface was irradiated at the Si-H frequency, 95% of the released molecules were H₂.

      Researchers also examined the role that quantum mechanics can play in the chemistry of complex molecules. Valentyn Prokhorenko of the University of Toronto and colleagues investigated whether the wave property of matter could influence the chemistry of retinal, a molecule in the protein bacteriorhodopsin. Bacteriorhodopsin is found in the rods of the eye, and the chemistry of retinal is critical for vision. As retinal responds to incoming light, one of the carbon-carbon double bonds in the molecule changes from the trans to the cis isomeric form. The researchers studied the reaction with laser-generated pulses of light that approximated sunlight. By modifying characteristics of the light pulses with optimization algorithms, they were able to alter the amount of cis-isomer produced by up to 20%. The technique helped reveal the molecular dynamics driving the chemistry of retinal and could be useful for studying other complex molecular systems.

Sarah Webb

Physics

Particle Physics.

      In 2006 a possible sighting was reported of a predicted but previously unobserved fundamental particle called the axion. The existence of the particle was postulated in 1977 to explain an anomalous result of the field equations of quantum chromodynamics, the theory that describes the binding of the elementary particles called quarks in protons and neutrons. The axion was believed to have no spin, no charge, and a very small mass, which would make it very difficult to detect. The sighting was based on an experiment by Emilio Zavattini and colleagues in the PVLAS (vacuum polarization with a laser) collaboration at the Italian Institute of Nuclear Physics, Trieste, in which they used a magnetic field to rotate the polarization of light in a vacuum. The result could be interpreted as a manifestation of the axion, but the properties of the particle appeared to be far different from those that had been originally postulated. Experiments were planned by several groups to confirm Zavattini's result.

      Gerald Gabrielse of Harvard University and colleagues used quantum electrodynamics—the theory that describes the electromagnetic interaction between electrically charged particles—and an experiment based on observations of an electron in a single-electron cyclotron to determine a more accurate value for the fine-structure constant. The fine-structure constant is a fundamental constant of nature that corresponds to the strength of electromagnetic interactions. The researchers were able to calculate the fine-structure constant to an accuracy of 0.7 parts per billion—10 times better than the previous most accurate measurement, which was made in 1987.

      There was a suggestion, however, that the constants of nature might not be so constant. Aleksander Ivanchik of the Ioffe Institute, St. Petersburg, and Patrick Petitjean of the Institute of Astrophysics, Paris, measured the wavelengths of absorption lines in quasar light that passed through very distant clouds of hydrogen when the universe was young. From the measurements, they calculated what the ratio of the mass of the proton to that of the electron would have been at that time. They then compared their measurements with those that Wim Ubachs and Elmer Reinhold of the Free University in Amsterdam made in a laboratory, and the results suggested that the ratio might have changed by about 0.002% over 12 billion years. A variation of this magnitude could have dramatic consequences for any grand unified theory of elementary particles. More detailed observation was required in order to confirm the result.

Photonics.

      The newly developing field of nanotechnology, which involves the construction of structures of nanometre dimensions, demanded some way of “seeing” structures that consisted of only a relatively few atoms. This goal became a possibility with coherent (in-phase) X-ray diffraction imaging. Using this technique, Mark A. Pfeifer and co-workers at the University of Oregon produced three-dimensional images that showed the electron-density distributions in 750-nm hemispherical lead particles and deformations in their atomic lattice. First the particles were illuminated with a beam of coherent X-rays whose source was high-intensity synchrotron radiation from the Advanced Photon Source at Argonne National Laboratory near Chicago. The diffraction pattern created by the scattering of the illuminating X-rays was then processed mathematically to produce the three-dimensional images. The technique was a substantial step toward the goal of being able to image the position and type of every atom in a nanocrystal.

      Researchers were seeking to develop light-emitting diodes (LEDs) as a source of UVC radiation—ultraviolet radiation with a relatively short wavelength (100 to 280 nm)—for a variety of applications, including germicidal irradiation to destroy bacteria, viruses, and fungi. Yoshitaka Taniyasu and co-workers at NTT Basic Research Laboratories, Atsugi, Japan, reported creating an LED that emitted ultraviolet light with a wavelength of only 210 nm, the shortest wavelength yet recorded for an LED. It was made from semiconductor materials based on aluminum nitride. If successfully developed, such LEDs could replace mercury or xenon electric-discharge lamps as UVC sources.

      A distance record for the transmission and detection of a laser pulse was established by David E. Smith and co-workers from the Goddard Space Flight Center, Greenbelt, Md. They sent laser pulses between an Earth-based observatory and an instrument aboard the Messenger spacecraft on a voyage to Mercury. The spacecraft was about 24 million km (15 million mi) away, and the experiment demonstrated the possibility of increased precision in measurements of solar system dynamics.

Condensed-Matter Physics.

      Many research groups were carrying out experiments that involved trapping and cooling a few thousand gas atoms to temperatures less than a millionth of a degree above absolute zero (0 K, –273.15 °C, or –459.67 °F). In the case of atoms with zero or integral intrinsic spin (atoms called bosons), the cooling creates a state of matter known as a Bose-Einstein condensate (BEC). One of the properties of a BEC is superfluidity—a state of zero viscosity. In the case of atoms with multiples of half-integral spin ( fermions), the cooling creates a fermionic concentrate. This concentrate can exhibit superfluidity if fermions of opposite spins (spin-up and spin-down) pair and form bosonlike objects, a phenomenon demonstrated conclusively in 2005 in an experiment by Martin W. Zwierlein and colleagues at the Massachusetts Institute of Technology. In 2006 Zwierlein and co-workers at the MIT-Harvard Center for Ultracold Atoms reported the first direct observation of the phase change that occurs when a fermionic gas enters into a superfluid state. The researchers used a fermionic concentrate that consisted of a cloud of lithium atoms suspended as a gas in a vacuum trap. The gas contained an unequal number of spin-up and spin-down atoms, and the pairing interaction between them was tuned by applying a magnetic field. As the temperature was lowered and the gas underwent the phase change, the gas cloud changed shape abruptly, and a higher-density central bump was formed. Such experiments were enabling the modeling of many other physical systems, most importantly metallic structures that might produce superconductivity at or above room temperature.

      The atom-by-atom construction of materials with special properties was being carried out by a number of laboratories. Yevhen Miroshnychenko and colleagues at the Institute for Applied Physics, Bonn, Ger., used “optical tweezers” (focused laser beams) to arrange and reorder strings of neutral atoms in a way that possibly could serve as a scalable memory for quantum information. Dale Kitchen of Princeton University and colleagues developed a technique in which a scanning tunneling electron microscope positioned magnetic atoms one by one on the surface of a semiconductor. Materials constructed in this manner might form the basis of a new breed of computer chip that would integrate both logic functions and storage.

Quantum Physics.

      The next generation of computing systems might well rely on a quantum phenomenon, such as the alignment of the spin of a single electron, to store data in the form of qubits. Such systems, which were commonly referred to as spintronic, by analogy with electronic, were undergoing development in a number of laboratories. Most investigations concerned small semiconductor structures called “quantum dots.” They typically consisted of an isolated clump of up to a few hundred atoms and were usually built up from heterostructures of gallium arsenide and aluminum gallium arsenide. Frank H.L. Koppens and fellow workers at the University of Technology, Delft, Neth., reported progress in making such a concept a reality. They set up an experiment with two quantum dots that each contained only a single electron, and they used the phenomenon of electron spin resonance to rotate a single spin in one of the two coupled dots. They were able to detect the rotation of the spin by measuring the variation in an electric current through the double dot.

      The coupling between groups of quantum dots posed a major problem, since in normal circumstances there was a fast dephasing of the electron spins, which caused information to be lost. Several groups were trying to overcome this problem. Alex Greilich and colleagues at the University of Dortmund, Ger., used a train of light pulses to synchronize the spins. Eric A. Stinaff's group at the Naval Research Laboratory, Washington, D.C., used a technique of optical coupling between pairs of indium-arsenide quantum dots by using an electric field. Although there was still some way to go before a functioning computer system based on this technology could be built, Mladen Mitic and colleagues at the University of New South Wales, Australia, succeeded in constructing a device called a quantum cellular automaton from four quantum dots of silicon that could store data in a way that was compatible with existing microchip technology.

      Quantum dots also had other uses. Gerasimos Konstantatos and colleagues at the University of Toronto developed a photodetector that consisted of an unpatterned layer of lead-sulfide quantum-dot nanocrystals. The material exhibited a sensitivity in the near infrared that was 10 times better than conventional photodetectors.

David G.C. Jones

Astronomy

Solar System.

 The year 2006 in astronomy would likely be remembered by many as the year in which astronomers demoted Pluto from planet to dwarf planet. (See Sidebar. (Astronomers Reclassify Pluto as a Dwarf Planet )). Nevertheless, it was also a year in which astronomers made a number of discoveries about the solar system, particularly in regard to the giant gas planets. A one-of-a-kind series of observations of Saturn was made by NASA's Cassini spacecraft when it passed through the planet's shadow on September 15. With the Sun blocked by Saturn, the spacecraft's imaging detectors were able to take images of the planet and its rings as they were backlit by the Sun. The images revealed two new rings—the first rings of Saturn to be discovered since the flyby of Voyager 1 in 1980. The brighter of the two rings coincided with the orbit of the two small co-orbital moons Janus and Epimetheus; the other coincided with the orbit of the moon Pallene. The icy ring particles were most likely by-products of collisions between meteoroids and the moons that lay within the rings. Cassini also found two ringlets, or bands of icy particles, in the gap between Saturn's two main rings. The ringlets had not been observed by Voyager 1 or Voyager 2, which lent credence to the idea that some features of the ringlets, and perhaps the ringlets themselves, were short-lived phenomena.

      One of the most spectacular planetary features in the solar system is Jupiter's Great Red Spot, which is about two to three times the diameter of the Earth and was first reported by Italian-born French astronomer Gian Domenico Cassini in 1655. Several smaller white storms appeared on Jupiter in the 1930s. By late 2000 they had merged into a single storm that was about the size of the Earth, and by early 2006 the storm had turned red. Jupiter's two red spots, in adjacent bands of the atmosphere, brushed by each other in July as they moved around Jupiter in opposite directions. A detailed understanding of the origin and persistence of these large-scale planetary weather patterns had not yet been worked out, but some astronomers speculated that the formation of the new red spot might signal a major climate change in Jupiter's atmosphere.

      In August astronomers at the University of Wisconsin at Madison reported the first definitive images of a dark spot on Uranus. The images, taken with the Hubble Space Telescope Advanced Camera for Surveys, showed an elongated feature that was 1,700 × 3,000 km (1,100 × 1,900 mi) in size.

Stars.

      By late 2006 more than 200 extrasolar planets had been detected in orbit around relatively nearby stars. Most had been found indirectly by tracking the motion of individual stars and detecting the small variations in velocity of a star caused by one or more planets in orbit around it. The planets detected by this method were typically 100–1,000 times the mass of the Earth (the mass of Jupiter is about 320 times that of the Earth), and none had been imaged directly. Since 2000 several extrasolar planets had been found through an entirely different observational technique—gravitational microlensing. The technique depended on an effect first discussed by physicist Albert Einstein. In his 1916 paper on general relativity, he showed how light that passed a massive object would be deflected by the object's gravity. In a later paper he showed that a star could act as a gravitational lens that would focus the light from more distant stars that lay along the same line of sight. Several astronomical groups—PLANET (Probing Lensing Anomalies NETwork), OGLE (the Optical Gravitational Lensing Experiment), and MOA (Microlensing Observations in Astrophysics)—were searching for such lensing events. In early 2006 the groups announced that they had detected the signature of a microlensing event produced by a planet with a mass only 5.5 times that of the Earth, which made it the first Earth-like planet detected outside the solar system. The planet is in orbit around a relatively low-mass red dwarf star about 20,000 light-years from Earth, and it orbits the star at a distance about two and a half times the distance between the Earth and the Sun. Red dwarf stars are the most abundant stars in the galaxy, so the discovery suggested that Earth-like planets might be quite common.

Galaxies and Cosmology.

      The 2006 Nobel Prize for Physics was awarded to John C. Mather of the NASA Goddard Space Flight Center, Greenbelt, Md., and George F. Smoot of the University of California, Berkeley, for two major contributions to cosmology. Using detectors aboard NASA's Cosmic Background Explorer (COBE) satellite, launched in 1989, they confirmed to a high precision that the universe is bathed in a blackbody microwave background radiation and that the radiation exhibits small spatial intensity fluctuations consistent with the formation of galaxies. (See Nobel Prizes .) These two observations provided very strong support for the idea that the universe evolved from a hot, dense explosive event, popularly called the big bang. Subsequent observations by other space missions and a number of ground-based telescopes provided further details about the nature of the big bang. NASA's Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001, found that the event took place about 13.7 billion years ago and that the density of the universe is very near its closure value, which in topological terms means that the universe is spatially flat.

      In March 2006 the WMAP team, headed by Charles Bennett of Johns Hopkins University, Baltimore, Md., announced the results of observations of the polarization, or preferred alignment, of the background radiation. The polarization they observed implied that galaxies first formed about 400 million years after the big bang. The observed power spectrum of fluctuations from point to point in space suggested, but did not necessarily prove, that the known universe began with a very rapid inflationary phase during which the universe expanded by a factor of 10²⁶ within a fraction of a second. The WMAP data also confirmed with unprecedented precision that the universe contains 4.4% ordinary (atomic) matter and 22% invisible (probably cold) dark matter. The remaining mass-energy content of the universe seemed to be a little-understood form of energy responsible for the accelerating expansion of the universe and commonly referred to by astronomers as dark energy.

Kenneth Brecher

Space Exploration

Manned Spaceflight.

       NASA selected Lockheed Martin to design and build Orion—NASA's next-generation Crew Exploration Vehicle. The selection capped a yearlong competition between Lockheed Martin and a partnership formed by Northrop Grumman and Boeing. The initial contract was worth $3.9 billion. Orion would be able to carry six crew members to the International Space Station (ISS) or four crew members on a lunar mission, with the first manned launch expected no later than 2014. Orion's two-stage launch vehicle, Ares I, was being designed by NASA and was expected to make its first test flight in 2009.

      The assembly of the International Space Station (ISS) resumed at a slow pace. Three space shuttle missions delivered supplies, equipment, and new truss segments, which included a new solar array. The first space shuttle mission also transported a crew member to the ISS to increase the size of the permanent ISS crew from two to three. (The eventual goal was a crew of seven.) The second space shuttle mission included three space walks and the use of both the space shuttle and the ISS robot arms to attach a 16-metric-ton solar array to the ISS. The solar-cell panels were extended slowly to avoid problems with sticking. The third space shuttle mission included four space walks, two of which involved connecting the new solar array to the ISS electrical system. The fourth space walk was added in order to overcome problems in retracting an old solar-panel array.

      None of the space shuttle launches in 2006 saw a repeat of the problems with damaging foam debris that had led to the destruction of the orbiter Columbia in 2003. As a precaution, damage inspections of the heat shield were made during each flight with cameras that were mounted on an extension to the shuttle robotic arm. An extra inspection of the heat shield was carried out at the end of the second mission, in September, after small objects were spotted drifting from the shuttle during preparations for reentry. No damage to the heat shield was found, and the objects were believed to have shaken loose from the cargo bay. After the flight, workers discovered an impact hole about 2.5 mm (0.1 in) wide on a shuttle-bay radiator panel. Although the puncture had not caused serious damage, it highlighted the ongoing hazard posed by small high-speed orbital debris and natural micrometeoroids.

      In 2006 two Soyuz missions carried replacement crews to the ISS. One of the missions also carried Anousheh Ansari, an Iranian-born American, as a paying passenger. She and her family sponsored the Ansari X Prize, which in 2004 had led to the first privately funded human spaceflights.

      In September Michael Griffin (Griffin, Michael ) (see Biographies) made the first-ever visit by a NASA administrator to China, where he discussed possible joint ventures in human spaceflight. Given the deliberate pace at which China was developing its program, however, the likelihood of such a venture in the near term was not high. The next human spaceflight by China, Shenzhou 7, was expected in 2007 or 2008 and was to feature China's first extravehicular activity in space.

       Bigelow Aerospace took a major step toward the privately funded construction of a space station when on July 12 it successfully launched its Genesis I test satellite atop a converted Russian ballistic missile. The craft, 4.4 m (14.4 ft) long, was pressurized in orbit to expand in diameter from 1.6 m to 2.5 m (5.2 ft to 8.2 ft). Bigelow planned eventually to build a habitat that would serve as a space motel and have more than 15 times the pressurized volume of Genesis I. Composite materials used in the skin of the inflatable structure were expected to provide protection from any impacts by orbital debris or micrometeoroids.

Space Probes.

      The U.S. space probe New Horizons was launched on Jan. 19, 2006, from Cape Canaveral, Florida, for a July 2015 flyby of Pluto and its largest moon, Charon. A flyby of Jupiter on Feb. 28, 2007, would help speed the craft on its way. New Horizons would be the first space probe to visit Pluto, which astronomers had come to recognize as an important member of a growing list of small icy worlds called Kuiper belt objects that populate the outer solar system.

      The return capsule from the NASA Stardust probe (launched in 1999) made a successful soft landing in Utah on January 15. The capsule carried collected samples of dust particles from Comet Wild 2 and of interstellar dust for scientific study. Japan's Hayabusa probe was feared lost late in 2005 following an attempt to retrieve material from the surface of asteroid Itokawa, but in 2006 mission controllers reestablished communications and attempted to prepare the spacecraft for a return flight to Earth.

 On March 10 NASA's Mars Reconnaissance Orbiter entered Mars orbit and—to reduce fuel requirements—gradually reached its operational orbit over the next six months by using atmospheric drag for aerobraking. Contact was lost with Mars Global Surveyor in November, and it appeared that its mission had come to an end. Among the spacecraft's findings during its nine years in orbit around Mars were images released in 2006 that showed crater walls with mineral deposits, suggestive of flowing water, that had formed within the previous five years.

      Europe's Venus Express probe (launched in 2005) entered into orbit around Venus on April 11 and achieved its operational orbit on May 7. The Messenger mission to Mercury (launched in 2004) flew past Earth in August 2005 and then Venus on Oct. 24, 2006; a second Venus flyby was scheduled for June 5, 2007, followed by three flybys of Mercury in 2008–09. The flybys would gradually reshape the probe's solar orbit so that it would be able to enter orbit around Mercury in March 2011.

Unmanned Satellites.

      Two environmental satellites, CloudSat and Calipso (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation), were launched together from Vandenberg Air Force Base, California, into polar orbit on April 28. CloudSat carried U.S.-Canadian radar equipment to map cloud tops. Calipso, developed by the U.S. and France, carried two lasers and an infrared radiometer to analyze atmospheric particles that affected the weather. CloudSat and Calipso were in virtually the same orbit as the older Aqua, Parasol, and Aura environmental satellites, and all of the satellites crossed the Equator within 15 minutes of each other so that diverse data could be taken nearly simultaneously.

      The Hinode and Solar-Terrestrial Relations Observatory (STEREO) missions were both designed to explore the Sun. Hinode was a Japanese-U.S.-U.K. satellite that carried a 50-cm (20-in) solar optical telescope, a 34-cm (13-in) X-ray telescope, and an extreme ultraviolet imaging spectrometer to observe changes in intense solar magnetic fields that were associated with solar flares and coronal mass ejections. It was launched on September 23 from Japan's Uchinoura Space Center (formerly known as Kagoshima) by an M-5 rocket into a Sun-synchronous Earth orbit that kept the satellite continuously in sunlight. The STEREO mission was launched on October 25 by a Delta II rocket from Cape Canaveral. It consisted of twin spacecraft that were designed to observe the Sun from separate locations in space and thus provide a stereoscopic view of solar activities. The Moon's gravity was used to pitch the satellites into different places along Earth's orbit, where one would orbit the Sun ahead of Earth and the other following Earth. After two years the two spacecraft would form a 90° angle with the Sun. Each spacecraft carried an ultraviolet telescope, a coronagraph, and other instruments.

      On February 22 Japan launched the Akari (Astro-F) satellite from Uchinoura. It carried a 67-cm (26-in) near- to far-infrared telescope, and its mission was to produce an infrared map of the entire sky. For its operation the telescope needed to be cooled by liquid helium, and the spacecraft carried a supply that was expected to last for 550 days. The Hubble Space Telescope, although aging—it was in the 16th year of a planned 15-year mission—continued its operations. Underscoring the need for a servicing mission, however, were a variety of problems, including two unexpected shutdowns of the Advanced Camera for Surveys. In 2004 NASA had canceled all future space shuttle flights to the Hubble Space Telescope because of safety concerns, but the agency reconsidered and in October announced that it had approved one final Hubble servicing mission. Tentatively scheduled for early 2008, the mission was expected to make it possible for the telescope to operate through 2013.

Launch Vehicles.

      The first test flight of the Falcon 1 launch vehicle, independently developed by SpaceX with funding from entrepreneur Elon Musk, took place March 24 on Kwajalein Atoll in the Pacific Ocean but failed just 25 seconds after liftoff. Corrosion between a nut and a fuel line had allowed the line to leak, which caused an engine fire. The next Falcon 1 launch attempt was set for early 2007. Despite its start-up difficulties, SpaceX won a $278 million contract from NASA for three demonstration launches of the company's Dragon spacecraft and Falcon 9 launcher in 2008–09. NASA also awarded a $207 million contract to Rocketplane-Kistler for development of its K-1 reusable rocket and a cargo module.

Dave Dooling

▪ 2006

Introduction

Researchers reported on the fast speed of electron transfers, the high temperature of collapsing bubbles, and the superfluidity of a fermionic condensate. Space probes parachuted onto Titan, slammed into a comet, and hovered over an asteroid. Astronomers discovered a remote solar system object larger than Pluto.

Chemistry

Industrial Chemistry.

      Acetylene is a starting material used in making many important products in the electronics and petrochemical industries. Storage of the highly reactive gas, however, is difficult, because the gas explodes when compressed under a pressure of more than two atmospheres (about 2 kg/cm²) at room temperature. In 2005 Susumu Kitagawa and colleagues at Kyoto (Japan) University reported the synthesis of a copper-organic microporous material that allowed acetylene to be compressed and stored safely at a pressure almost 200 times higher. Greater amounts of the gas thus could be stored in smaller containers. The new material was Cu₂(pzdc)₂(pyz). Pzdc is pyrazine-2,3-dicarboxylate, and pyz is pyrazine. The compound contains nanoscale-dimensioned channels that adsorb large amounts of acetylene at room temperature. Unlike conventional adsorbants, such as activated carbons and zeolites, the new compound showed a selective adsorption of acetylene (C₂H₂) compared with carbon dioxide (CO₂), its molecular cousin. Kitagawa's group said that the discovery could be used as the basis for the design and synthesis of metal-organic compounds that could hold other gases. Two prime candidates were nitrogen oxides (NO_x) and sulfur oxides (SO_x), air pollutants that must be removed from industrial emissions.

Applied Chemistry.

 Individual carbon nanotubes, which resemble minute bits of string, can be assembled to form ribbons or sheets that are ultrathin but extraordinarily strong, light, and electrically conductive. Many trillions of these microscopic fibres must be assembled in order to make useful commercial or industrial products. In one technique, similar to that used for making paper, nanotubes dispersed in water were allowed to collect on a filter, dried, and then peeled off the filter—a process that typically took about a week. Ray H. Baughman and colleagues at the University of Texas at Dallas in 2005 reported the development of a dry process for assembling carbon nanotube sheets 5 cm (2 in) wide at rates of 7 m (23 ft) per minute. Nanotubes were first gathered into an aerogel, a highly porous solid with extremely low density, and then were compressed into a sheet. The nanotube sheets made by this process had been used as a medium for the microwave bonding of plastics and for such objects as flexible light-emitting diodes and electrically conducting film. Baughman said that their laboratory method appeared to be suitable for scaling up to an industrial process that could make nanotube sheets available commercially.

      Chemists at the University of California, Los Angeles, made the first nanoscale valve, which could be opened and closed on demand to trap and release molecules. Jeffrey I. Zink, who headed the research group, said that the valve had potential applications in new drug-delivery systems that would be small enough to work inside living cells. It joined a wide array of microscopic gears, shafts, motors, and other microelectromechanical systems that had been produced with nanotechnology. The moving parts of the valve were formed by rotaxanes, molecules in which a ring component fits around the central portion of a separate dumbbell-shaped component and can move up and down in a linear motion. The rotaxane molecules were attached by one end to openings of minute holes, a few nanometres in diameter, on the surface of a piece of porous silica. When the movable ring structure of the rotaxane molecule was in the down position, it blocked the hole and trapped molecules. When the ring structure was in the up position, it allowed the molecules to escape. The energy for the operation of the switch was obtained through redox reactions.

Environmental Chemistry.

       Green chemistry, or “sustainable chemistry,” is the effort to use techniques that minimize pollution in chemistry. One major focus was the development of chemical reactions that reduced or eliminated the use of toxic substances and the production of toxic by-products. A notable advance in this area in 2005 concerned the Barton-McCombie deoxygenation, an important reaction used by organic chemists to replace hydroxyl (–OH) groups with hydrogen atoms. The ingredients for the reaction had traditionally included tin hydrides that were not only toxic but also expensive and difficult to handle. John L. Wood and co-workers at Yale University reported the development of a less-toxic deoxygenation reaction, in which water and trimethylborane were used in place of the tin hydride. The new reaction also works under mild conditions because of the low energy that is needed to break the O–H bond when water forms a chemical complex with trimethylborane.

 Nanoparticles, such as buckyballs (soccer-ball-shaped molecules [C₆₀] made of 60 carbon atoms), are ultrasmall particles whose unusual properties sparked substantial interest for their potential use in commercial and industrial products. Their properties also led to concern about their potential hazard to the environment and how they should therefore be regulated. Scientists had assumed that buckyballs—because they are insoluble—posed no potential hazard to living organisms and their environment. Joseph Hughes of the Georgia Institute of Technology and co-workers reported, however, that buckyballs form into clumps called nano-C60 upon contact with water and that nano-C60 is readily soluble. The researchers also found that even at low concentrations the nanoparticles inhibited the growth of soil bacteria, which potentially would have a negative environmental effect. Hughes suggested that the antibacterial property of nano-C60 might be harnessed for beneficial uses.

Physical Chemistry.

      For more than 30 years, scientists had been trying to verify the existence of a “liquid” magnetic state. In theory, such a state would occur when the magnetic spins of the electrons in a material fluctuated in a disorderly fluidlike arrangement in contrast to the ordered alignment of magnetic spins that produces magnetism. Liquid magnetic states might be related to the way that electrons flow in superconducting materials. Satoru Nakatsuji and co-workers at Kyoto University synthesized a material, nickel gallium sulfide (NiGa₂S₄), that might demonstrate its existence. The Japanese team and researchers from Johns Hopkins University, Baltimore, Md., and the University of Maryland at College Park studied a polycrystalline sample of the material that had been cooled to an extremely low temperature. They found that the triangular arrangement of the atoms in the material appeared to prevent the alignment of the magnetic spins of the electrons. The scientists concluded that for an instant the material appeared to have been a magnetic liquid, but they said that verification would be needed.

      The transfer of electrons from one atom to another is a key step in photochemical reactions, including those that underlie photosynthesis and commercial processes such as photography and xerography. Alexander Föhlisch of the University of Hamburg and co-workers reported a new and more accurate measurement of the time required for electron transfer. Their study of sulfur atoms deposited on the surface of ruthenium metal found that electrons jumped from the sulfur to the ruthenium in about 320 attoseconds (billionths of a billionth of a second, or 10⁻¹⁸ second). For the experiment the researchers beamed X-rays at the sulfur, exciting an inner-shell, or core, electron so that it jumped to a higher energy level and left an empty “core hole” in its place. The electron then moved onto the ruthenium metal in less time than it took for the hole to be filled by another electron, a process known to take 500 attoseconds. Föhlisch believed that the research would enable studies of electrodynamics on the attosecond scale. Knowledge of how electrons move would be a crucial step for the development of spintronic computing, in which information is stored in the spin state of electrons.

      In sonochemistry, high-frequency sound waves are used to introduce energy into a liquid-reaction medium. The energy forms bubbles in the liquid, a phenomenon called acoustic cavitation. The bubbles quickly collapse and release tremendous amounts of energy in a burst of heat and light. Some scientists believed that the collapse could be exploited to produce “desktop” nuclear fusion. Ken Suslick and David Flannigan of the University of Illinois at Urbana-Champaign reported the first direct measurement of the process that takes place inside a single collapsing bubble in a sonochemical experiment. They recorded the spectra of light emitted from the collapse, much as astronomers use spectra to measure the temperature of stars, and determined that the gases in the collapsing bubble reached a temperature of 15,000 K, more than two times hotter than the surface of the Sun. The experiment showed that a plasma was formed but did not provide evidence for nuclear fusion.

Organic Chemistry.

      The growing public health problem caused by the emergence of antibiotic-resistant bacteria was encouraging pharmaceutical chemists to search for new antibiotics. One common way of finding new antibiotics was to modify the complex molecular structures of old standbys, such as tetracycline and erythromycin, because slight alterations in their structure could enable an antibiotic to slip past the defenses that had evolved in resistant bacteria. After 50 years of research, all the tetracycline antibiotics in use were either natural products or semisynthetics—that is, products made by modifying the structure of the natural product. In 2005 Mark G. Charest and co-workers in the department of chemistry and chemical biology at Harvard University reported a method for synthesizing a broad range of structural variants of tetracycline. The synthetic-chemical breakthrough involved 14- to 18-step processes that began with benzoic acid, a widely available and inexpensive compound.

Michael Woods

Physics

Particle Physics.

      The Standard Model of particle physics describes the basic composition of nature in terms of fundamental particles, such as quarks and electrons, and fundamental forces, which act between these particles through the exchange of massless particles. Quarks are bound tightly together in composite particles such as protons and neutrons and have never been observed directly. Nevertheless, the mass of a quark can be estimated through a complex calculation that involves the known mass of a composite particle such as the proton and an assumed value for the force that binds the quarks together. A good test of the Standard Model, therefore, is to use this value to predict the mass of a new type of composite particle. In 2005 this calculation was carried out for the first time on a so-called charmed B meson—a bound state of two types of quark—by a team from Glasgow (Scot.) University, Ohio State University, and Fermi National Accelerator Laboratory (Fermilab), near Chicago. Only days after the prediction was published, Darin Acosta and fellow experimentalists associated with the Tevatron accelerator at Fermilab found 19 examples of a meson whose mass agreed well with the theoretical prediction—a result that was seen as a strong vindication of the model.

      There were still problems in particle physics to be solved, however. Researchers at the High Energy Accelerator Research Organization (KEK) at Tsukuba, Japan, and the BaBar Experiment at Stanford Linear Accelerator Center (SLAC), Menlo Park, Calif., discovered a number of new perplexing particles, including the Y(3940) and the Y(4260). A few appeared to be composite particles that consisted of four quarks, but some researchers speculated that they might be completely new types of particles.

      The existence of pentaquarks (particles made up of five quarks bound together), which a number of laboratories reported to have found in 2003, came to appear more doubtful in 2005. The Large Acceptance Spectrometer collaboration at Jefferson Laboratory, Newport News, Va., conducted the most precise experiments made to date for detecting pentaquarks but found no evidence for them.

      SLAC researchers who analyzed the results of experiments in which accelerated electrons were scattered off electrons in a target material found a small asymmetry that depended on whether the accelerated electron had a left- or right-handed spin. The asymmetry was the first observed example of the violation of parity (the principle that physical phenomena are symmetrical) in electron-electron interactions, and its magnitude was in agreement with theoretical predictions based on the Standard Model.

Optics and Photonics.

      It had become possible to observe physical processes with extremely high time resolution. The observational technique involved exciting the system of interest with a “pump” pulse of electromagnetic radiation and then probing it with a precisely timed second pulse. In the visible region of the electromagnetic spectrum, laser pulses with lengths of several femtoseconds (one femtosecond = 10⁻¹⁵ second) could be produced, but in the extreme ultraviolet and X-ray region, pulses as short as 0.2 femtosecond (or 200 attoseconds) could be realized. The temporal evolution of a system could be followed as a function of the time delay between the pulses. Such a setup was used by Ferenc Krausz at the Technical University of Vienna and co-workers to observe directly the time variation of the electric field in a light wave at a frequency of approximately 10¹⁵ Hz. Alexander Föhlisch of the University of Hamburg and co-workers used the technique to study ultrafast electron transfer in a solid—an important process in photochemistry and electrochemistry. (See Chemistry.) At the same time, Tsuneto Kanai and co-workers from the University of Tokyo developed a similar technique that might make it possible to investigate molecular structures to a precision of a fraction of a nanometre (one-billionth of a metre). These techniques were expected to become increasingly important in the study of atomic and molecular processes. The extension of their application depended on the production of coherent (in-phase) sources of radiation in the X-ray region of the spectrum. Jozsef Seres from the Technical University of Vienna and co-workers built a source of coherent one-kiloelectronvolt X-rays (at a wavelength of about one nanometre). It relied on the generation of high-order harmonics in a jet of helium gas ionized by a five-femtosecond laser pulse.

      Mario Paniccia and associates from Intel Corp. succeeded in producing the first continuous-wave silicon laser based on the Raman effect, the phenomenon in which the wavelength of light shifts when the light is deflected by molecules. Pumped by an external diode laser, the device emitted continuous radiation at a wavelength of 1,686 nanometres with power in the milliwatt range. The creation of lasers from relatively inexpensive silicon components held promise for the development of many new applications. Other devices were being developed that did away with the external pump laser. A group of researchers headed by Federico Capasso of Harvard University produced one such device, an electrically pumped laser made from alloys of aluminum, gallium, indium, and arsenic. It worked by means of a “quantum cascade” of electrons that passed through hundreds of precisely grown layers of silicon. The device produced electromagnetic radiation with a wavelength of 9 micrometres, and the researchers planned to modify it in order to produce radiation with a wavelength between 30 and 300 micrometres, a region of the spectrum for which no cheap and practical lasers existed.

Superconductivity.

      The discovery of superconductors (materials in which electrical resistance can be reduced to essentially zero) had long been an empirical process, but in 2005 work conducted by F. Lévy and colleagues at the Atomic Energy Commission of France suggested a possible path to follow for devising totally new superconductors. Working with a ferromagnetic material called URhGe, they found that the critical point, or temperature, at which the material loses its ferromagnetic properties could be varied by applying pressure to a block of the material. As the pressure was increased, the critical point moved to lower and lower temperatures so that fluctuations in the magnetic properties of the material became predominantly quantum mechanical rather than thermal—a so-called quantum critical point. At the quantum critical point, the application of a strong magnetic field produced superconducting phenomena.

Quantum Physics.

      The next development in computing might well involve quantum computing—the storage and transport of qubits, quantum-system states that can be used to represent bits of data. A great advantage of quantum-computing devices is that their interaction might not be limited by the speed of light; through the phenomenon called quantum entanglement, it might be possible for two qubit devices to interact instantaneously. There were many candidates for quantum-mechanical systems upon which such devices could be based, including atoms, trapped ions, or “ quantum dots” (tiny isolated clumps of semiconductor atoms with nanometre dimensions). Although practical systems to store and manipulate qubits had not yet been constructed, a number of laboratories had produced devices that might form part of such a system. Sébastien Tanzilli of the University of Geneva and colleagues built an interface between states of alkaline atoms and photons at wavelengths suitable for transmission along optical fibres, and Robert McDermott of the University of California, Santa Barbara, and colleagues employed a Josephson junction (a type of superconducting switching device) to measure the qubit states of two interconnected quantum devices virtually simultaneously. Hans-Andreas Engel and Daniel Loss of the University of Basel, Switz., suggested a mechanism by which the spin states of a pair of electrons in a quantum dot could be measured without the destruction of the spin states. This mechanism might well form the basis for a qubit memory device.

Condensed-Matter Physics.

      Experiments that involved cooling a few thousand gas atoms to temperatures less than a millionth of a degree above absolute zero (0 K, −273.15 °C, or −459.67 °F) had by 2005 become almost commonplace. A cooled gas that consists of atoms with zero or integral intrinsic spin (atoms called bosons) yields a state of matter known as a Bose-Einstein condensate (BEC); the atoms act together as one “superparticle” described by a single set of quantum-state functions. For atoms with multiples of half-integral spins (atoms called fermions), a similar cooling process can take place to produce fermionic condensates. These atoms, however, cannot fall to the same state (as described by the Pauli exclusion principle) but instead tidily fill up all available states starting from the lowest energy. In this case it was postulated that atoms should pair up and each strongly interacting pair would act like a boson. A series of experiments had suggested that such pairing did take place, but the first conclusive evidence of it was obtained in 2005 by Martin Zwierlein and colleagues at the Massachusetts Institute of Technology. They produced a rotating sphere of a fermionic gas with ultracold lithium atoms and observed the formation of a framework of minute vortices, a phenomenon unambiguously associated with superfluids (a fluid with a vanishingly small viscosity). The formation of a superfluid is characteristic of BECs and showed that pairing had occurred.

David G.C. Jones

Astronomy

      For information on Eclipses, Equinoxes and Solstices, and Earth Perihelion and Aphelion in 2006, see Table (Earth Perihelion and Aphelion, 2006).

Solar System.

      In planetary space science, the year 2005 began with the precision landing of the Huygens (European Space Agency) space probe on Saturn's moon Titan on Jan. 14, 2005. The probe had been released from the Cassini ( NASA) spacecraft, which had been in orbit around Saturn since July 2004. Huygens parachuted through the atmosphere of Titan for about 2.5 hours and then continued to take measurements for about another 70 minutes while on the surface. Titan is perpetually covered in clouds, and the mission provided the first opportunity to examine the moon's atmospheric layers and surface geology directly. The probe revealed deep surface channels, which were probably carved by flowing liquid methane. The surface temperature is far too low (−180 °C, or −290 °F) to allow water to exist in liquid form. Grapefruit-sized objects that were shown lying on the surface were probably composed of water ice.

      On July 4, 2005, after a journey of more than 431 million km (268 million mi), NASA's Deep Impact space probe fired a 370-kg (816-lb) copper projectile, or impactor, into the nucleus of Comet Tempel 1, which was only about 14 km (8.7 mi) wide and 4 km (2.5 mi) long. The crash excavated a crater about 30 m (about 100 ft) deep and 100 m (about 325 ft) across. Cameras aboard the main spacecraft took pictures before, during, and after the strike, which produced a bright flash of light as matter was ejected from the comet. The large cloud of ejected material was observed by some 80 ground-based telescopes at radio, infrared, optical, and ultraviolet wavelengths. Preliminary analyses of the observations were at odds with the standard “dirty snowball” model of comets, which had described comets as agglomerates of graphite and silicate dusts held together by ices such as frozen carbon dioxide, water, and methane. The ejected material behaved more like fine dust particles, which suggested that the comet “may resemble an icy dirt ball more than it does a dirty snowball,” according to Deep Impact research team member Carey Lisse of the University of Maryland. Other scientists said that the data implied that the object had a layered structure. Overall, astronomers concluded that Tempel 1 was an extraordinarily fragile object that was only weakly held together by gravity.

      For several years the status of Pluto as the most distant planet of the solar system had been questioned because of the discovery of other similar icy bodies in the Kuiper Belt, which lies beyond the orbit of the planet Neptune and extends well beyond the orbit of Pluto. Pluto—discovered in 1930—was known to have one moon, called Charon, which was detected by ground-based telescopes in 1978. In May 2005 a team of astronomers, who used the Advanced Camera for Surveys on the Hubble Space Telescope, discovered that Pluto has not one but three moons. The two newly discovered moons have diameters estimated to be between 32 and 70 km (20 and 45 mi) and are about two to three times as far as from Pluto as Charon. The existence of two additional moons lent strength to the claim by some astronomers that Pluto should still be viewed as a planet in its own right. Then, in the summer of 2005, astronomers Michael E. Brown of the California Institute of Technology, Chadwick Trujillo of the Gemini Observatory in Hilo, Hawaii, and David Rabinowitz of Yale University announced the discovery of the largest object found in the outer solar system since the discovery of Neptune and its moon Triton in 1846. The object was originally recorded in images taken in October 2003 with the 122-cm (48-in) Schmidt telescope on Mt. Palomar, near San Diego, and the astronomers designated the object 2003 UB313. Observations in January 2005 showed that the object had been slowly moving and that it was more than twice the distance from the Sun as Pluto. By analyzing these observations, the team was able to conclude that the diameter of the object is at least 1.5 times that of Pluto. The object, unofficially called Xena, moves in a highly elliptical orbit that is inclined by about 44 degrees to the plane in which most of the planets move, and it takes about 560 years to orbit the Sun. While using the giant Keck II telescope on Mauna Kea, Hawaii, in September, the team spotted a small moon that orbits Xena. Whether Pluto and Xena are, indeed, the 9th and 10th planets in the solar system or merely exotic members of the Kuiper Belt, their very existence could be expected to help scientists unravel the mysteries of how the solar system was formed.

Stars.

      For more than a decade, astronomers had been finding planets around stars other than the Sun, and by late 2005 at least 160 such extrasolar planets had been detected. Since a planet is small compared with its parent star, it was extraordinarily difficult to detect extrasolar planets directly in photographic images. Instead, every extrasolar planet had been found indirectly by looking for and detecting the wobble it induced in the motion of its parent star, as shown by shifts in the star's spectra or, in a few cases, by the small amount of light the planet blocked when passing in front of the star. In March 2005 two separate groups reported the direct detection of extrasolar planets. Each team used the infrared Spitzer Space Telescope to record the thermal radiation from hot Jupiter-sized planets just as they passed in front of and behind their central star. One object, called TrES-1, was found to have a surface temperature of about 790 °C (1,454 °F), with an atmosphere rich in carbon monoxide. The other planet, called HD 209458b, had a temperature of about 960 °C (1,760 °F). Both were far too hot to support any life like that known on Earth.

 The year 2005 brought with it a host of spectacularly detailed images of the remnants of supernovae that had exploded in the Milky Way galaxy during the past millennium. Supernova explosions produce the heavy chemical elements, leave behind magnetized and rapidly rotating neutrons stars, and are likely sources of the highly energetic particles called cosmic rays. In 1572 the Dutch astronomer Tycho Brahe noticed a “new star” in the sky, which faded from sight several months after its appearance. NASA's Chandra X-ray Observatory produced the most detailed image to date of the remnant of Tycho's supernova explosion. Studies made by a group from Rutgers University at Piscataway, N.J., used the data to offer the first strong evidence that supernovae accelerate heavy subatomic particles, which make up the preponderance of cosmic rays. Perhaps even more spectacular than these findings was a photograph of the Crab Nebula, the remnant of a supernova that exploded on July 4, 1054. It was produced from a mosaic of images taken with the Hubble Space Telescope and showed in great detail the complex structure of filaments and wisps within the nebula.

Galaxies and Cosmology.

       Gamma-ray bursts were first detected in the late 1960s. These extremely powerful bursts of photons last from less than a second to several minutes. Their cause and origin were subject to a great deal of theoretical conjecture until the late 1990s, when distant galaxies were definitively identified as a source of long-lived gamma-ray bursts. Long-lived bursts were thought to be associated with supernova explosions that occurred with the death of massive stars. The year 2005 brought a host of new observations of gamma-ray bursts and insights into their nature. In January detectors aboard NASA's Swift spacecraft recorded the X-rays from the relatively long-lived burst designated GRB 050117. Within about three minutes of the burst, Swift was able to point its X-ray imaging telescope in the direction of the burst and, for the first time, recorded an X-ray image of such an event. During the year Swift also recorded for the first time the precise location of two relatively short-lived gamma-ray bursts, GRB 050509B and GRB 050709. On the basis of their positions, both events were shown to have arisen in relatively nearby galaxies, which meant that the luminosities of the events were approximately a thousand times less than those of long-lived gamma-ray bursts detected from distant galaxies. Some astronomers thought that the short bursts arose from the merger of compact objects, such as when two neutron stars coalesced and produced jets of high-energy particles and radiation. On September 4 the Swift satellite recorded its 68th burst event of the year, GRB 050904. A team of astronomers led by Nobuyuki Kawai of the Tokyo Institute of Technology used the infrared Subaru Telescope on Mauna Kea to determine that the source of the burst lay about 12.8 billion light years from Earth, which made it the most distant such event recorded to date. The burst occurred a mere 900 million years after the universe was formed and suggested that supernovae existed early in the history of the universe.

Kenneth Brecher

Space Exploration

      For launches in support of human spaceflight in 2005, see Table (Human Spaceflight Launches and Returns, 2005).

      In March 2005 Michael Griffin, a former NASA manager, was named to succeed Sean O'Keefe as NASA administrator. Griffin quickly made radical changes such as the cancellation of much of the space research program, including the study of the effects of zero-g (microgravity) environments on both humans and physical phenomena. Most of the cuts were intended to make it possible to fund the Vision for Space Exploration program announced by Pres. George W. Bush in 2004. The program included the return of humans to the Moon by 2020 to determine what lunar resources could be utilized for the purpose of beginning human exploration of Mars and beyond. Key elements were to be the creation of an infrastructure to support long-term exploration and the use of “go-as-you-pay” funding rather than set political deadlines. In September 2005 NASA presented its plans for the spacecraft it would develop for the post-space-shuttle era. They included a four-person Crew Exploration Vehicle (CEV) and a heavy-lift launch vehicle. The CEV would resemble the Apollo Command/Service Module of the 1960s and '70s but would be large enough to carry four to six persons. It would have a two-stage launch vehicle, the first stage powered by a space-shuttle-derived solid-rocket booster and the second powered by a space-shuttle main engine. The heavy-lift launch vehicle (which could be used for launching cargo or a manned spacecraft) would also use shuttle-derived components—two solid-rocket boosters and five main engines powered by fuel from a redesigned external tank—and would be able to place up to 100 metric tons into orbit. These spacecraft were also to be used as building blocks for manned lunar and Mars missions. In October 2005 NASA announced the selection of two contractors, Lockheed Martin and a team formed by Northrop Grumman and Boeing, to produce preliminary designs. An accelerated development schedule was planned to lead to a 2012 launch.

Manned Spaceflight.

      In July the U.S. space shuttle program resumed flight with launch of the orbiter Discovery. It was the first space shuttle flight since the loss of the orbiter Columbia and its crew of seven astronauts during its descent for landing on Feb. 1, 2003. The shedding of foam from the external tank that had occurred just seconds after liftoff of the Columbia led to damage of the high-temperature heat-shield tiles on the leading edge of the left wing that doomed the craft. Despite a range of engineering design changes to the insulating foam on the shuttle's external tank since the accident, video cameras installed on Discovery to monitor its launch showed a section of foam from the external tank breaking off and whipping backward through the slipstream after the separation of the boosters. The foam lost during the Discovery launch did not strike the vehicle, but the incident required NASA to reevaluate the production program for external tanks and to postpone the next space shuttle launch until 2006.

      The remainder of Discovery's mission, STS-114, went well. It docked to the International Space Station (ISS) two days after launch, and fresh supplies and experiment gear were delivered to the ISS. The Discovery crew used a camera on the orbiter's robotic arm to inspect the heat shield for damage. No holes from impacts with lost foam were found, but the camera revealed two areas where felt insulating pads had been pulled from between heat-shield tiles and the orbiter's aluminum skin. Because of uncertainties about excessive heating that might occur around the protrusions during reentry, two astronauts were dispatched on a spacewalk and gingerly removed the strips from the tiles. It was the first time that astronauts had worked around the orbiter's belly; all previous spacewalks had been in or above the payload bay.

      Two crew-exchange missions, Soyuz TMA-6 and 7, were flown to the International Space Station (ISS). Each carried an American and Russian replacement for American and Russian crew members who had completed a six-month stay on the ISS. In addition, the TMA-6 mission carried an Italian scientist and TMA-7 a space tourist.

       China continued its manned space program with its second manned mission, Shenzhou 6, which carried two taikonauts (astronauts). The first manned mission, Shenzhou 5, lasted one day and carried a single taikonaut. Although it had started its space program cautiously, China announced long-range plans that included complex rendezvous maneuvers, assembly of a space station, and possible manned missions to the Moon.

Space Probes.

 One of the most notable events in space exploration in 2005 was the collision on July 4 of the Deep Impact impactor probe with the short-period comet Tempel 1. The 370-kg (816-lb) impactor, which had been released by the main Deep Impact spacecraft the day before, slammed into the comet at a relative speed of 37,000 km/hr (23,000 mph). To obtain information about the composition of the comet nucleus, high-resolution infrared and medium-resolution visible cameras on the main Deep Impact spacecraft observed the collision and the material that it ejected from the comet. The impactor was largely made of pure copper to ensure clean spectral data of the material. The collision was also observed by the Hubble Space Telescope, the Spitzer Infrared Space Telescope, the Chandra X-Ray Observatory, and many ground-based observatories. (See Astronomy (Physical Sciences ).)

      The Spirit and Opportunity rovers on Mars continued their work more than a year after the completion of their primary 90-day missions. The European Space Agency's Mars Express orbiter deployed the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument, which was designed to use microwave pulses to search for radar signatures of subsurface water. NASA's Mars Global Surveyor and Mars Odyssey continued their observations of the planet and were to be joined in early 2006 by the Mars Reconnaissance Orbiter (MRO). The MRO, launched August 12, carried instruments for studying the atmosphere of Mars and for searching for signs of water on the planet. Its shallow subsurface radar was to probe the surface to a depth of 1 km (0.6 mi) to detect variations in electrical conductivity that might be caused by water.

      The Huygens probe, which was released in December 2004 by the Cassini spacecraft in orbit around Saturn, parachuted to the surface of Titan, Saturn's largest moon, on Jan. 14, 2005. Data that Huygens transmitted during its final descent and for about 70 minutes from the surface included 350 pictures that showed a shoreline with erosional features and a river delta that scientists believed had been formed by liquid methane. In error one radio channel on the satellite was not turned on, and data were lost concerning the winds Huygens encountered during its descent. As the Cassini spacecraft continued to orbit Saturn, it made several flybys of the moons Titan, Mimas, and Enceladus. During the flybys Cassini used its radar mapper and instruments for infrared, visible, and ultraviolet observations to study surface features on the moons.

       Japan's Hayabusa probe (formerly called MUSES-C) arrived at asteroid Itokawa (named after Hideo Itokawa, Japan's rocket pioneer) on September 12 and became only the second spacecraft to have visited an asteroid. Hayabusa then hovered above the asteroid, which is only 600 m (about 2,000 ft) long, and mapped its surface in preparation for several descents to collect surface samples that it would return to Earth. A 600-g (21-oz) MINERVA lander released by Hayabusa was to have studied the asteroid as it hopped around the surface, but the small probe was lost after it was released on November 12. Hayabusa attempted brief landings on November 20 and November 26. It was unclear whether it succeeded in collecting any soil samples, and control and communications problems with the spacecraft raised doubts whether it would be able to return to Earth.

      Europe's Venus Express spacecraft was launched November 9 by a Russian Soyuz-Fregat rocket and was scheduled to go into orbit around Venus in April 2006. Near-infrared and other instruments were to study the structure and composition of the middle and upper Venusian atmosphere.

Unmanned Satellites.

      Japan's Suzaku (Astro-E2) spacecraft, launched in July, was designed to complement the U.S. Chandra X-Ray Observatory and Europe's XMM-Newton spacecraft. Suzaku was equipped with X-ray instruments to study hot plasmas that occurred in star clusters, around black holes, and other regions. The mission of Gravity Probe B ended in October when the last of its liquid-helium coolant ran out. The satellite carried high-precision quartz gyroscopes whose precession (shift in rotational axis) provided extremely accurate measurements of the subtle effects predicted by Einstein's general theory of relativity. China launched the Shijian 7 spacecraft July 6 on a three-year mission to study the space environment. The U.S. Department of Defense launched the XSS-11 experimental satellite, which was designed to approach to within 500 m (1,640 ft) of target spacecraft, including several dead American satellites, and inspect them. NASA's DART (Demonstration of Autonomous Rendezvous Technology) spacecraft made a successful rendezvous with a target satellite, but during its final approach a propulsion system failure aborted the mission at a distance of 91 m (300 ft) from the target.

Launch Vehicles.

      Europe's most powerful rocket to date, the Ariane 5 ECA, became operational in 2005, with launches on February 12 and November 16. Using liquid-propellant engines and solid-propellant boosters, it was capable of lifting a 9,600-kg (21,000-lb) payload to geostationary transfer orbit. The premier flight of the Ariane 5 ECA, in 2002, had failed shortly after liftoff.

Dave Dooling

▪ 2005

Introduction

Chemistry

Nuclear Chemistry.

      The periodic table of the elements once contained only 92 naturally occurring elements, from hydrogen (the lightest building block of matter, with atomic number 1) to uranium (the heaviest, with atomic number 92). To this group, scientists have added many artificially created elements beginning with neptunium in 1940. These elements are very heavy and are produced in nuclear reactions that combine the nuclei of lighter elements. Atoms of many of the new elements exist only very briefly before decaying into other atoms. By 2003 the periodic table contained 114 elements.

      In 2004 scientists in the United States and Russia announced the synthesis of two new superheavy elements, elements 113 and 115. Their interim names pending the confirmation of their discovery were ununtrium (113) and ununpentium (115), names derived from scientific Latin indicating their atomic numbers. Scientists of the Lawrence Livermore National Laboratory, Livermore, Calif., and the Joint Institute for Nuclear Research, Dubna, Russia, announced the result. At a particle accelerator in Dubna, they had smashed calcium atoms (atomic number 20) into americium atoms (atomic number 95) to produce an atom with an atomic number of 115, which then decayed into an atom with an atomic number of 113.

      Both new elements had very short half-lives. It took just a fraction of a second for ununpentium to decay to ununtrium, which itself survived for a second before decaying. Researchers said the discovery strengthened expectations concerning the existence of an “island of stability,” an area at the outer reaches of the periodic table and theorized to contain superheavy elements with a longer half-life, possibly long enough for commercial or industrial applications.

Carbon Chemistry.

      Fullerenes are hollow cagelike structures of carbon atoms that debuted in 1985 with the discovery of C₆₀, or buckminsterfullerene. Since then, scientists had made a variety of fullerenes, including cylindrical structures termed carbon nanotubes. Synthesis of certain highly sought smaller fullerenes, however, remained elusive.

      In 2004 Xie Su Yuan and associates of the State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen, China, reported the synthesis of one such fullerene, C₅₀, which they described as the “little sister” of C₆₀. Like C₆₀, it has a ball-like shape, but it is surrounded by a ring of 10 chlorine atoms. The synthesis of C₅₀ involved introducing carbon tetrachloride, the source of the chlorine atoms, into the fabrication process typically used to make fullerenes.

      Predictions suggested that fullerenes smaller than C₆₀ might have unusual electronic, magnetic, and mechanical properties because of the high curvature of their surface. The process developed by the researchers produced relatively large amounts of C₅₀, which enabled them to begin studying its properties. The researchers believed the process could be used to make stable forms of other small fullerenes that they hoped to study.

Topological Chemistry.

      Beginning in the 1960s, chemists synthesized a variety of elegantly shaped molecules that resembled knots, interlinked rings, or other structures. Two independent research groups took this work, referred to as topological chemistry, to a striking new level of complexity. In one project Kelly S. Chichak and colleagues at the University of California, Los Angeles, reported the synthesis of a molecular Borromean ring—three rings linked together in such a way that cutting one link also releases the other two. (The Borromean ring was named for the Borromeo family, which used it as its family crest in 15th-century Tuscany; the rings also symbolized a giant's heart in Nordic mythology and the holy trinity in Christianity.) Synthesis of the Borromean ring was a tour de force, since closing one molecular ring through another so the rings were linked together like segments of a chain was in itself a notable accomplishment. In another research project Leyong Wang and associates at Johannes Gutenberg University, Mainz, Ger., reported synthesis of two molecules, each of which contained four molecular rings that were mutually interlinked. Far from being mere gimmicks, scientists stated that such structures might eventually have application in nanomachines and other forms of nanotechnology.

Physical Chemistry.

      The trend toward ever-smaller portable digital music players, cell phones, and other electronic devices sparked concern whether a molecular size barrier existed that would limit further miniaturization of digital memory devices and other electronics components that used thin layers of ferroelectric materials. Such materials show an electric polarization that can be quickly switched from one state to another—from a “1” to a “0,” for instance—in ways that make them ideal for digital applications. Scientists believed there might be a critical thickness below which the materials would lose their ferroelectric properties. Dillon D. Fong and colleagues of Argonne National Laboratory near Chicago reported the first experimental evidence that ferroelectric materials remain ferroelectric down to a thickness of 1.2 billionth of a metre and would therefore not impose a limit to miniaturization in ultrasmall electronic devices.

      The innermost structure of metals, ceramics, and other materials is important because it largely determines the strength, conductivity, and other key properties of the material. In metals, for example, the smaller the average grain size in the microstructure is, the greater is the strength of the metal. Chemists and materials scientists used powerful X-ray diffraction devices to study the three-dimensional microstructure of materials. In a major advance in efforts to characterize the microstructure of materials, Søren Schmidt and associates of Risø National Laboratory in Roskilde, Den., added a fourth dimension—time—to those studies. They developed a modification to the three-dimensional X-ray diffraction microscope at the European Synchrotron Radiation Facility in Grenoble, France, producing a four-dimensional microscope. They used the microscope to watch the formation of crystals in a sample of aluminum as it was put under stress and deformed. The initial findings challenged the widely accepted idea that new grains in the crystalline structure of a metal grow in a smooth spherical fashion. Scientists planned to use the microscope to study the underlying mechanisms of solidification, precipitation, and other phenomena that affect the properties of a wide range of materials.

Astrochemistry.

      Phosphorus is central to life. It forms the backbone of DNA and RNA molecules, is part of the adenosine triphosphate (ATP) molecules that serve as an energy source for life processes, and forms cell membranes and other structures, yet phosphorus is much rarer than the other chemical elements that were needed for life to emerge on the primordial Earth. For every phosphorus atom in the oceans, there are 974 million carbon atoms, 633 million nitrogen atoms, 49 million hydrogen atoms, and 25 million oxygen atoms. In addition, the most common terrestrial phosphorus-bearing mineral, apatite, releases only minute amounts of phosphorus when mixed with water.

      So where did terrestrial life get its phosphorus? At the 228th national American Chemical Society meeting in Philadelphia, Matthew A. Pasek of the University of Arizona reported a possible solution to the long-standing mystery: meteorites. Meteorites bear several phosphorus-containing minerals, the most important of which is the iron-nickel phosphide called schreibersite. Pasek and colleagues showed that schreibersite mixed with water at room temperature yields several phosphorus compounds. Among them was P₂O₇, a compound similar to the phosphate in ATP.

      Previous experiments had formed P₂O₇, but only at high temperature and other extreme conditions. Researchers said the identification of meteorites as rich sources of phosphate that could be readily released into water solution allowed some informed speculation on the origin of life on Earth. On the basis of this finding, life on Earth probably originated near a freshwater source where a meteorite had recently fallen, and the meteorite was probably an iron meteorite, which has up to 100 times as much schreibersite as other types of meteorites.

Applied Chemistry.

      Scientists reported the first use of multiphoton absorption photopolymerization (MAP) to build intricate three-dimensional nanostructures that might become the basis for microscopic machines and electronic devices. A research group headed by John T. Fourkas of Boston College reported the development of an acrylate resin that made it possible to fabricate microstructures on a biological material without damage. The resin, similar to Plexiglas, was hardened at the focal point of a laser beam that was directed over the resin in a three-dimensional scanning pattern to build up structures that were 1,000 times smaller than the diameter of a human hair. Unhardened resin was then washed away. In a dramatic demonstration of the size of the features that could be produced, Fourkas fabricated various structures on the surface of a human hair, including microscopic three-dimensional letters spelling the word “hair.” Fourkas envisioned eventually using MAP to build sensors, drug-delivery systems, and other structures directly on skin, blood vessels, and even inside living cells. He emphasized that such applications of MAP would require much additional research. The current research, however, brought them closer to reality.

Michael Woods

Physics

Particle Physics.

      In 2004 experimenters at the University of Tokyo's Super-Kamiokande Laboratory expanded and quantified the results of their investigation of the neutrino for which they were awarded the Nobel Prize for Physics in 2002. Neutrinos, the most elusive of stable fundamental particles, exist as three types: muon-neutrinos, tau-neutrinos, and electron-neutrinos. Super-Kamiokande experiments in the 1990s were the first to suggest an oscillation between muon-neutrinos and tau-neutrinos—that is, a conversion of one type of neutrino to another. This phenomenon implied that neutrinos had mass (albeit a very small mass), contrary to the prevailing view that neutrinos were massless particles. According to theory, the probability that a muon-neutrino would change into the tau type and vice versa depended on its energy, the distance it had traveled, and the relative masses of the two neutrino types. New data showed a sinusoidal variation in the number of muon-neutrinos detected, which confirmed the theory and enabled the relative masses of the two neutrino types to be calculated.

      Another fundamental particle that gave physicists headaches was the muon. The generally accepted theory of fundamental particles, called the Standard Model, very precisely predicted the value of a property of these particles called the magnetic moment. Physicists at the Brookhaven National Laboratory, Upton, N.Y., conducted an experiment to make exact measurements of the magnetic moment of negatively charged muons and announced results that flouted the predicted value.

      On the other hand, physicists were able to refine the precision of other predictions that the Standard Model was able to make. The predictions involved calculations using parameters, such as particle masses, whose values constrain other parts of the model. The DØ collaboration, formed by physicists from 19 countries working with the Tevatron proton-antiproton collider at Fermi National Accelerator Laboratory (Fermilab), near Chicago, measured the mass of the top quark to a greatly improved precision of around 2%. Among the benefits anticipated with this greater precision were improved predictions concerning characteristics of the yet-to-be-observed Higgs boson, the particle postulated to account for the fact that fundamental particles have mass.

Condensed-Matter Physics.

      Experiments that involved cooling a few thousand gas atoms to a temperature closely approaching absolute zero (0 K, −273.15 °C, or −459.67 °F) were being pursued in a number of laboratories. When a cooled gas consists of atoms with zero or integral intrinsic spin (atoms classified as bosons), the result is a state of matter known as a Bose-Einstein condensate. Rather than existing as independent particles, the bosons become one “superparticle” described by a single set of quantum state functions. When the cooled gas consists of atoms with an intrinsic spin of ¹/₂, ³/₂, ⁵/₂, and so on (atoms classified as fermions), the atoms cannot fall to the same condensed state, as described by the Pauli exclusion principle. Instead, they tidily fill up all available states starting from the lowest energy. Physicists were studying such fermionic condensates in an attempt to observe a phenomenon called Cooper pairing. Cooper pairing of electrons (which are fermions) in some solids and liquids at low temperatures produces superconductivity (the complete lack of electrical resistance) and superfluidity (the lack of viscosity). In the case of fermionic condensates, physicists believed that a similar phenomenon should be possible in which pairs of atoms would strongly interact, forming a Cooper pair that would have the properties of a boson. The production and study of fermionic condensates exhibiting Cooper pairing was expected to help unravel the theory underlying superconductivity and superfluidity, and many laboratories were involved in the race to develop such condensates.

      Early in 2004 Rudolf Grimm and colleagues of the University of Innsbruck, Austria, reported producing fermionic condensates that had very low viscosity. This property was necessary but not sufficient evidence that the production of Cooper pairing had been achieved. At JILA (formerly the Joint Institute for Laboratory Astrophysics), Boulder, Colo., Deborah Jin and co-workers also worked with a fermionic condensate. In an earlier experiment they had used a magnetic field to bind potassium atoms into loose molecule-like associations that could then form a Bose-Einstein condensate. In a new experiment they adjusted the magnetic field to prevent the molecular associations but still observed a pairing of atoms that formed a condensate. Although the group did not yet claim that Cooper pairing was taking place, it was clear that one or another laboratory would shortly produce conclusive evidence for the production of Cooper pairing in this new form of matter.

Quantum Physics.

      The phenomenon of quantum teleportation was quickly changing from being an exotic by-product of quantum theory to becoming a practical application in computing and information transfer. Teleportation concerns the instantaneous transfer of information from one place to another. It circumvents the restriction on exceeding the speed of light (a restriction imposed by relativity theory) by making use of the phenomenon called entanglement. If two quantum systems are prepared together, so that their states are “entangled,” then separated to an arbitrarily large distance, measurement of the state of one system will instantaneously define the state of the second system. The state is said to represent a qubit, or quantum bit, of information.

      Two scientific teams using different systems achieved teleportation of the quantum states of ions (electrically charged atoms). Previous experiments had demonstrated teleportation only with the quantum states of beams of light. The ion-teleportation experiments consisted essentially of preparing the initial quantum state of one particle and then teleporting that state to a second particle at the push of a button. Mark Riebe and co-workers at the Institute for Experimental Physics, University of Innsbruck, used three calcium ions trapped together at an ultrahigh vacuum. One ion constituted the source, and the second served essentially as carrier of information to the third, the receiver. Murray Barrett and his colleagues at the National Institute of Standards and Technology, Boulder, Colo., produced similar results with beryllium ions, using a different form of trap and experimental layout. Although there are many types of particles that might function as the basis of practical devices for storing and transporting qubits, including photons and atoms, trapped ions, or quantum dots, tiny isolated clumps of semiconductor atoms with nanometer dimensions, it was generally agreed that the ion-trap setup used in these experiments was one of the most promising candidates.

      Meanwhile, advances continued to be made in experiments on teleportation of light. Rupert Ursin and co-workers at the Institute for Experimental Physics, University of Vienna, described teleportation of photons over a distance of 600 m (about 2,000 ft) and Zhao Zhi and co-workers at the University of Science and Technology of China demonstrated five-photon entangled states, an important step on the road to the development of quantum communication. Other experimenters were considering the transfer of quantum information via the interaction of matter and light. Physicist Boris Blinov and colleagues in the department of physics at the University of Michigan succeeded in observing entanglement between a trapped ion and an optical photon.

      On the other hand, Irinel Chiorescu and colleagues at Delft (Neth.) University of Technology coupled a two-state system—made up of three in-line Josephson junctions—to a superconducting quantum interference device (SQUID) on the same semiconductor segment. The SQUID served as a detector for the quantum states, and entangled states could be generated and controlled. The experiment pointed the way to the possible use of solid-state quantum devices for controlling and manipulating quantum information. Such experiments were made possible by advances in a number of fields, from precision laser spectroscopy to techniques involving ultralow temperature and ultrahigh vacuum. In the midst of this experimental ferment, it was not yet clear which path might eventually lead to the building of large-scale quantum computers, overcoming the inherent restrictions of electronic devices.

      Experimental techniques in microscopy reached a level of sophistication that made it possible to study the spin of a single electron a short distance below the surface of a solid. Dan Rugar and co-workers at the IBM Almaden Research Center, San Jose, Calif., combined the techniques of magnetic resonance imaging and atomic force microscopy to create a technique called magnetic resonance force microscopy (MRFM). They mounted a micromagnetic probe on a tiny cantilever a short distance above the surface of the material being studied. The probe generated a magnetic-field gradient so large that the interaction between the probe's magnetic field and that of a single electron produced a measurable mechanical force on the probe. The new technique not only dramatically increased the resolution of magnetic resonance imaging but also held promise for helping make use of atomic spin for qubits in information storage.

      Anton Zeilinger and co-workers at the Institute for Experimental Phases of the University of Vienna carried out an experiment concerning the transition between the quantum and classical realms of physics. It demonstrated the fallacy of the common tendency to separate qualitatively the quantum behaviour of extremely small particles, such as electrons, from the classical behaviour of everyday objects, such as billiard balls. Using relatively large cagelike carbon C₇₀ molecules, Zeilinger's group observed a smooth transition between quantum and classical behaviour. They heated the molecules and sent them through a series of gratings onto a detector, in a rerun of the seminal two-slit experiment that showed the quantum nature of fundamental particles such as electrons. At low temperatures the molecules formed an interference pattern at the detector—a manifestation of quantum behaviour. As the temperature of the molecules was increased, however, there was a swift but smooth transition to behaviour like that of classical objects.

      This experiment demonstrated that the division between the quantum and classical realms is not a function of the size of the particle but most likely a function of the interaction of the particle with the outside world (in this case the emission of radiation by the heated molecules).

David G.C. Jones

Astronomy

      For information on Eclipses, Equinoxes and Solstices, and Earth Perihelion and Aphelion in 2005, see Table (Earth Perihelion and Aphelion, 2005).

Solar System.

      Two NASA spacecraft, the Mars rovers Spirit and Opportunity, touched down on the red planet in early 2004. Spirit landed in a crater called Gusev, which in area was about the size of the state of Connecticut. Opportunity landed on the opposite side of the planet, in a crater on the Martian equatorial plain called the Meridiani Planum. The mission of each rover was to study the chemical and physical composition of the surface at various locations in order to help determine whether water had ever existed on the planet and to search for other signs that the planet might have supported some form of life. Using an alpha-particle spectrometer, Spirit revealed that the chemical composition of the soil in the area where it had landed was similar to that found previously by Mars landers at other sites. This finding suggested that winds on Mars widely dispersed the dusty material found on its surface. Opportunity uncovered evidence that the rocks in the crater where it landed had been deposited in salty water at least 5 cm (2 in) deep that had been flowing at 10–50 cm per second.

      On June 30, following a seven-year, 3.5-billion-km (2.2-billion-mi) journey, the Cassini spacecraft arrived at Saturn, and it became the first spacecraft to enter into orbit around the planet. Cassini's mission, slated to last four years, was to study not only the planet but also its elaborate ring system and its moons. It carried a probe, called Huygens, that was scheduled to be released December 25 and land on Saturn's giant moon Titan three weeks later. The first images of the ring system obtained by Cassini in orbit around Saturn were more detailed than any that had been obtained by previous spacecraft. Among the features they showed were wave patterns thought to be caused by the gravitation of Saturn's moons. The rings appeared to be composed primarily of water ice mixed with dust that was similar in composition to the material detected on the moon Phoebe. While making its one close approach to Phoebe, Cassini revealed that the surface of the moon was heavily cratered. The cratering supported the idea that some of Saturn's smaller moons might have been formed from material that was ejected from Phoebe in a collision with a passing comet or asteroid. As Cassini passed within 339,000 km (211,000 mi) of Titan, onboard infrared detectors provided detailed images of its methane clouds. The appearance of the clouds was seen to change significantly over a period of only a few hours.

      On March 15 Michael E. Brown of the California Institute of Technology and collaborators Chad Trujillo of the Gemini Observatory, on Mauna Kea, Hawaii, and David Rabinowitz of Yale University announced the discovery of the most distant object of the solar system that had ever been observed, at a distance of 13 billion km (8.1 billion mi). Its discoverers named the new object Sedna, after the Inuit goddess said to live in a cave at the bottom of the Arctic Ocean. The new object was about three-quarters the size of Pluto and somewhat larger than the planetoid (planetlike object) Quaoar, which was discovered by the same group in 2002. Sedna was found to have a highly elliptical orbit, which took it from 76 times the Earth–Sun distance to about 900 times that distance and back in a period of 10,000 years. Observations of Sedna quickly raised a number of puzzling questions. Astronomers had thought that all objects in the outer solar system would be icy and therefore white or gray in appearance, but Sedna was almost as red as Mars. Its extremely elliptical orbit resembled the orbits of objects thought to exist in the Oort cloud, a distant cloud of icy objects that had been postulated by Dutch astronomer Jan Oort more than a half century before to account for the origin of comets. Sedna, however, was observed at a distance 10 times closer than the predicted inner edge of the Oort cloud. The proposal that Sedna had been kicked toward the inner solar system by the gravitation of a passing star was just one of several ideas that was being explored to account for its orbit.

      For many Earth-bound skywatchers, the astronomical event of the year was the transit of Venus on June 8, a rare event in which the planet was seen to pass directly between Earth and the Sun. During the transit Venus was visible for six hours as a small dark disk that crossed the bright disk of the Sun. The previous transit of Venus had occurred on Dec. 6, 1882. The next Venus transit would occur in only eight years, but the one following it would be more than a century later, in 2117. The transits of Venus were once of great importance to astronomers because careful timings of the events permitted the calculation of the distance between Earth and the Sun.

Stars.

      Over the past decade, more than 135 exoplanets (planets outside the solar system) had been detected in orbit around a wide variety of stars. Almost all of the planets had a mass in the range of 100 to 1,000 times that of Earth, and all of them were probably gas giants, such as Jupiter and Saturn. The presence of a planet in orbit around a star had usually been determined by studying variations in the speed of the star as it moved through space. In 2004, for the first time, a group of astronomers, using a network of 10-cm (4-in)-diameter telescopes at the Astrophysical Institute of the Canary Islands, Spain, discovered a Jupiter-sized planet in orbit around a star by detecting a periodic decrease in the brightness of the star as the planet passed in front of it.

      Small rocky planets, such as Earth, were believed to have at most a mass about 10 times that of Earth. Exoplanets with a mass in that range had been found, but they were in orbit around millisecond pulsars, an unlikely habitat for life. In 2004 three separate groups announced that they had detected exoplanets with a mass ranging from 14 to 40 times the mass of Earth. These planets, therefore, would likely be icy giant planets, such as Uranus and Neptune. Studies by George Rieke and collaborators from the University of Arizona, using NASA's Spitzer Infrared Space Telescope, in Earth orbit, found that of 266 young stars they had studied, 71 were surrounded by a disk of dusty debris. The observation suggested that there might exist many stars with small rocky planets. Other astronomers using the Spitzer Space Telescope detected a gap in the ring system surrounding the young star CoKu Tau 4, which suggested that there was a Jupiter-like planet in orbit around the star. Finally, a group of astronomers who used the advanced adaptive optics system on the European Southern Observatory's Very Large Telescope in Chile might have obtained the first near-infrared image of an exoplanet. The Jupiter-sized object orbited a relatively young nearby brown dwarf star of very low mass, called 2M1207. The various studies of exoplanets gave an indication that exoplanets were ubiquitous, and they gave further impetus for the search for Earth-like exoplanets in the Milky Way Galaxy.

Galaxies and Cosmology.

      Over the previous six years, a consistent picture of the origin and evolution of the universe had emerged from two kinds of observational evidence. Visible-light observations of Type Ia supernovae—exploding stars that all had roughly the same intrinsic luminosities—indicated that the galaxies in which they were found were moving away from one another at ever-increasing speeds. This observation implied that the rate of expansion of the universe was increasing with time. Detailed independent observations of minute fluctuations in the microwave background radiation left from the Big Bang provided confirmation of the accelerating expansion rate. Taken together, these observations also indicated that only 5% of the universe consisted of normal atomic matter, 70% consisted of dark energy, and roughly 25% consisted of an unknown cool dark matter. In 2004 observations made with three of NASA's Great Observatories—the Hubble Space Telescope, the Chandra X-Ray Observatory, and the Spitzer Infrared Space Telescope—helped confirm and clarify these findings. Using the Chandra Observatory, Andrew Fabian and collaborators from the University of Cambridge made detailed observations of distant clusters of galaxies that were 1 billion–8 billion light-years from Earth. The hot gases that filled the space between the member galaxies of the cluster emitted a prodigious amount of X-rays. By analyzing the X-ray spectra of 26 such clusters, the team concluded that they contained dark energy and matter in agreement with the earlier—and completely independent—studies.

      On March 9 NASA reported the first results from a study of an image obtained from the Hubble Space Telescope that showed objects in the universe more distant than had been seen before. The image required a total exposure of one million seconds (11.6 days) and was made by using both the Hubble's Advanced Camera for Surveys and the Near Infrared Camera and Multi-Object Spectrometer. Called the Hubble Ultra Deep Field, the image contained an estimated 10,000 galaxies that lay in a small patch of the sky that extended only one-tenth the angular diameter of the moon. The galaxies were estimated to have been formed only 400 million–800 million years after the Big Bang. The infrared, visible, microwave, and X-ray observations indicated that the age of the universe was about 13.7 billion years, give or take some 200 million years.

Kenneth Brecher

Space Exploration

      For Launches in Support of Human Space Flight in 2004, see Table (Human Spaceflight Launches and Returns, 2004).

 The era of privately funded human space travel arrived in 2004 with successful suborbital flights to the edge of space to claim the $10 million Ansari X Prize. Earlier in the year, the United States had announced plans to return humans to the Moon, to press onward to Mars in the coming decades, and to retire the aging space shuttle and withdraw from most activities aboard the International Space Station (ISS) once the station had been completed.

Manned Spaceflight.

      SpaceShipOne (SS1) captured headlines as it claimed the Ansari X Prize. The prize, founded by American space visionary Peter Diamandis, was modeled after the Orteig Prize, which helped spur Charles Lindbergh's nonstop solo transatlantic flight in 1927. The purpose of the Ansari X Prize was to open human space flight to commercial ventures for travel, tourism, and commerce. To win, a spacecraft had to carry at least one person (but be capable of flying three) to the edge of space (an altitude of 100 km [62 mi]), return safely to Earth, and then repeat the trip within two weeks.

      Several groups lined up to compete for the prize, but early on, the Mojave Aerospace Ventures team, led by the American aviation pioneer Burt Rutan (builder of the world-circling Voyager aircraft) and backed by American Microsoft billionaire Paul Allen, was the odds-on favourite. Rutan designed SS1, based in Mojave, Calif., as a lightweight three-person craft to be carried by an aircraft called White Knight to an altitude of 14 km (8.7 mi) and then released so that it could be pushed into space by its own hybrid rocket. After two earlier supersonic flights, SS1 became the first private spacecraft when it flew 124 m (407 ft) beyond the 100-km boundary on June 21 in a demonstration flight. Although minor difficulties were encountered, the flight proved the basic design of the spacecraft. The attempt for the Ansari X Prize by SS1 began on September 29 with a flight to 103 km (64 mi), and it was completed on October 4 with a flight to 112 km (69.6 mi). For 2006 a second competition, the X Prize Cup, was planned with the goal of decreasing turnaround time and increasing the altitude and number of passengers. British entrepeneur Sir Richard Branson, owner of Virgin Atlantic airlines, teamed with Rutan to form Virgin Galactic and plan space tourism with a five-passenger version of SS1. Real-estate magnate Robert T. Bigelow took the wraps off plans to build inflatable space stations and offered a $50 million America's Space Prize for establishing a reliable manned orbital transport service. Legislation to regulate the new space-tourism industry was introduced in the U.S. Congress but stalled over discussions concerning crew and passenger safety requirements that would have had the effect of stifling the new business.

      Efforts by NASA to resume space shuttle flights continued slowly, and the date for the next mission slipped to mid-2005. The immediate cause of the 2003 Columbia accident was the detachment of foam insulation from a support on the external tank; the foam then smashed through critically important heat- shield tiles on the leading edge of the left wing. To prevent a repetition of the accident, NASA replaced the insulation with electrical heaters at the point where the detachment occurred on the Columbia. Preparations for resuming space shuttle flights were slowed after the Kennedy Space Center was damaged by three hurricanes in August and September.

      In the aftermath of the loss of Columbia, NASA restricted future shuttle missions, including those supporting the ISS. It also canceled service missions to the Hubble Space Telescope, which prompted an outcry by the international astronomy community. NASA relented and in June announced plans to develop a robotic spacecraft that would be able to service the telescope, including the installation of new cameras and replacement gyroscopes. The robot would use a Canadian-made Special Purpose Dexterous Manipulator, a remotely controlled arm that was originally developed for the ISS. A service mission scheduled for 2007 would keep the Hubble operating until the launch of the James Webb Space Telescope, planned for 2011. Meanwhile, the ISS crew was reduced to two persons, the number for which the Russian Soyuz-TMA and Progress-M spacecraft could carry supplies. The next Chinese manned space flight, Shenzhou 6, was expected in 2005.

Space Probes.

      Scrutiny of Mars intensified with the successful landings of two U.S.-built surface rovers, Spirit and Opportunity, on January 3 and January 25, respectively. Within a few days of landing, each rover had begun exploring the Martian surface. Each was designed for a nominal 90-day mission but functioned so well that operations were extended several times. As 2004 neared a close, NASA planned to continue operating the two landers until they failed to respond to commands from Earth. By October, Spirit had traveled more than 3.6 km (2.2 mi) and Opportunity more than 1.6 km (1 mi). Through January, the European Space Agency (ESA) tried in vain to establish contact with its Beagle 2 lander, sent to the surface on Dec. 25, 2003, from the Mars Express orbiter. An investigation into the loss of the lander revealed a number of management shortfalls that might have led to its failure. Meanwhile, the orbiter started returning a series of striking images of the Martian surface after settling into orbit on January 28. Data from onboard instruments indicated the presence of trace quantities of methane over an area containing water ice. This finding was taken as a possible sign of microbial life on Mars. (See Special Report (Mystique of Mars ).) Japan's attempt to put its Nozomi (“Hope”) Mars probe into orbit on Dec. 9, 2003, failed, and the craft ended up in an orbit around the Sun.

      ESA launched its first lunar probe, Small Missions for Advanced Research and Technology (SMART)-1, on Sept. 27, 2003. The 370-kg (82-lb) probe had a xenon-ion engine that generated only 7 g (0.2 oz) of thrust, but it was sufficient to nudge SMART-1 from its first stop (the L1 libration point between Earth and Sun) into lunar orbit, planned around November 15. Once there, SMART-1 was to scan the Moon for signs of water in polar craters and to map terrain and minerals.

       Saturn received its first permanent visitor from Earth—the Cassini-Huygens spacecraft—on June 30, after a nearly seven-year journey. The Cassini orbiter, developed by the United States, would spend four years studying Saturn and its moons. During this time it was scheduled to make numerous flybys of the moons, including a series of 44 flybys of Titan. The orbiter's Huygens probe, developed by ESA to study Titan, was released December 25 and was to parachute through Titan's methane atmosphere for a landing on its surface on Jan. 14, 2005—the first attempted landing on any celestial body beyond Mars. Huygens was expected to provide data on the atmospheric structure of Titan and could possibly return some images from the surface.

      The first attempt since the early 1970s to bring to Earth materials collected from outer space ended as a near-total failure when the Genesis spacecraft crashed into the Utah desert on September 8. The spacecraft had been launched on Aug. 8, 2001, and spent 884 days orbiting the Sun with ultrapure sample plates exposed to collect a few micrograms (less than a millionth of an ounce) of the particles that make up the solar wind. The intent was to determine directly the composition of the Sun in order to provide more certain results than those obtained by means of spectral data from telescopic observations. Genesis was to have been recovered by helicopter as it parachuted to Earth. The parachutes did not deploy, apparently because, as investigations later suggested, drawings for the craft's gravity sensors were reversed. Despite damage to the sample capsule, the Genesis science team said it could salvage some specimens.

      ESA launched its Rosetta craft on a 10-year mission to obtain sample materials from Comet 67P/Churyumov-Gerasimenko. The expectation was that, like the Rosetta Stone, the craft would help decode ancient history—in this case, the history of the solar system. The 654-million-km (406-million-mi) cruise was to involve three gravity-assisted flybys of Earth and one of Mars before arriving at the comet in 2014. Rosetta would then deploy a 100-kg (220-lb) probe, Philae, that would use two harpoons to anchor itself to the surface of the comet. Data would be collected by an alpha-particle spectrometer and a set of six panoramic cameras, and a drill would be used to extract samples for chemical analysis. Messenger, the second-ever mission to Mercury, was launched by the U.S. on August 3. (The first mission, in 1974–75, was a flyby of Mercury by Mariner 10.) To alter the trajectory of Messenger in preparation for insertion in orbit around Mercury in 2011, the spacecraft was to fly past Earth once, Venus twice, and Mercury three times.

Unmanned Satellites.

       Gravity Probe B (GP-B) was launched April 20 into polar orbit. It carried four gyroscopes of ultraprecision 4-cm (1.6-in) polished quartz spheres spinning in liquid helium. Measurements during its one-year mission were to test Einstein's general theory of relativity. Specifically, they would prove or disprove the frame dragging effect—a very subtle phenomenon in which the rotation of a body (in this case, Earth) slowly drags the space-time continuum with it.

       China launched two space-physics satellites into Earth orbit: Double Star 1, launched into an equatorial orbit on Dec. 29, 2003, and Double Star 2, launched into polar orbit on July 25, 2004. The two satellites carried identical instruments made by Chinese and European scientists to measure the density, speed, mass, and electrical charge of plasmas and neutral gases in space. Aura, the latest in the NASA series of Earth observation satellites, was launched July 15 into polar orbit. Aura carried instruments to measure the chemical makeup and activity in Earth's stratosphere and troposphere, including concentration levels of ozone and of gases that destroy ozone. Swift, a satellite designed to swing into the proper orientation to catch the first few seconds of gamma-ray bursts, was launched on November 20.

Launch Vehicles.

      The privately funded SpaceX Falcon launch vehicle moved closer to operational status with the placement of the first flight unit on the launch pad at Vandenberg Air Force Base, California, for a launch planned in 2005. The Falcon was to be able to place into orbit a 680-kg (1,500-lb) payload for about $6 million, saving half the cost of using other launch vehicles, in part by using a recoverable first stage. SpaceX planned to develop a larger Falcon V vehicle to compete with the Delta family of launchers.

      The Delta IV heavy-lift launch vehicle was launched for the first time on December 21. It had a 4.6-m (15-ft) core rocket and two identical boosters, each powered by RS-68 liquid hydrogen engines derived from the space shuttle main engine. The last Atlas 2 rocket was launched on August 31. Atlas started as an intercontinental ballistic missile and, like other missiles, was drafted into use as a space launcher in the 1950s. The Atlas 2 rocket retained the missile's basic design.

Dave Dooling

▪ 2004

Introduction

Chemistry

Nuclear Chemistry.

      In 2003 the International Union of Pure and Applied Chemistry approved darmstadtium as the official name and Ds as the symbol for element 110 on the periodic table. Scientists working at the Society for Heavy Ion Research, known as GSI, in Darmstadt, Ger., synthesized element 110 for the first time in 1994 and proposed the name. It took some years, however, to verify their work and approve the proposal. Darmstadtium replaced the element's interim name, ununnilium (scientific Latin for 110 with an -ium suffix), which had appeared in classroom textbooks and periodic tables.

Carbon Chemistry.

      All-carbon fullerene molecules, such as the soccer-ball-patterned buckminsterfullerene (C₆₀), have cage structures with open interiors that are ideal for holding metal atoms or small gas molecules. During the year chemists continued to look for ways to trap such substances inside fullerenes in an effort to make new materials that would have scientific or industrial applications.

      Koichi Komatsu and colleagues at Kyoto (Japan) University reported synthesis of a fullerene derivative that readily accepts and holds a molecule of hydrogen (H₂). Prepared from C₆₀, the molecule has a tailored “mouth”—an opening in its cage—that is slightly larger than previous versions. Other researchers had made fullerene derivatives that could incorporate hydrogen in as much as 10% yield. Komatsu's derivative, in contrast, can be filled to 100% yield. In laboratory tests no hydrogen leaked from a sample of the filled molecules during more than three months of monitoring at room temperature. The hydrogen was released slowly, however, when the molecules were heated to temperatures above 160 °C (320 °F). Researchers sought to develop materials that could safely hold and release hydrogen, which because of its high flammability poses an explosion hazard, for possible applications in new generations of hydrogen-fueled vehicles. Molecular encapsulation and slow release could solve that problem.

      A strand of spider silk is five times as strong as a strand of steel of identical mass. That strength underpinned ongoing research to make commercial amounts of spider silk for cables, supertough fabrics, and other uses. Ray Baughman of the University of Texas at Dallas and co-workers reported synthesis of long carbon-nanotube composite fibres that match spider silk's strength. Nanotubes consist of carbon atoms bonded into a hexagonal-mesh framework similar to that of graphite; the framework is rolled into a seamless cylinder barely a nanometre in diameter.

      Baughman's composite fibres appeared to be tougher than any natural or synthetic organic fibre described to date, and they were able to be woven into textiles. The researchers developed a process for spinning the solid fibres from a gel material consisting of nanotubes and a polymer, polyvinyl alcohol. They produced composite fibres the width of a human hair at a rate of about 70 cm (2.3 ft) per minute and yielded individual strands as long as 100 m (330 ft).

      The researchers then used their spun carbon-nanotube fibres to make supercapacitors, electronic devices capable of storing large amounts of electricity. In addition, they wove the supercapacitors, which had the same energy-storage density as large commercial supercapacitors, into conventional fabrics. The fibre capacitors showed no decline in performance during 1,200 charge-discharge cycles. The investigators cited a number of promising electronic-textile applications for the fibres, including electromagnetic shields, sensors, antennae, and batteries.

Inorganic Chemistry.

      A relatively new group of crystalline ionic compounds, called electrides, was stirring excitement among chemists and materials scientists. The electrons in electrides do not congregate in localized areas of specific atoms or molecules, nor are they delocalized like the electrons in metals. Rather, the electrons are trapped in sites normally occupied by anions, negatively charged atoms or groups such as the chloride ion (Cl⁻) and the hydroxyl ion (OH⁻).

      The trapped electrons act like the smallest possible anions, which opens the door to important practical applications—for example, powerful reducing agents or materials with unusual electrical, magnetic, or optical properties. Scientists had been unable to explore those possibilities because all electrides made in the past were fragile organic complexes. They decomposed at temperatures above −40 °C (−40 °F) and could not withstand exposure to air or water.

      Satoru Matsuishi and Hideo Hosono of the Japan Science and Technology Corp., Kawasaki, and colleagues reported an advance that promised to simplify future research on electrides. They synthesized an inorganic electride that is stable at room temperature. The material, having the formula [Ca₂₄Al₂₈O₆₄]⁴⁺(4e⁻), in which the four electrons (e⁻) counterbalance the positively charged (4+) ion, also withstands exposure to air and moisture. Matsuishi's group made it by removing almost all of the oxygen anions (O²⁻) trapped in cavities in the internal structure of a single crystal of 12CaO∙7Al₂O₃. The vacant cavities filled with electrons to a density typical of electrides; in the process the colour of the crystal changed from colourless to green and then to black. The researchers believed that the new compound would point the way to other stable electrides with practical applications.

Organic Chemistry.

      Chemists missed the mark when they picked the original name—inert gases—for a family of six elements that compose group 18 of the periodic table. They thought that helium, neon, argon, krypton, xenon, and radon were inert and never combined with other elements to form chemical compounds. That notion was upset in the 1960s when researchers made the first xenon compounds and the group's preferred name changed to the noble gases. Xenon, for instance, forms a variety of inorganic compounds with oxygen and fluorine.

      Leonid Khryashtchev and co-workers of the University of Helsinki, Fin., reported making the first true organic compound incorporating a noble gas, krypton (Kr). It is the compound HKrCCH, in which a krypton atom is bonded to a carbon atom and a hydrogen atom. They synthesized minute amounts of the compound by focusing ultraviolet light on acetylene (HC≡CH) trapped inside a krypton matrix that had been chilled to within a few degrees of absolute zero. Khryashtchev believed that the landmark reaction could open a window on a new area of krypton chemistry.

“Green” Chemistry.

      Catalysts speed up chemical reactions that otherwise would not occur or would occur at a snail's pace. They play an indispensable behind-the-scenes role in the manufacture of hundreds of consumer products, ranging from gasoline to medicines. Chemists face big problems, however, in separating a certain class of catalysts from the products after the reaction is done. Called homogeneous catalysts, they are usually dissolved in the same liquid that contains the reactants. When the reaction finishes, the liquid holds not only the desired products but also the catalyst. Separating the catalyst can be expensive and time-consuming.

      During the year R. Morris Bullock and Vladimir K. Dioumaev of Brookhaven National Laboratory, Upton, N.Y., developed a self-separating, reusable catalyst. The catalyst dissolves in the reactants but is insoluble in the product; at the end of the reaction, it precipitates from solution, which makes it easy to recover and reuse. Although the chemists demonstrated the catalyst—an organometallic tungsten-containing complex—in only one specific case, they hoped that the results would lead to a general method for developing self-separating catalysts for a variety of reactions of practical interest.

      Bullock and Dioumaev noted that self-precipitating catalysts would be a major advance in “green” chemistry, the effort to replace chemical processes potentially damaging to the environment with friendlier alternatives. Separating homogeneous catalysts from products often requires the use of toxic solvents, which require special disposal methods. Catalysts that automatically separate would reduce or eliminate the need for solvents.

Applied Chemistry.

      The traditional chemical process for making hydrogen is amenable to industrial-scale production of that clean-burning fuel, but it is far from ideal for small-scale hydrogen production, such as for use in fuel cells in homes or motor vehicles. Termed reforming, the industrial process uses steam and hydrocarbons such as methane as raw materials and requires catalysts and temperatures above 800 °C (1,500 °F).

      Zhong L. Wang and Zhenchuan Kang of the Georgia Institute of Technology reported an advance toward a better small-scale hydrogen-production technology. It involved oxides of the rare-earth elements cerium, terbium, and praseodymium. Scientists had long known that these compounds can make hydrogen from water vapour and methane in a continuous “inhale-exhale” cycle. The oxides have a unique internal crystalline structure, which allows up to 20% of their oxygen atoms to leave and return without damaging the crystalline lattice. Integrated into a hydrogen-production system, the oxides would permit oxygen atoms to move out and back in as the oxygen participated in a two-step temperature-governed cycle of oxidation and reduction reactions that produce hydrogen. The built-in oxygen supply would decrease the amount of water vapour needed for the process.

      Wang and Kang discovered that doping, or supplementing, the rare-earth oxides with iron atoms lowered the temperatures at which the hydrogen-production cycle could be run. The doped lattice structures “exhale” oxygen atoms at about 700 °C (1,300 °F) and “inhale” them at 375 °C (700 °F). Lowering the latter temperature a little more, to about 350 °C (660 °F), would permit use of solar energy as part of the heat source, Wang noted.

Michael Woods

Physics

Particle Physics.

      In 2003 independent teams of scientists involved in technically quite different high-energy particle experiments at the Jefferson National Accelerator Facility, Newport News, Va., and the Institute of Theoretical and Experimental Physics, Moscow, reported evidence for a new particle, the theta-plus (Θ⁺), made of an unprecedented five quarks. Their findings corroborated evidence for the particle announced the previous year by researchers at the SPring-8 accelerator facility near Osaka, Japan.

      It had been known for decades that protons and neutrons, the familiar particles that compose atomic nuclei, are made of still smaller particles called quarks. The standard model, the theory encompassing the fundamental particles and their interactions, does not preclude the existence of five-quark particles, or pentaquarks. Until the latest findings, however, only particles made up of three quarks (e.g., protons and neutrons) or of two quarks (unstable, short-lived particles known as mesons) had ever been observed. The new experiments all pointed to the fleeting existence of a pentaquark with a mass of 1.54 GeV (billion electron volts), which decayed into a neutron and a K-meson (kaon). The results agreed with theoretical predictions of the particle made by Russian physicists in 1997.

      Although the existence of quarks was well established, individual “free” quarks—quarks not bound into particles—remained to be observed. Experiments at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) in which gold nuclei moving at 99% of the speed of light were collided head-on into one another continued to show intriguing hints of the production of free quarks as part of a so-called quark-gluon plasma. Gluons are the massless field particles that hold quarks together in particles. Physicists expected that at sufficiently high collision energies, the protons and neutrons in the gold nuclei would liberate their quarks and gluons to form an extremely hot, dense “soup” of nuclear matter. Such a quark-gluon plasma was believed to have existed in the first instant after the big-bang birth of the universe.

Condensed-Matter Physics.

      Experiments that involve cooling a few thousand atoms of a gas to temperatures closely approaching absolute zero (0 K, −273.15 °C, or −459.67 °F) provided fascinating results once again in 2003. When the cooled gas consists of atoms having zero or integral-number intrinsic spins (such atoms are called bosons), the result is a state of matter known as a Bose-Einstein condensate (BEC), which was first created in the laboratory in 1995. Rather than existing as independent particles, the atoms in a BEC become one “superparticle” described by a single set of quantum state functions. In a technological achievement for low-temperature physics, Aaron Leanhardt, Wolfgang Ketterle, and co-workers from the Massachusetts Institute of Technology (MIT)–Harvard University Center for Ultracold Atoms trapped sodium atoms in a “container” of magnetic fields, cooled them to form a BEC, and ultimately brought 2,500 of them to the lowest temperature documented to date—about 500 picokelvins (500 trillionths of a kelvin). The previous low-temperature record had been 3 nanokelvins (3 billionths of a kelvin), six times higher.

      Gases consisting of atoms having intrinsic spins that are multiples of half integers (such atoms are known as fermions) also can be cooled similarly, but their properties (as described by the Pauli exclusion principle) do not allow them to fall into the same condensed state. Instead, they fill up all available states starting from the lowest energy. A common example is the stepwise buildup of electrons, which are fermions, in successive orbitals around the nucleus of an atom. At first sight the behaviour of ultracold fermions might seem less interesting than that of bosons but for one possible phenomenon—Cooper pairing. It should be possible for two fermionic atoms to pair in a strongly interacting way. This atom pair would function similarly to the paired electrons called Cooper pairs, which are responsible for superconductivity in some materials when they are cooled to low temperatures. Strongly interacting fermions—not only electrons but also protons, neutrons, and quarks—were involved in some of the most important unanswered questions in science from astrophysics and cosmology to nuclear physics. The controlled production of paired fermionic atoms could give new insight into these questions and lead to novel and useful quantum effects.

      By midyear six research teams had succeeded in chilling gases of fermions to their lowest energy states, an important step toward achieving Cooper pairing of atoms. Deborah Jin and colleagues at JILA, Boulder, Colo., worked with potassium atoms, as did Massimo Inguscio and researchers at the University of Florence. Using lithium atoms were Randall Hulet's team at Rice University, Houston, Texas; Christophe Salomon's group at the École Normale Supérieure, Paris; John Thomas's group at Duke University, Durham, N.C.; and Ketterle's team at MIT. No team produced evidence of pairing, but Cindy Regal and co-workers of the JILA group succeeded in forcing fermion atoms to combine into a molecule-like state called a magnetic Feshbach resonance. Some researchers hoped that this fleeting interaction would serve as a stepping-stone from which the atoms could be coaxed further to form Cooper pairs. In terms of fundamental physics, gases of ultracold fermionic atoms might well prove more important than BECs.

Photonics and Optical Physics.

      A new generation of relatively compact pulsed lasers under development had the potential to produce hitherto undreamed-of power —in the petawatt region (a petawatt is 10¹⁵ W). A complex system involving compressing, amplifying, stretching, amplifying, and then compressing again converted relatively long-duration low-power laser pulses with energies of hundreds of joules into very short, femtosecond (10^–15 second), high-power pulses. Many laboratories were working on such devices, which promised to make possible laser-driven fusion reactions and to reproduce in the laboratory the conditions that existed near the birth of the universe. A leader in the field was Victor Yanovsky's group at the University of Michigan, which reported having produced a sharply focused pulse with a power density of 10²¹ W/cm². Groups also were working on techniques to use such pulses to control electronic processes.

      The refraction of light took on new interest as a number of researchers developed ways of making materials with negative refractive indexes. On entering such a material, electromagnetic radiation such as light would be bent through a negative, rather than a positive, angle; i.e., its change in direction would be opposite that normally observed. C.G. Parazzoli and co-workers of the Boeing Co. and A.A. Houck and colleagues at MIT built systems that exhibited this phenomenon, as did Ertugrul Cubukcu and co-workers from Bilkent University, Ankara, Turkey. In related work Matthew Bigelow and colleagues of the University of Rochester, N.Y., demonstrated the ability to control the propagation of light—slowing it down or speeding it up—as it traveled through a crystalline material at room temperature by altering the material's refractive index.

Quantum Physics.

      Many research teams continued to investigate the application of quantum phenomena to computing. Operation of quantum computers would involve the storage and transfer of so-called qubits, states of quantum systems that could be used to represent bits of data. The great advantage of such devices was that the transfer of information might not be limited by the speed of light. The bizarre phenomenon of quantum entanglement allows two systems—for example, subatomic particles or atoms—in the same quantum state to be separated by an arbitrary distance but to remain connected in such a way that they reflect each other's condition. Two entangled qubit devices would thus be in contact instantaneously. By 2003 scientists had used entanglement to achieve “quantum teleportation”—the transfer of the quantum state of a particle from point to point (albeit without physical transfer of the particle itself)—on a small scale, but practical systems to store and manipulate qubits without destroying their coupled states remained to be constructed. There were many different candidates on which to base entangled systems, including photons, atoms, trapped ions, and quantum dots, the last being tiny isolated clumps of semiconductor atoms with dimensions measured in nanometres (billionths of a metre).

      During the year Markus Aspelmeyer and colleagues of the University of Vienna reported the first long-distance demonstration of quantum entanglement across open space. They showed that photons of light remained coupled and able to communicate their states over a distance of 600 m (more than a third of a mile). The concept of entanglement was now well established, and it appeared increasingly likely that qubit systems would provide the next major leap forward in computing.

David G.C. Jones

Astronomy

Solar System.

       Earth Perihelion and Aphelion, 2004For Eclipses, Equinoxes and Solstices, and Earth Perihelion and Aphelion in 2004, see Table (Earth Perihelion and Aphelion, 2004).

      On the morning of August 27, Mars and Earth made their closest approach in 60,000 years—a “mere” 56 million km (35 million mi) apart. As many people on Earth delighted in the excellent viewing opportunities offered by the event, the exploration of Mars by robotic spacecraft missions continued apace. NASA's Mars Global Surveyor, which had been orbiting Mars since 1997, found more than 500 examples of new types of geologic features on the Red Planet, including evidence of landslides near regions of former volcanic activity and erosion gullies possibly formed by flowing water in the past. It also provided evidence that the planet's core is at least partially liquid iron. NASA's Mars Odyssey spacecraft, which began its observations from orbit in late 2001, continued mapping high levels of hydrogen near the planet's surface, which was suggestive of the presence of large amounts of water ice. Several new spacecraft missions to Mars also were launched during the year. (See Space Exploration (Physical Sciences ).)

      Ever since Galileo pointed his five-centimetre (two-inch)-diameter telescope at Jupiter in 1610 and discovered four moons of the giant planet, astronomers had sought out heretofore-unseen satellites of the solar system's planets. In 2003 a bevy of new moons were discovered. Using the Keck telescopes in Hawaii, David C. Jewitt and Scott S. Sheppard of the University of Hawaii discovered 21 new satellites of Jupiter. This brought the number of its moons known at year's end to 61. The same astronomers also found another moon of Saturn, which brought its known total to 31. In addition, a group of astronomers led by Matthew J. Holman of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., announced the discovery of three new moons of Neptune, which brought its known total to 11; these were the first new finds for Neptune since 1989, when the Voyager 2 spacecraft discovered several moons during its flyby of the giant planet. All of the moons are small (a few kilometres in diameter) and have orbits suggesting that they were captured by their respective planets rather than being formed with them.

Stars.

      Since the early 1990s more than 100 extrasolar planets had been discovered revolving around relatively nearby individual stars—stars up to about 100 light-years distant. Astronomers detected most of them indirectly by observing subtle gravitational effects on the parent stars as they were tugged to and fro by the unseen bodies. The year 2003 brought announcements of a variety of new extrasolar planets, some of them comparatively far from Earth. At the start of the year, a Jupiter-mass planet was detected when it passed in front of the star it was orbiting, slightly dimming its light. Called OGLE-TR-56b, it is about 5,000 light-years away and was the first extrasolar planet to be initially detected by its transiting. Another study resulted in the identification of what was likely the oldest planet found to date. This planet orbits a star in a binary system that contains both a radio-emitting pulsar, named PSR B1620-26, and a white dwarf. Furthermore, this stellar-planetary system resides in the globular star cluster M4, which is about 7,000 light-years away and is estimated to be 12.5 billion–13 billion years old. A major implication of the discovery was that at least some planets formed very early in the history of the universe.

      Most stars are assumed to be spherical objects. Their shape, nevertheless, is difficult to discern directly because of their relatively small angular diameters as seen from Earth. For a long time only the Sun presented a large-enough target to establish its shape directly. It is spherical to better than one part in 100,000. In 2003 astronomers using the European Southern Observatory's Very Large Telescope Interferometer at Cerro Paranal in Chile found that one of the brightest stars in the night sky, the magnitude-zero Achernar (Alpha Eridani) in the constellation Eridanus, is highly oblate. A team led by Armando Domiciano de Souza of the University Astrophysical Laboratory at Nice, France, found that the star is so flattened by rotation that its radius is 50% larger at its equator than at its poles. The star has a measured surface rotational speed of 225 km (140 mi) per second with respect to Earth's line of sight, too slow to account for the observed oblateness. Astronomers concluded either that the star has its polar axis tipped toward Earth and is actually rotating near its breakup speed of 300 km (186 mi) per second or that it has an interior that rotates much faster than its surface.

Galaxies and Cosmology.

      Earth's solar system lies in the plane of the Milky Way Galaxy, an average-size spiral galaxy comprising about 100 billion stars plus gas and dust. The Milky Way Galaxy has long been known to be one of several dozen galaxies in the Local Group, which includes the Andromeda Galaxy and the Magellenic Clouds. In 2003 a team of astronomers from France, Italy, Australia, and the U.K. announced the discovery of a new member of the Local Group. It was named the Canis Major Dwarf Galaxy after the constellation in which it appears to lie. Its discovery was made possible by the Two-Micron All Sky Survey (2MASS), a project initiated in the late 1990s in which automated telescopes in Arizona and Chile systematically scanned the entire sky in three infrared wavelengths. 2MASS allowed astronomers to peer through the clouds of dust that pervade the plane of the Milky Way Galaxy. The newly discovered galaxy lies some 25,000 light-years from Earth's solar system and about 42,000 light-years from the centre of the Milky Way, which makes it the closest galaxy to the Milky Way found to date. It contains only about a billion stars, which are being tidally disrupted by the enormous gravitational field of the Milky Way Galaxy.

      Another unanticipated aspect of the Milky Way Galaxy was uncovered in studies carried out by the Sloan Digital Sky Survey (SDSS). A detailed mapping project making use of a special-purpose 2.5-m (100-in) telescope at Apache Point Observatory in New Mexico, the SDSS involved observation of the positions and brightnesses of more than 100 million stars and galaxies at five visible and infrared wavelengths. Within the data acquired to date, Brian Yanny of Fermi National Accelerator Laboratory, Batavia, Ill., Heidi Jo Newberg of Rensselaer Polytechnic Institute, Troy, N.Y., and collaborators found evidence for a huge structure containing as many as 500 million stars forming a ring around the Milky Way Galaxy with a radius of about 60,000 light-years. Independent studies by a group of European astronomers led by Annette Ferguson of the University of Groningen, Neth., suggested that the ring may be slightly elliptical. The ring had not been seen in visible light because it lies in the same plane as the dusty disk of the Milky Way. Early studies of stars populating the ring indicated that they were not initially part of the Milky Way Galaxy, which implies that they are debris from another galaxy that collided with the Milky Way Galaxy and then disintegrated. Both the 2MASS and the SDSS galaxy studies underscored the continuing dynamic evolution of the Milky Way Galaxy and its neighbouring galaxies in the Local Group.

      Scientists' picture of the origin and evolution of the universe has grown enormously since its expansion was first theorized to exist and subsequently detected in the 1920s. The big-bang model posits that the universe began with a hot, dense explosive phase resulting in the formation of a few elements—mainly hydrogen and helium—and giving rise to galaxies and to radiation detected today primarily at microwave wavelengths with a temperature of about 3 K (−454 °F). Studies of supernovas carried out in the past five years implied that the universe is currently expanding at an accelerating rate, driven by some gravitationally repulsive “dark energy” originally hypothesized in 1917 (for quite different reasons) by Albert Einstein. In 2001 NASA launched the Wilkinson Microwave Anisotropy Probe (WMAP) to study the microwave background radiation with greater precision than had been previously achieved. This radiation was observed to be coming from all directions in the sky. Fluctuations in its overall intensity as small as one part in a million were key to unraveling the origin of both the large- and small-scale structures of the universe. The radiation comes from a time when the universe was only a few thousand years old and when galaxies were just beginning to form.

      In February NASA scientists announced the first results from WMAP, which included strong confirmation that the universe is composed of about 4% ordinary (baryonic) matter—such as hydrogen and helium—with the rest being roughly 23% nonbaryonic dark (nonluminous) matter of some kind and 73% dark energy. Other WMAP results suggested that the big bang occurred about 13.7 billion years ago, give or take 200 million years. WMAP also provided the first evidence that the earliest stars formed between 100 million and 400 million years after the big bang.

Kenneth Brecher

Space Exploration

Manned Spaceflight.

      The space community was shattered by the tragic loss on Feb. 1, 2003, of the U.S. space shuttle orbiter Columbia and its seven-person crew just minutes before it was to land at the Kennedy Space Center in Florida. (For Obituaries of Columbia astronauts, see Michael P. Anderson (Anderson, Michael P. ), David M. Brown (Brown, David M. ), Kalpana Chawla (Chawla, Kalpana ), Laurel Blair Salton Clark (Clark, Laurel Blair Salton ), Rick D. Husband (Husband, Rick D. ), William C. McCool (McCool, William C. ), and Ilan Ramon (Ramon, Ilan ).) The orbiter, which had made the shuttle program's first flight into space in 1981, was concluding its 28th mission (STS-107) when it broke apart over Texas at approximately 9:00 am Eastern Standard Time at an altitude of 60 km (40 mi), showering debris across southeastern Texas and southern Louisiana. Disintegration of the craft was recorded by television cameras and U.S. Air Force radar. Its major components and the remains of the crew were recovered over the following month.

      Destruction of the Columbia followed by almost exactly 17 years the loss of the Challenger in a launch accident on Jan. 28, 1986. Ironically, the cause of the Columbia catastrophe soon was determined to be launch-related as well. Films showed that a piece of insulating foam broke loose from the external propellant tank and struck the leading edge of the left wing approximately 81 seconds after liftoff. Bits of foam had detached in past missions without serious mishap, and at the time of the Columbia launch, NASA engineers did not think that the foam carried enough momentum to cause significant damage. In fact, as demonstrated in postaccident tests, the foam was capable of punching a large hole in the reinforced carbon-carbon insulation tiles that protected the shuttle's nose and wing leading edges from the extreme heat of atmospheric entry. Although some engineers had wanted ground-based cameras to take photos of the orbiting shuttle to look for damage, the request did not get to the right officials.

      During Columbia's atmospheric entry, hot gases penetrated the damaged tile section and melted major structural elements of the wing, which eventually collapsed. Data from the vehicle showed rising temperatures within sections of the left wing as early as 8:52 am, although the crew knew of their situation for perhaps only a minute or so before vehicle breakup. Subsequent investigation by NASA and the independent Columbia Accident Investigation Board uncovered a number of managerial shortcomings, in addition to the immediate technical reason (poor manufacturing control of tank insulation and other defects), that allowed the accident to happen.

      The most palpable result of the accident was a grounding of the remaining three shuttles—Discovery, Atlantis, and Endeavour (the last built to replace Challenger)—until NASA and its contractors could develop means to prevent similar accidents, which perhaps would include kits for repairs in orbit. The shuttle Return to Flight mission was STS-114, scheduled for late 2004. At the same time, NASA gave new emphasis to its Orbital Space Plane (OSP) concept, a smaller reusable craft designed to carry as many as four astronauts (but not large cargo) into low Earth orbit. The OSP likely would not be ready until 2008–10, and funding was uncertain.

      Assembly of the International Space Station (ISS) in Earth orbit was suspended after the Columbia accident until shuttle flights could resume. Limited research was conducted by rotating two-person crews launched in Russian Soyuz spacecraft.

       Human Spaceflight Launches and Returns, 2003China entered the human spaceflight arena on October 15 with the launch of Shenzhou 5 carrying Yang Liwei, a People's Liberation Army pilot, on a 21-hour, 14-orbit mission. Four unmanned Shenzhou flights over four years had tested the spacecraft in orbital missions. In its general outline the vehicle resembled the Soyuz, but it relied heavily on Chinese-developed technologies and manufacturing. The next Shenzhou mission was expected to have a three-person crew and to last longer. Previously only the U.S. and Russia had had the capability to launch humans into space. (For Human Spaceflight Launches and Returns in 2003, see Table (Human Spaceflight Launches and Returns, 2003).)

Space Probes.

      Exploration of Mars and other planets continued apace, with the Red Planet being the target for several new orbiters and landers. Japan's Nozomi, launched in 1998, would have been first to arrive (December 14), but problems with its propulsion system prevented it from being put into Mars orbit. The European Space Agency's (ESA's) Mars Express, which was launched on June 2 from Kazakhstan, went into Mars orbit on December 25. Its lander, named Beagle 2 for the 19th-century ship that carried Charles Darwin, likely reached the Martian surface the same day, but it was not heard from by the end of the year. NASA's twin Spirit and Opportunity rovers were launched on June 10 and July 7, respectively, and were scheduled to land in January 2004.

      Mars Express carried a colour stereo camera, an energetic neutral atoms analyzer to study how the solar wind erodes the atmosphere, a mineralogical mapping spectrometer, a radar instrument for subsurface and ionospheric sounding, and atmospheric and radio science experiments. Beagle 2 was to have descended by parachute and airbag cushions to a site in Isidis Planitia, a sedimentary basin that may have been formed by water. The 33-kg (73-lb) lander was equipped with a robotic arm to acquire soil and rock samples for X-ray, gamma-ray, and mass spectroscopy analysis.

      For landing its Spirit and Opportunity rovers, NASA returned to the parachute-and-enveloping-airbag design successfully used by the Pathfinder/Sojourner mission in 1997. Once deployed, each 18-kg (40-lb), six-wheel, golf-cart-size robot was to range as far as 500 m (0.3 mi) from the landing site. Each rover carried a panoramic colour stereo camera, a drill to make small holes for microscopic images of unweathered rock surfaces, and infrared, gamma-ray, and alpha-particle spectrometers to assay the chemistry of rocks and soil.

      Japan launched the Hayabusa (MUSES-C) spacecraft on May 9 for a June 2005 rendezvous with the near-Earth asteroid 1998 SF36. It was to orbit the asteroid for several months and then pass near the surface and collect samples vaporized by metal pellets fired into the surface. Hayabusa would return to Earth in 2007 and drop for retrieval a capsule containing the samples. NASA's Galileo spacecraft ended almost eight years of highly successful exploration of Jupiter and its moons with a programmed fiery plunge into the giant planet's atmosphere on September 21.

Unmanned Satellites.

      The Spitzer Space Telescope, the last of NASA's four Great Observatories for space-based astrophysics, was launched on August 25. The spacecraft, formerly called the Space Infrared Telescope Facility, was renamed Spitzer for the American astrophysicist Lyman Spitzer, Jr., who first proposed the idea of stationing large telescopes in space. To remove the spacecraft from Earth's thermal and radiation effects, it was placed in a solar orbit having a period of revolution that caused it to drift slowly away from Earth as the two orbited the Sun. Spitzer carried an 85-cm (33.5-in) primary mirror that focused infrared light on three instruments—a general-purpose infrared camera, a spectrograph sensitive to mid-infrared wavelengths, and an imaging photometer taking measurements in three far-infrared bands. Together the instruments covered a wavelength range of 3–180 μm (micrometres; the red end of human vision cuts off at about 0.77 μm). To avoid interference from its own heat, the telescope was cooled to 5.5 K (5.5° above absolute zero) and the detectors to 1.5 K, by liquid helium. Spitzer was expected to spend 2.5–5 years gathering information on the origin, evolution, and composition of planets and smaller bodies, stars, galaxies, and the universe as a whole.

      At the other end of the spectrum, ESA's International Gamma-Ray Astrophysics Laboratory (INTEGRAL) started returning science data following its Oct. 17, 2002, launch by Russia. It carried gamma-ray and X-ray imagers and spectrometers to study the most energetic events in the universe. Among several other astronomy-oriented launches in 2003 was Canada's Microvariability and Oscillations of Stars (MOST; June 30), an orbiting telescope for studying physical processes in stars and properties of extrasolar planets.

Launch Vehicles.

      Brazil's space program suffered a major setback when its VLS-1 launcher exploded on the launchpad at its Alcântara facility on August 22, killing 21 engineers and technicians. One of its four solid-propellant boosters appeared to have ignited prematurely and destroyed the vehicle. Two previous attempts to launch the vehicle, in 1997 and 1999, had ended in failures after liftoff, with no injuries. The first U.S. Delta IV Heavy Evolved Expendable Launch Vehicle moved to the launchpad on December 10, with launch scheduled for July 2004. Equipped with three powerful liquid-fueled (hydrogen-oxygen) engines, it was designed to carry more than 23,000 kg (51,000 lb) into low Earth orbit and more than 13,000 kg (29,000 pounds) into geosynchronous transfer orbit.

      Competitors moved closer to the launchpad in the X Prize contest, which was advertised as a $10 million incentive “to jumpstart the space tourism industry through competition.” The winning vehicle had to be privately financed and built, to carry at least one person (but be capable of flying three) to the edge of space (100 km, or 62 mi) and back, and to repeat the trip within 14 days. By 2003 the contest, inaugurated in 1996, had registered at least 25 teams, whose designs involved various vertical and horizontal takeoff-and-landing strategies. American aviation pioneer Burt Rutan's company Scaled Composites, for example, was developing SpaceShipOne (SS1), which would be carried to a high launch altitude by a twin-engine jet aircraft, rocket into space, and then glide to a landing. On December 17, SS1 broke the sound barrier at an altitude of nearly 21 km (68,000 ft) during its first powered flight near Mojave, Calif.

Dave Dooling

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