space exploration

space exploration
Investigation of the universe beyond Earth's atmosphere by means of manned and unmanned spacecraft.

Study of the use of rockets for spaceflight began early in the 20th century. Germany's research on rocket propulsion in the 1930s led to development of the V-2 missile. After World War II, the U.S. and the Soviet Union, with the aid of relocated German scientists, competed in the "space race," making substantial progress in high-altitude rocket technology (see staged rocket). Both launched their first satellites (see Sputnik; Explorer) in the late 1950s (followed by other satellites and unmanned lunar probes) and their first manned space vehicles (see Vostok; Mercury) in 1961. A succession of longer and more complex manned space missions followed, most notably the U.S. Apollo program, including the first manned lunar landing in 1969, and the Soviet Soyuz and Salyut missions. Beginning in the 1960s, U.S. and Soviet scientists also launched unmanned deep-space probes for studies of the planets and other solar system objects (see Pioneer; Venera; Viking; Voyager; Galileo), and Earth-orbiting astronomical observatories (see, for example, Hubble Space Telescope), which permitted observation of cosmic objects from above the filtering and distorting effects of Earth's atmosphere. In the 1970s and '80s the Soviet Union concentrated on the development of space stations for scientific research and military reconnaissance (see Salyut; Mir). After the dissolution of the Soviet Union in 1991, Russia continued its space program, but on a reduced basis owing to economic constraints. In 1973 the U.S. launched its own space station (see Skylab), and since the mid 1970s it has devoted much of its manned space efforts to the space shuttle program and, more recently, to developing the International Space Station in collaboration with Russia and other countries.

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▪ 1995

      During 1994 the design of the international space station was established as the United States and Russia moved to combine forces in a single orbital facility. The DC-X launch vehicle program was revived, and Japan and Europe introduced new launch vehicles. In addition, an international campaign was begun to understand the effect of solar and space phenomena on the Earth.

Manned Flight.
      Only seven space shuttle missions were flown during 1994, two carrying the same Space Radar Laboratory to survey the Earth at different times of the year. Three missions studied materials and life sciences in space, and two continued detailed observations of the Earth's atmosphere.

      The U.S. National Aeronautics and Space Administration (NASA) confirmed on January 13 that the repairs to the Hubble Space Telescope made in late 1993 were successful and that the Hubble's optics were working as planned. There was a marked increase in the sharpness of images taken by the wide-field camera.

      On the first mission of the year, February 3-11, Discovery carried the Wake Shield Facility and Spacehab-02. The crew comprised commander Charles F. Bolden, Jr., pilot Kenneth S. Reightler, Jr., and mission specialists Franklin R. Chang-Diaz, N. Jan Davis, Ronald M. Sega, and Sergey K. Krikalev. The Wake Shield Facility was a satellite designed to be released by the shuttle and retrieved a few days later. As it orbited on its own, its wake would create a near-perfect vacuum in which high-quality semiconductor films could be grown. A problem with its guidance system, however, prevented the astronauts from releasing the facility. Six metal balls, from 5 to 15 cm (2 to 6 in) in diameter, were released for use as calibration targets by ground-based radar. Krikalev was the first Russian to be launched aboard a U.S. spacecraft as the two nations initiated their joint space station program.

      Materials sciences were the focus of the mission of Columbia (March 4-18). The crew comprised commander John H. Casper, pilot Andrew M. Allen, and mission specialists Pierre J. Thuot, Charles D. Gemar, and Marsha S. Ivans. The U.S. Microgravity Payload comprised several automated devices for processing materials in the weightlessness of space. In one, a furnace processed samples of mercury cadmium telluride, an alloy that is valued as a detector of infrared radiation but that suffers from defects when fabricated on Earth. Another device observed how dendrites—branchlike structures—grow in transparent crystalline materials. Columbia also carried the Shuttle Solar Backscatter Ultraviolet instrument to measure ozone in the upper atmosphere. Inside Columbia the crew assembled scale models of solar array supports to measure vibration and stress.

      The Earth was given a close examination by the Space Radar Laboratory (SRL), a special mapping radar flown twice by the shuttle Endeavour. On the first mission (April 9-20) the crew comprised commander Sidney M. Gutierrez, pilot Kevin P. Chilton, and mission specialists Linda M. Godwin, Jay Apt, Michael R. Clifford, and Thomas David Jones. On the second (September 30-October 11) the commander was Michael A. Baker, the pilot Terrence W. Wilcutt, and the mission specialists Thomas David Jones, Steven L. Smith, Peter J.K. Wisoff, and Daniel W. Bursch. The SRL uses synthetic aperture radar, which mathematically combines a series of radar echoes to generate an image that otherwise would require a single, larger antenna. SRL was the first space radar to use three bands of radio frequencies—C, L, and X—at once to obtain more detailed images. At some frequencies the radar penetrated the ground and revealed structures such as ancient streambeds.

      A laser probed the atmosphere on the mission of Discovery (September 9-20). The crew comprised commander Richard N. Richards, pilot L. Blaine Hammond, Jr., and mission specialists Carl J. Meade, Mark C. Lee, Susan J. Helms, and Jerry M. Linenger. The Laser In-Flight Technology Experiment focused lasers on the Earth's atmosphere through a 1.5-m reflector telescope and then measured the return signal. This allowed scientists to measure the speed of aerosols and dust in atmospheric conditions that ranged from clear air to tropical storm Debby. Discovery also deployed and retrieved the Spartan 201-II satellite, which observed the Sun's corona. Lee and Meade tested a miniature backpack designed to rescue astronauts should they drift away from the space station, which would not be able to maneuver to retrieve them.

      The second International Microgravity Laboratory on Columbia (July 8-23) carried life and materials sciences experiments provided by Japan's National Space Development Agency and the German Space Agency. The crew comprised commander Robert D. Cabana, pilot James D. Halsell, Jr., mission specialists Carl E. Waltz, Leroy Chiao, Richard J. Hieb, and Donald A. Thomas, and payload specialist Chiaki Naito-Mukai. In addition to tests on the crew, the life sciences experiments included observation of the hatching of newts and the behaviour of goldfish and carp, some with their balance organs removed. Materials experiments included the cooling of samples of molten metal alloys to below freezing while they were suspended in an electromagnetic field. Such experiments are limited on Earth.

      Earth was surveyed yet again when Atlantis carried the third Atmospheric Laboratory for Applications and Space Science (ATLAS-3; November 3-14). The crew comprised commander Donald R. McMonagle, pilot Curtis L. Brown, and mission specialists Ellen E. Ochoa, Scott E. Parazynski, Joseph R. Tanner, and Jean-François Clervoy of the European Space Agency (ESA). ATLAS instruments observe the Sun as a source of energy for the Earth's atmosphere and as a light source whose changes reveal the presence of certain chemicals in the atmosphere.

      Shuttle missions planned for 1995 were scheduled to carry the Astro cluster of ultraviolet telescopes on a new survey of the universe (January), rendezvous with the Mir space station (February) and then dock with it (May), recarry the Wake Shield Facility and launch a special Space Free-Flier Unit (October), launch the seventh Tracking and Data Relay Satellite (June), carry the second U.S. Microgravity Laboratory (September), and retrieve the Free-Flier Unit (September). The first woman pilot on a shuttle mission, Eileen Collins, was to fly the Mir rendezvous. The Microgravity Laboratory mission was planned to last a record 16 days.

      NASA started plans to replace the shuttles' 1970s "green screen" electronic displays and 1960s electromechanical displays with high-resolution, full-colour liquid crystal displays (LCDs). Such "glass cockpits" would allow information to be displayed with much greater flexibility and detail. Each display would be driven by a powerful microcomputer.

      Operations continued aboard Russia's Mir space station with the launch on January 8 of Soyuz TM-18, which carried Viktor Afanesyev, Yury Usacho, and Valery Polyakov to the station. Polyakov was scheduled to stay in space for 429 days. Soyuz TM-17 departed from Mir on January 14, returning with Vasily Tisbiliyev and Aleksandr Serebrov. On July 1, Soyuz TM-19 carried two cosmonauts to Mir and returned with Afanesyev and Usacho. On October 4 Ulf Merbold, a German astronaut from the ESA who had flown twice on the U.S. space shuttle, was launched aboard Soyuz TM-20 to spend 30 days aboard Mir as part of the Euro-Mir program. A second European astronaut was to begin a 135-day stay in August 1995.

      NASA completed the redesign of its space station program by including the Russian Space Agency as a partner equal with the ESA, the Japanese National Space Agency, and the Canadian Space Agency. The new International Space Station Alpha was to be assembled during a five-year period beginning in 1997. Most of its design was based on earlier work on the Freedom program, which experienced severe cost overruns and was finally not funded further by the U.S. Congress.

      Alpha was to have a wingspan of 110 m (328 ft) across its main truss, which would support the solar power panels that were to extend 88 m (289 ft) from tip to tip. Alpha's total weight in orbit was to be 377 metric tons. Its orbit would be at an altitude of about 352 km (218 mi) at an inclination of 51.6° to the Equator.

      The program was to be developed in three phases, the first of which started with Krikalev's flight aboard the shuttle. The shuttle Atlantis was to rendezvous with Mir in May 1995 and then dock and exchange Russian and U.S. crew members in October 1995; nine more docking missions were planned. The second phase would assemble enough of Alpha during 1997-98 for a crew of three to operate aboard the station. Phase 3 would add more modules and round out the station's capabilities by June 2002.

Planetary Probes.
      During the Apollo 11 25th-anniversary year, the Moon was explored by Clementine, a modest spacecraft built by the U.S. Ballistic Missile Defense Organization (BMDO) and carrying an array of ultrasmall, lightweight sensors designed to detect and track missiles and warheads from space. Because budget and arms treaty concerns ruled out flying a special target vehicle, the BMDO decided to test Clementine around the Moon. It was launched on January 25 and, after some swing-by maneuvers, arrived in lunar orbit on February 19. Before its departure on May 1 for asteroid 1620 Geographos (later canceled by a computer failure), Clementine returned some 1.8 million images of the Earth and the Moon. It was able to map the Moon's polar regions and found a crater that is in perpetual darkness, an encouraging sign that water may be locked in the soil.

      Ulysses, the international solar polar mission, sailed beneath the Sun's south pole during the summer. Ulysses recorded solar winds blowing at 3.2 million km/h (2 million mph).

      The first two probes for NASA's new Discovery program to explore the planets with a series of low-cost missions were readied, and NASA sought proposals for at least one more. The first Discovery mission, to be launched in February 1996, was to be the Near Earth Asteroid Rendezvous (NEAR); the probe would rendezvous with asteroid 433 Eros, a 36-km (22-mi)-long block of silicate rock, in January 1999. Mars Pathfinder, to be launched in 1997, was to land a small probe on the surface of Mars and deploy a miniature rover to demonstrate technologies for a network of environmental survey probes.

      Looking farther into the future, NASA and Carnegie Mellon University, Pittsburgh, Pa., successfully tested the Dante II robot on the slope of Mt. Spurr, a semiactive volcano 128 km (79 mi) west of Anchorage, Alaska. The 771-kg (1,696-lb), 3-m (9.8-ft)-tall robot had eight legs that moved in groups of four. It carried several TV cameras to let scientists view the terrain as the robot explored the volcano from July 29 to August 5. The robot eventually stumbled and fell, and a rock climber had to attach a harness to it so that a helicopter could retrieve it. NASA officials said that they were pleased with the results, however, and would continue development of the robot.

      Investigators concluded that communication with the Mars Observer spacecraft was lost because of a slow leak that allowed fuel and oxidizer to mix and explode when the probe's thrusters were to be turned on just before it arrived at Mars in August 1993. The Magellan spacecraft ended its survey of Venus in a blaze of glory when engineers ordered the spacecraft to lower its orbit into the upper reaches of the planet's atmosphere. Magellan, orbiting Venus since 1990, was failing slowly and running out of attitude-control propellant. Its final experiment provided scientists with information about the density of the upper atmosphere.

Unmanned Satellites.
      The U.S. launched the second in a series of satellites designed to study Earth-Sun interactions such as the events that knocked out two Canadian communications satellites on January 20. NASA's Wind satellite was placed in orbit on November 1 as part of the International Solar-Terrestrial Physics (ISTP) program. The first satellite was Japan's Geotail probe, launched in July 1992 to study the tail of the Earth's geomagnetic field. The Earth is surrounded by belts of radiation trapped by its magnetic field. These, in turn, form a shield around which the solar wind must flow as it streams away from the Sun. The shield is not "bulletproof," as was demonstrated in January when a coronal mass ejection (CME) sprayed large quantities of charged particles from the Sun's corona into space. When these particles arrived at the Earth on January 20, the electronics on two Canadian communications satellites were knocked out of service for several hours; one was permanently damaged. A similar CME was spotted on April 14 by Japan's Yohkoh (Sunbeam) satellite. This allowed the Space Environment Laboratory in Boulder, Colo., to warn utility systems to take precautions against induced currents from a geomagnetic storm that formed auroras visible as far south as Boulder. Three more satellites in the ISTP program were scheduled to be launched in 1995.

      In a spectacular cosmic show observed by the Galileo spacecraft and the Hubble Space Telescope, Jupiter was pummeled by Comet Shoemaker-Levy 9 from July 16 to 22. (See Sidebar (Comet Shoemaker-Levy 9: A Spectacular Good-bye )). Also during the year astronauts learned that Earth is often pummeled by meteors. The U.S. Department of Defense revealed that several of its secret satellites had detected at least 136 meteor explosions in the Earth's upper atmosphere during 1975-92. The blasts had energies as great as the 15-kiloton atomic bomb that was dropped on Hiroshima, Japan.

      GOES-8, the first of a new generation of Geostationary Operational Environmental Satellites, was launched on April 13 and soon started work. It had been delayed for several years because of design problems. The "GOES-NEXT" series, as it was known, would provide improved weather observations. The first of a new generation of military communications satellites, Milstar 1, was launched February 7. Critics claimed that it was unnecessary because it was designed to provide secure communications in case of a nuclear war.

Launch Vehicles.
      Japan introduced its new H-2 launch vehicle, Europe prepared to launch its first Ariane 5 in late 1995, and the U.S. finally pulled its revolutionary DC-X demonstrator from the brink of cancellation. Both the H-2 and Ariane 5 were designed to give their respective nations advanced space launch capabilities, including manned flight. The first H-2, launched on February 4 from Tanegashima, Japan, carried an Orbital Reentry Experiment to test a thermal protection system planned for the unmanned H-2 Orbital Plane Experiment to be flown in 1999. The first Ariane 5 launch was set for October or November 1995.

      The DC-X, designed to demonstrate technologies for single-stage-to-orbit launches, flew missions on June 20 and June 27, thus demonstrating its capability to be reused quickly. However, the last (and fifth) flight resulted in a fire that damaged the vehicle but that also demonstrated its automated capability to abort its mission. NASA officials said that an orbital demonstrator could fly as early as 1999. NASA was also discussing a joint industry-government partnership for a similar vehicle to replace the space shuttle in the 21st century.

      The last of an old reliable line of U.S. booster rockets, Scout, was launched May 9. Its capability was largely duplicated by the Pegasus air-launched rocket family. The U.S. government also decided to allow limited use of demobilized ballistic missiles as space launchers. Meanwhile, U.S. firms worked with new Russian companies to determine whether robust Russian rocket engines could be used to upgrade U.S. launch vehicles. Israel won permission to market its Shavit space launcher, which was derived from its Jericho ballistic missile. The U.S. government was concerned that sensitive U.S. missile technology might be improperly distributed through such sales.


      This updates the articles space exploration; telescope.

▪ 1994

      The United States experienced a frustrating year in space exploration in 1993 as several key satellites were lost, space shuttle launches were delayed five times, and the space station was drastically cut in size. In addition, a daring project to pioneer a new launch vehicle was killed before its final test flight. In December, however, an ambitious mission to repair the Hubble Space Telescope was completed successfully.

Manned Flight.
      The first manned mission of 1993, by Endeavour (STS-54, January 13-19), deployed the sixth Tracking and Data Relay Satellite (TDRS 6) and a two-stage rocket to boost it to a geostationary orbit (one in which it would remain over the same place on the Earth's surface). Endeavour also carried a diffuse X-ray spectrometer to scan the heavens in search of broad, faint X-ray sources. The crew included commander John Casper, pilot Donald McMonagle, and mission specialists Gregory Harbaugh, Susan Helms, and Mario Runco, Jr. Harbaugh and Runco walked in space to test equipment for maneuvering around the space station during construction. Inside, Runco and other crew members used toys to demonstrate physics principles as they talked with students at elementary schools.

      Discovery (STS-56, April 8-17) carried the second in the Atmospheric Laboratory for Applications and Science (ATLAS-2) series to monitor yearly changes in 30 to 40 gases, including ozone, in the Earth's middle atmosphere. The crew comprised commander Kenneth Cameron, pilot Stephen Oswald, and mission specialists Kenneth Cockrell, Michael Foale, and Ellen Ochoa (the first Hispanic woman in space). The launch was postponed once, at T-11 seconds, by computer malfunction, and while in flight there was an hours-long blackout during which virtually all the data sent from the shuttle were lost. The crew deployed and retrieved (April 11-13) a Spartan satellite carrying a coronascope and an ultraviolet telescope to study the Sun's outer atmosphere.

      Columbia (STS-55, April 26-May 6) carried Spacelab D-2 after two launch delays due to equipment problems, including an on-pad shutdown after ignition. The crew included commander Steven Nagel, pilot Terence Henricks, flight engineer Charles Precourt, mission specialists Bernard Harris and Jerry Ross, and payload specialists Hans Schlegel and Ulrich Walter (both of Germany). Among the passengers were 240 tadpoles and 240 fish larvae flown to test how their bodies adjusted to weightlessness in space. Most died in orbit. The crew and scientists on the ground also operated a robot to test teleoperations techniques that could allow remote, unmanned operations. The mission was extended a day to allow additional experiments. When bad weather at the Kennedy Space Center in Florida delayed their landing another day, the crew had to fly with most of the cabin lights out in order to conserve power. This mission brought the shuttle program's cumulative flight time to one year.

      The commercially developed Spacehab module was first flown in Endeavour (STS-57, June 21-July 1), the crew of which consisted of commander Ronald J. Grabe, pilot Brian Duffy, flight engineer Nancy Sherlock, and mission specialists G. David Low, Janice Voss, and Peter ("Jeff") Wisoff. Spacehab, which added working space similar to that in the main cabin, was to fly twice a year, largely with National Aeronautics and Space Administration (NASA)-sponsored experiments. Endeavour also chased down the European Retrievable Carrier (EURECA), a satellite deployed in August 1992 by Atlantis to carry out automated materials experiments. A space walk by Low and Wisoff tested techniques for the Hubble Space Telescope repair mission and secured one of EURECA's antennae.

      Two satellites were deployed and one retrieved by Discovery (STS-51, September 12-22). Members of the crew were commander Frank Culbertson, pilot William Readdy, flight engineer Daniel W. Bursch, and mission specialists James Newman and Carl Walz. On the first day the crew deployed the Advanced Communications Technology Satellite (ACTS), which was designed to experiment with communications in the radio spectrum. The next day the crew released Germany's Orbiting Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS) to operate autonomously until September 19, when it was retrieved for return to Earth. ORFEUS also carried a movie camera to film the shuttle in space. Newman and Walz walked in space to test additional techniques and tools for the Hubble repair mission.

      Toward the end of the year, Columbia carried the second Space Life Sciences mission (STS-58; October 18-November 1). The crew consisted of commander John Blaha, pilot Rick Searfoss, mission specialists Rhea Seddon, Bill McArthur, David Wolf, and Shannon Lucid, and payload specialist Martin Fettman (the first veterinarian in space). Also aboard were 48 rats, six of which were sacrificed and dissected the day before landing to preserve subtle changes in their inner ears that might be lost after their return to Earth. Another 15 had blood and fluid samples withdrawn during the mission. Several were injected with erythropoietin to stimulate the production of red blood cells and counter the anemia that plagues long-duration space travelers. Most experiments used the crew as test subjects. In one, bungee cords pulled a subject to the deck while electrodes measured changes in the body's reaction to falling. In another, the subject placed his or her head in a rotating dome painted with dots while a TV camera recorded how the eye compensated for the perceived motion. Wolf set a record for the fastest heart rate—196 beats per minute—in space as he pedaled on a bicycle ergometer. The two-week mission was the longest flown by a shuttle.

      In a mission regarded by many as NASA's biggest challenge since the Apollo landings on the Moon, Endeavour was launched from Cape Canaveral on December 2 to repair the Hubble Space Telescope. The crew consisted of commander Richard Covey, pilot Kenneth Bowersox, and mission specialists Tom Akers, Jeffrey Hoffman, Story Musgrave, Claude Nicollier, and Kathryn Thornton. During a record five space walks, Hoffman and Musgrave alternated with Akers and Thornton to correct the Hubble's flawed vision, which had been caused by improper grinding and polishing of its primary mirror. They replaced faulty gyroscopes and worn solar panels, repaired a computer, and, most important, installed a new wide-field planetary camera and a device containing 10 tiny mirrors to compensate for the defective primary mirror. On December 10 the Hubble was released into space, and on December 13 Endeavour returned to the Kennedy Space Center. Donald K. ("Deke") Slayton, 69, died June 13 of a brain tumour. He had been grounded in 1962 by a heart arrhythmia but finally flew on the Apollo-Soyuz Test Project in 1975. (See OBITUARIES (Slayton, Donald Kent ).)

      In the face of severe budget problems, the European Space Agency (ESA) in September canceled its plans to develop a small shuttlecraft with the Russian space agency. ESA had earlier reduced its Hermes shuttlecraft program to a project to develop a suit for space walks, an Apollo-style crew capsule, and an automated transfer vehicle.

      Russia continued to operate its Mir 1 space station. On February 4 cosmonauts aboard Mir released an aluminum-coated plastic film mirror that unfurled to a diameter of 20 m (65 ft). Mirrors several kilometres wide were proposed to illuminate cities and increase daylight for crops. Cosmonauts Anatoly Solovyev and Sergey Abdeyev returned to Earth aboard Soyuz TM-15 on February 1 after having spent six months aboard Mir. Their primary duties included a number of space walks to rejuvenate the seven-year-old station and extend its life. They were replaced on January 26 by Gennady Manakov and Aleksandr Poleshchuk, launched aboard Soyuz TM-16. After placing a docking target on Mir for the U.S. shuttle Atlantis to use during a planned 1995 visit, Manakov and Poleshchuk returned to Earth on July 22.

      The plans by NASA to build a permanently manned space station were sharply curtailed yet again, and the craft even lost its Reagan-era name, Freedom, as the U.S. moved to add Russia to the international team that comprised ESA, the Canadian Space Agency, and Japan's National Space Development Agency. Soon after taking office, U.S. Pres. Bill Clinton ordered NASA to examine three redesign options intended to bring the station's price down from more than $30 billion to a range of $9 billion to $16 billion.

      NASA was unable to reduce the price as low as Clinton wanted, and on June 17 the president selected a combination of two options. The new design eliminated five of the major truss elements that served as the station's backbone and used a 14-m (47-ft)-long common module for laboratory, habitat, and docking. Russia's Soyuz TM spacecraft would be used as a lifeboat. The first element of space station Alpha was to be launched in October 1997.

      More changes were portended on September 2 when U.S. Vice Pres. Al Gore and Russian Prime Minister Viktor Chernomyrdin signed an agreement that would join the U.S. and Russian space station programs. In the first phase, NASA would start using Mir for a fee of $100 million a year. NASA would receive up to two man-years of crew time and joint development of life-support and electrical power systems and a new space suit. In the second phase, NASA's Alpha design would be adjusted to use Russia's Mir 2, now under construction, as the core module and habitat. On December 16 the U.S. and Russia formally became partners and announced that a Russian astronaut would fly in the shuttle Discovery in six weeks.

      To streamline its management, NASA canceled contracts with its four principal space station contractors and renegotiated a new single contract, which was won by Boeing Defense and Space Group. The U.S. Congress canceled NASA's project to build an advanced solid rocket motor to help the shuttle boost modules for the station.

Planetary Probes.
      What was to be the year's premier planetary event became an embarrassing spell of silence as Mars Observer failed to check in after starting preparations to orbit Mars. The spacecraft's radio was shut down on August 21 to protect it while gas lines were opened and propellant tanks pressurized for the burn to insert the craft into orbit. But hours later the spacecraft failed to reestablish contact with Earth controllers. NASA worked for five days to summon the spacecraft and then listened until August 30, hoping that it would go into a "safe mode" programmed into the probe in case contact was lost. Nothing was ever heard. Mars Observer was to have mapped the surface of Mars in great detail over a period of one Martian year.

      Frustration continued with the Galileo spacecraft as it coasted on the final leg of its journey to Jupiter. Galileo's high-gain antenna had refused to open because dry lubricant had slipped away during the long prelaunch wait. Extensive thermal and mechanical hammering by ground controllers failed to spring it loose, and so scientists started planning to retrieve pictures and data at much lower rates than expected. On August 28 Galileo took about 150 images of asteroid Ida as it zipped past at a distance of 2,400 km (1,490 mi) and a speed of 44,800 km/h (28,000 mph). Galileo was scheduled to arrive at Jupiter in 1995.

      At Venus the Magellan spacecraft neared the end of its life mainly because of reduced funding. Flight controllers commanded it to lower its orbit by a series of delicate braking passes through the upper atmosphere of Venus. This yielded data on the density of the Venusian atmosphere. In its lower orbit Magellan would allow scientists to map Venus' gravity field with greater detail.

      Possibly the first detection of interstellar dust was made by the Ulysses solar-polar spacecraft. Instruments indicated that dust struck Ulysses at 30 km per second (19 mi per second) as it arced above the solar system's equator.

      Two robot tests demonstrated remote exploration techniques. On January 1 NASA's eight-legged Dante tried to explore the inside of Mt. Erebus, Antarctica's only active volcano. The test ended after Dante moved just a few metres and broke the fibre-optic cable connecting it to a ground station that linked it to operators in Greenbelt, Md. Additional tests were planned in 1994. A Russian Marsokhod was driven a few metres across the rugged Kamchatka Peninsula in August by operators at McDonnell Douglas' Huntington Beach, Calif., facilities. They were linked by a communications satellite and used a virtual reality system to produce three-dimensional images of the terrain.

      An intriguing probe of the Moon and asteroid 1620 Geographos was to be launched in January 1994 by the Ballistic Missile Defense Organization with the use of a low-cost Clementine probe, a cheaper way of observing those bodies than launching dedicated targets for the spacecraft's miniature sensors. A Clementine 2 mission was being considered for an October 1994 launch to make up for part of the Mars Observer loss.

Launch Vehicles.
      The year's bright spot in launch vehicles was the DC-X, which demonstrated technologies for a single-stage vehicle that would be launched into orbit and then land vertically. In a series of three tests in August through October, the DC-X was launched to altitudes of a few hundred metres and successfully landed. Congress cut funding, however, before the team could attempt the most demanding final flight, which would have taken the DC-X to an altitude of 8,500 m (28,000 ft), made a U-turn, and returned to the landing site.

      Efforts to develop small launchers for small satellites picked up momentum during the year as NASA contracted the Universities Space Research Association (USRA) to launch three satellites atop older Minuteman missiles that were being retired by the military. USRA would have to hold three launches within three years to demonstrate this "quicker, cheaper" approach. Early in the year Russia demonstrated a similar approach by orbiting a satellite with an old SS-25 missile. Lockheed Missiles & Space Co. formed a partnership with Russia's Khrunichev Enterprise to market the Russian Proton rocket; other U.S. companies teamed with Russian counterparts to market rocket engines that included descendants of the Sputnik 1 launch engine. The U.S. market was opened to Russian launch vehicles under an agreement related to the space station plan.

      Efforts by India to gain cryogenic (very low-temperature) space-rocket technology from Russia were thwarted by U.S. objections that this would enhance India's nuclear weapons program. On September 20 India's first Polar Satellite Launch Vehicle failed to place a remote-sensing satellite in a high-enough orbit. Also, South Africa announced that it had discontinued its Arniston satellite launcher as part of the dismantling of its nuclear bomb program.

      During the year three major satellites were lost because of launch mishaps: a navy communications satellite on March 26 when its Atlas-Centaur second stage misfired; an ocean surveillance satellite on August 2 when a booster on its Titan IV launch vehicle burned through and destroyed the vehicle; and the Landsat 6 Earth observation satellite on October 5 shortly after it separated from its Titan II launcher (the Titan was not considered responsible for this loss). In addition, contact was lost with the National Oceanic and Atmospheric Administration's Nova 11 advanced polar-orbiting weather satellite on August 21. Plans to launch the Commercial Experiment Transporter (COMET) were postponed by cost overruns.

Unmanned Satellites.
      Scientists announced that the Cosmic Background Explorer (COBE) satellite had produced the strongest evidence yet for the big bang theory of the creation of the universe. COBE's observations in the infrared spectrum revealed that the universe had the same temperature in every wavelength it observed and in every direction.

      Germany's ROSAT, carrying an X-ray telescope, indicated that the universe might have enough "dark matter" to halt its expansion. On February 20 Japan launched the Asuka satellite carrying U.S. and Japanese instruments to examine stars in the X-ray region of the spectrum. The Hubble Space Telescope returned a number of interesting scientific finds despite its problems. These included an apparent collision between two galaxies and new estimates of the masses of Pluto and its moon, Charon.


      See also Astronomy ; Earth Sciences ; Industrial Review: Aerospace (Industrial Review ); Telecommunications (Industrial Review ); Military Affairs ; Television and Radio .

      This updates the articles space exploration; telescope.

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 the investigation, by means of manned and unmanned spacecraft, of the reaches of the universe beyond Earth's atmosphere and the use of the information so gained to increase knowledge of the cosmos and benefit humanity.

 Humans have always looked at the heavens and wondered about the nature of the objects seen in the night sky. With the development of rockets and the advances in electronics and other technologies in the 20th century, it became possible to send machines and animals and then people above Earth's atmosphere into outer space. Well before technology made these achievements possible, however, space exploration had already captured the minds of many people, not only aircraft pilots and scientists but also writers and artists. The strong hold that space travel has always had on the imagination may well explain why professional astronauts and laypeople alike consent at their great peril, in the words of Tom Wolfe (Wolfe, Tom) in the The Right Stuff (1979), to sit “on top of an enormous Roman candle, such as a Redstone, Atlas, Titan or Saturn rocket, and wait for someone to light the fuse.” It perhaps also explains why space exploration has been a common and enduring theme in literature and art. As centuries of speculative fiction in books and more recently in films make clear, “one small step for man, one giant leap for mankind” was taken by the human spirit many times and in many ways before Neil Armstrong (Armstrong, Neil) stamped humankind's first footprint on the Moon.

      Achieving spaceflight enabled humans to begin to explore the solar system and the rest of the universe, to understand the many objects and phenomena that are better observed from a space perspective, and to use for human benefit the resources and attributes of the space environment. All of these activities—discovery, scientific understanding, and the application of that understanding to serve human purposes—are elements of space exploration. (For a general discussion of spacecraft, launch considerations, flight trajectories, and navigation, docking, and recovery procedures, see spaceflight.)

Overview of recent space achievements

Motivations for space activity
      Although the possibility of exploring space has long excited people in many walks of life, for most of the latter 20th century only national governments could afford the very high costs of launching people and machines into space. This reality meant that space exploration had to serve very broad interests, and it indeed has done so in a variety of ways. Government space programs have increased knowledge, served as indicators of national prestige and power, enhanced national security and military strength, and provided significant benefits to the general public. In areas where the private sector could profit from activities in space, most notably the use of satellites as telecommunication relays, commercial space activity has flourished without government funding.

      In the years after World War II, governments assumed a leading role in the support of research that increased fundamental knowledge about nature, a role that earlier had been played by universities, private foundations, and other nongovernmental supporters. This change came for two reasons. First, the need for complex equipment to carry out many scientific experiments and for the large teams of researchers to use that equipment led to costs that only governments could afford. Second, governments were willing to take on this responsibility because of the belief that fundamental research would produce new knowledge essential to the health, security, and quality of life of their citizens. Thus, when scientists sought government support for early space experiments, it was forthcoming. Since the start of space efforts in the United States, the Soviet Union, and Europe, national governments have given high priority to the support of science done in and from space. From modest beginnings, space exploration has expanded under government support to include multibillion-dollar exploratory missions in the solar system and to major space-based astronomical observatories such as the Hubble Space Telescope.

      Soviet leader Nikita Khrushchev (Khrushchev, Nikita Sergeyevich) in 1957 used the fact that his country had been first to launch a satellite as evidence of the technological power of the Soviet Union and of the superiority of communism. He repeated these claims after Yury Gagarin (Gagarin, Yury Alekseyevich)'s orbital flight in 1961. Although U.S. President Dwight D. Eisenhower (Eisenhower, Dwight D.) had decided not to compete for prestige with the Soviet Union in a space race, his successor, John F. Kennedy (Kennedy, John F.), had a different view. On April 20, 1961, in the aftermath of the Gagarin flight, he asked his advisers to identify a “space program which promises dramatic results in which we could win.” The response came in a May 8, 1961, memorandum recommending that the United States commit to sending people to the Moon, because “dramatic achievements in space…symbolize the technological power and organizing capacity of a nation” and because the ensuing prestige would be “part of the battle along the fluid front of the cold war.” From 1961 until the collapse of the Soviet Union in 1991, competition between the United States and the Soviet Union was a major influence on the pace and content of their space programs. Other countries also viewed having a successful space program as an important indicator of national strength.

 Even before the first satellite was launched, U.S. leaders recognized that the ability to observe military activities around the world from space would be an asset to national security. Following on the success of its photoreconnaissance satellites, which began operation in 1960, the United States built increasingly complex observation and electronic-intercept intelligence satellites. The Soviet Union also quickly developed an array of intelligence satellites, and later a few other countries instituted their own satellite observation programs. Intelligence-gathering satellites have been used to verify arms-control agreements, provide warnings of military threats, and identify targets during military operations, among other uses.

      In addition to providing security benefits, satellites offered military forces the potential for improved communications, weather observation, navigation, and position location. This led to significant government funding for military space programs in the United States and the Soviet Union. Although the advantages and disadvantages of stationing force-delivery weapons in space have been debated, as of the early 21st century, such weapons have not been deployed, nor have space-based antisatellite systems—that is, systems that can attack or interfere with orbiting satellites. The stationing of weapons of mass destruction in orbit or on celestial bodies is prohibited by international law.

 Governments realized early on that the ability to observe Earth from space could provide significant benefits to the general public apart from security and military uses. The first application to be pursued was the development of satellites for assisting in weather forecasting. A second application involved remote observation of land and sea surfaces to gather imagery and other data of value in crop forecasting, resource management, environmental monitoring, and other applications. The U.S. and Soviet governments also developed their own satellite-based global positioning system (GPS)s, originally for military purposes, that could pinpoint a user's exact location, help in navigating from one point to another, and provide very precise time signals. These satellites quickly found numerous civilian uses in such areas as personal navigation, surveying and cartography, geology, air traffic control, and the operation of information-transfer networks. They illustrate a reality that has remained constant for the past half century—as space capabilities are developed, they often can be used for both military and civilian purposes.

      Another space application that began under government sponsorship but quickly moved into the private sector is the relay of voice, video, and data via orbiting satellites. Satellite telecommunications has developed into a multibillion-dollar business and is the one clearly successful area of commercial space activity. A related commercial space business is the provision of launches for private and government satellites. In 2004 a privately financed venture sent a piloted spacecraft to the lower edge of space for brief suborbital flights. Although technically a much less challenging achievement than carrying humans into orbit, its success was seen as an important step toward opening up space to commercial travel and eventually to tourism. Suggestions have been made that in the future other areas of space activity, including remote sensing and the capture of solar energy to provide electric power on Earth, could become successful businesses.

      Most space activities have been pursued because they serve some utilitarian purpose, whether increasing knowledge or making a profit. Nevertheless, there remains a powerful underlying sense that it is important for humans to explore space for its own sake, “to see what is there.” Although the only voyages that humans have made away from the near vicinity of Earth—the Apollo flights to the Moon—were motivated by Cold War competition, there have been recurrent calls for humans to return to the Moon, travel to Mars, and visit other locations in the solar system and beyond. Until humans resume such journeys of exploration, robotic spacecraft will continue to serve in their stead to explore the solar system and probe the mysteries of the universe.

Major milestones
 The first artificial Earth satellite, Sputnik 1, was launched by the Soviet Union on October 4, 1957. The first human to go into space, Yury Gagarin (Gagarin, Yury Alekseyevich), was launched, again by the Soviet Union, for a one-orbit journey around Earth on April 12, 1961. Within 10 years of that first human flight, American astronauts walked on the surface of the Moon. Apollo (Apollo program) 11 crew members Neil Armstrong (Armstrong, Neil) and Edwin (“Buzz”) Aldrin (Aldrin, Edwin Eugene, Jr.) made the first lunar landing on July 20, 1969. A total of 12 Americans on six separate Apollo missions set foot on the Moon between July 1969 and December 1972. Since then, no humans have left Earth orbit, but several hundred men and women have spent as many as 438 consecutive days in space. Starting in the early 1970s, a series of Soviet (Russian from December 1991) space stations (space station), the U.S. Skylab station, and numerous space shuttle flights have provided Earth-orbiting bases for varying periods of human occupancy and activity. With the development of the International Space Station (ISS) in the 1990s, the intent has been to have humans living and working in space on a permanent basis. The first ISS crew took up long-duration residence on November 2, 2000.

 Since 1957 Earth-orbiting satellites and robotic spacecraft journeying away from Earth have gathered valuable data about the Sun, Earth, other bodies in the solar system, and the universe beyond. Robotic spacecraft have landed on the Moon, Venus, Mars, and the asteroid Eros, have visited the vicinity of all the major planets, and have flown by the nuclei of comets, including Halley's Comet, traveling in the inner solar system. Scientists have used space-derived data to deepen human understanding of the origin and evolution of galaxies, stars, planets, and other cosmological phenomena.

       Significant milestones in space explorationOrbiting satellites also have provided, and continue to provide, important services to the everyday life of many people on Earth. Meteorologic satellites deliver information on short- and long-term weather patterns and their underlying causes. Other Earth-observation satellites remotely sense land and ocean areas, gathering data that improve management of Earth's resources. Telecommunications satellites allow essentially instantaneous transfer of voice, images, and data on a global basis. Satellites operated by the United States and Russia give precision navigation, positioning, and timing information that has become essential to many terrestrial users. Earth-observation satellites have also become extremely useful to the military authorities of several countries as complements to their land, sea, and air forces. (For a list of significant milestones in space exploration, see the table (Significant milestones in space exploration).)

      As the many benefits of space activity have become evident, other countries have joined the Soviet Union and the United States in developing their own space programs. They include a number of western European countries operating both individually and, after 1975, cooperatively through the European Space Agency, as well as China, Japan, Canada, India, Israel, and Brazil. By the start of the 21st century, more than 30 countries had space agencies or other government bodies with substantial space activities.

History of space exploration

Prelude to spaceflight
Precursors in fiction and fact
      Since ancient times, people around the world have studied the heavens and used their observations and explanations of astronomical phenomena for both religious and practical purposes. Some dreamed of leaving Earth to explore other worlds. For example, the French satirist Cyrano de Bergerac (Cyrano de Bergerac, Savinien) in the 17th century wrote Histoire comique des états et empires de la lune (1656) and Histoire comique des états et empires du soleil (1662; together in English as A Voyage to the Moon: With Some Account of the Solar World, 1754), describing fictional journeys to the Moon and the Sun. Two centuries later, the French author Jules Verne (Verne, Jules) and the English novelist and historian H.G. Wells (Wells, H.G.) infused their stories with descriptions of outer space and of spaceflight that were consistent with the best understanding of the time. Verne's De la Terre à la Lune (1865; From the Earth to the Moon) and Wells's The War of the Worlds (1898) and The First Men in the Moon (1901) used sound scientific principles to describe space travel and encounters with alien beings.

      In order to translate these fictional images of space travel into reality, it was necessary to devise some practical means of countering the influence of Earth's gravity. By the beginning of the 20th century, the centuries-old technology of rockets had advanced to the point at which it was reasonable to consider their use to accelerate objects to a velocity sufficient to enter orbit around Earth or even to escape Earth's gravity and travel away from the planet.

      The first person to study in detail the use of rockets (rocket) for spaceflight was the Russian schoolteacher and mathematician Konstantin Tsiolkovsky (Tsiolkovsky, Konstantin Eduardovich). In 1903 his article “Exploration of Cosmic Space by Means of Reaction Devices” laid out many of the principles of spaceflight. Up to his death in 1935, Tsiolkovsky continued to publish sophisticated studies on the theoretical aspects of spaceflight. He never complemented his writings with practical experiments in rocketry, but his work greatly influenced later space and rocket research in the Soviet Union and Europe.

      In the United States, Robert Hutchings Goddard (Goddard, Robert Hutchings) became interested in space exploration after reading works such as The War of the Worlds. Even as a young man, he dedicated himself to working on spaceflight. In his 1904 high-school graduation speech, he stated that “it is difficult to say what is impossible, for the dream of yesterday is the hope of today and the reality of tomorrow.” Goddard received his first two patents for rocket technology in 1914, and, with funding from the Smithsonian Institution, he published a theoretical treatise, A Method of Reaching Extreme Altitudes, in 1919. Goddard's claim that rockets could be used to send objects as far as the Moon was widely ridiculed in the public press, including The New York Times (which published a retraction on July 17, 1969, the day after the launch of the first manned mission to the Moon). Thereafter, the already shy Goddard conducted much of his work in secret, preferring to patent rather than publish his results. This approach limited his influence on the development of American rocketry, although early rocket developers in Germany took notice of his work.

      In the 1920s, as a professor of physics at Clark University in Worcester, Massachusetts, Goddard began to experiment with liquid-fueled rockets. His first rocket, launched in Auburn, Massachusetts, on March 16, 1926, rose 12.5 metres (41 feet) and traveled 56 metres (184 feet) from its launching place. The noisy character of his experiments made it difficult for Goddard to continue work in Massachusetts. With support from aviator Charles A. Lindbergh (Lindbergh, Charles A.) and financial assistance from the philanthropic Daniel Guggenheim (Guggenheim, Meyer) Fund for the Promotion of Aeronautics, he moved to Roswell, New Mexico, where from 1930 to 1941 he built engines and launched rockets of increasing complexity.

      The third widely recognized pioneer of rocketry (rocket), Hermann Oberth (Oberth, Hermann), was by birth a Romanian but by nationality a German. Reading Verne's From the Earth to the Moon as a youth inspired him to study the requirements for interplanetary travel. Oberth's 1922 doctoral dissertation on rocket-powered flight was rejected by the University of Heidelberg for being too speculative, but it became the basis for his classic 1923 book Die Rakete zu den Planetenräumen (“The Rocket into Interplanetary Space”). The work explained the mathematical theory of rocketry, applied the theory to rocket design, and discussed the possibility of constructing space stations and of traveling to other planets.

      In 1929 Oberth published a second influential book, Wege Zur Raumschiffahrt (Ways to Spaceflight). His works led to the creation of a number of rocket clubs in Germany as enthusiasts tried to turn Oberth's ideas into practical devices. The most important of these groups historically was the Verein für Raumschiffahrt (VfR; “Society for Spaceship Travel”), which had as a member the young Wernher von Braun (Braun, Wernher von). Although his work was crucial in stimulating the development of rocketry in Germany, Oberth himself had only a limited role in that development. Alone among the rocket pioneers, Oberth lived to see his ideas become reality: he was Braun's guest at the July 16, 1969, launch of Apollo 11.

Other space pioneers
      Although Tsiolkovsky, Goddard, and Oberth are recognized as the most influential of the first-generation space pioneers, others made contributions in the early decades of the 20th century. For example, the Frenchman Robert Esnault-Pelterie (Esnault-Pelterie, Robert) began work on the theoretical aspects of spaceflight as early as 1907 and subsequently published several major books on the topic. He, like Tsiolkovsky in the Soviet Union and Oberth in Germany, was an effective publicist regarding the potential of space exploration. In Austria, Eugen Sänger (Sänger, Eugen) worked on rocket engines and in the late 1920s proposed developing a “rocket plane” that could reach a speed exceeding 10,000 km (more than 6,000 miles) per hour and an altitude of more than 65 km (40 miles). Interested in Sänger's work, Nazi Germany in 1936 invited him to continue his investigations in that country.

Early rocket development

 It was space exploration that motivated the members of the German VfR to build their rockets, but in the early 1930s their work came to the attention of the German military. In 1932 Wernher von Braun (Braun, Wernher von), at age 20, became chief engineer of a rocket-development team for the German army. After Adolf Hitler came to power in 1933, Braun was named the civilian head of that team, under the military command of Walter Robert Dornberger (Dornberger, Walter Robert). To give Braun's engineers the needed space and secrecy for their work, the German government erected a development and test centre at Peenemünde on the coast of the Baltic Sea. There they developed, among other devices, the V-2 (V-2 missile) (originally designated the A-4) ballistic missile. First launched successfully in 1942, the V-2 was used on targets in Europe beginning in September 1944. Although built as a weapon of war, the V-2 later served as the predecessor of many of the rockets used in the early space programs of the United States and the Soviet Union. As World War II neared its end in early 1945, Braun and many of his associates chose to surrender to the United States, where they believed they would likely receive support for their rocket research and space exploration plans. Later in the year, they were taken to the United States, as were their engineering plans and the parts needed to construct a number of V-2s. The German rocket team played a central role in the early development of space launchers for the United States.

      In 1936, as Braun was developing rockets for the German military, several young American engineers led by graduate student Frank Malina began working on rocketry at the Guggenheim Aeronautical Laboratory of the California Institute of Technology (GALCIT). Malina's group was supported by the eminent aerodynamicist Theodore von Kármán (Kármán, Theodore von), GALCIT's director, and it included Chinese engineer Qian Xuesen (Ch'ien Hsüeh-sen), who in the 1950s returned home to become one of the pioneers of rocketry in China. In 1943 Malina and his associates began calling their group the Jet Propulsion Laboratory (JPL), a name that was formally adopted the following year. JPL soon became a centre for missile research and development for the U.S. Army. Following World War II, those weapons were adapted for use in early U.S. space experiments. After 1958, when it became part of the newly established National Aeronautics and Space Administration (NASA), JPL adapted itself to being the leading U.S. centre for solar system exploration.

      In the U.S.S.R., the government took an interest in rockets as early as 1921 with the founding of a military facility devoted to rocket research. Over the next decade, that centre was expanded and renamed the Gas Dynamics Laboratory. There in the early 1930s, Valentin Glushko (Glushko, Valentin Petrovich) carried out pioneering work on rocket engines. Meanwhile, other rocket enthusiasts in the Soviet Union organized into societies that by 1931 had consolidated into an organization known as GIRD (the abbreviation in Russian for “Group for the Study of Reactive Motion”), with branches in Moscow and Leningrad. Emerging as leaders of the Moscow branch were the aeronautical engineer Sergey Korolyov (Korolyov, Sergey Pavlovich), who had become interested in spaceflight at a young age, and the early space visionary Fridrikh Tsander. Korolyov and a colleague, Mikhail Tikhonravov, on August 17, 1933, launched the first Soviet liquid-fueled rocket. Later that year, the Moscow and Leningrad branches of GIRD were combined with the Gas Dynamics Laboratory to form the military-controlled Rocket Propulsion Research Institute (RNII), which five years later became Scientific-Research Institute 3 (NII-3). In its early years, the organization did not work directly on space technology, but ultimately it played a central role in Soviet rocket development.

      Korolyov was arrested in 1937 as part of the Soviet leader Joseph Stalin's great purges (Stalin, Joseph) of intellectuals and was sent to a Siberian prison. After Stalin recognized the imprudence of removing the best technical people from the Soviet war effort, Korolyov was transferred to a prison-based design bureau, where he spent most of World War II working on weapons, although not on large rockets. By the end of the war, Stalin had become interested in ballistic missiles, and he sent a team, which included Korolyov, on visits to Germany to investigate the V-2 program. A number of German engineers were relocated to the Soviet Union in the aftermath of the war, but they did not play a central role in postwar Soviet rocket development; most returned to Germany in the early 1950s.

Preparing for spaceflight
 Between 1946 and 1951, the U.S. Army (United States Army, The) conducted test firings of captured German V-2 rockets at White Sands, New Mexico. These sounding-rocket flights reached high altitudes (120–200 km [75–125 miles]) before falling back to Earth. Although the primary purpose of the tests was to advance rocket technology, the army invited American scientists interested in high-altitude research to put experiments aboard the V-2s. An Upper Atmosphere Research Panel, chaired by the physicist James Van Allen (Van Allen, James A.), was formed to coordinate the scientific use of these rocket launchings. The panel had a central role in the early years of American space science, which focused on experiments on solar and stellar ultraviolet radiation, the aurora, and the nature of the upper atmosphere. As the supply of V-2s dwindled, other U.S.-built sounding rockets such as the WAC Corporal, Aerobee, and Viking were put into use. In other countries, particularly the Soviet Union, rocket-based upper-atmosphere research also took place after World War II.

      In the early 1950s, scientists began planning a coordinated international investigation of Earth, to be called the International Geophysical Year (IGY), that would be held in 1957–58 under the auspices of the International Council of Scientific Unions. By this time, progress in rocket development had advanced such that orbiting of an artificial satellite around Earth by 1957 seemed feasible. At the urging of American scientists, IGY planners in 1954 called for scientifically instrumented satellites to be launched as part of IGY activities. Soon thereafter, the governments of the Soviet Union and the United States each announced plans to do so.

      In the years following World War II, the United States and the U.S.S.R. became political and military competitors in what soon was being called the Cold War. Because the Soviet Union was a closed society, U.S. leaders gave high priority to developing technology that could help gather intelligence on military preparations within the Soviet borders. As orbiting satellites neared realization, the idea of equipping such satellites with cameras and flying them over Soviet territory became more attractive to U.S. planners, and the U.S. Air Force began work on a reconnaissance satellite project. Still unresolved, however, was the question of whether it would violate national sovereignty to fly over a country's territory in orbit, above most of the atmosphere. One reason the U.S. government had committed itself to the IGY satellite program was that it wanted to establish the principle that outer space was not subject to claims of territorial sovereignty and thus that an orbiting satellite could pass freely over any point on Earth. Such overflights were essential if reconnaissance satellites were to have intelligence value.

 As scientific and military planners contemplated initial space projects and engineers worked on developing the needed launch vehicles, the idea that humans would soon begin the exploration of space entered popular imagination. In Europe, since the 1930s, the British Interplanetary Society had been actively promoting the idea that human space travel was soon to happen. American movies such as The Day the Earth Stood Still (1950), Destination Moon (1950), and When Worlds Collide (1951) contained vivid images of such journeys. Reports were widespread of sightings of unidentified flying objects (unidentified flying object) (UFOs), which were thought by some to be spacecraft from alien worlds.

      Authors such as Isaac Asimov (Asimov, Isaac), Robert A. Heinlein (Heinlein, Robert A), and Arthur C. Clarke (Clarke, Sir Arthur C.) both discussed the reality of space technology in popular writings and constructed believable science-fiction stories based on its use. A central figure in popularization efforts within the United States was Braun (Braun, Wernher von). A charismatic spokesman for the idea of space travel, Braun, in a series of talks, books, magazine articles, and television appearances during the 1950s, reached millions of people with his ideas for establishing orbiting space stations and human travel to the Moon and Mars. The efforts of Braun and other popularizers helped create a receptive climate for initial government proposals to undertake space activities and, particularly, to put humans in space.

From Sputnik to Apollo
The first satellites
  Although Soviet plans to orbit a satellite during the IGY had been discussed extensively in technical circles, the October 4, 1957, launch of Sputnik 1 came as a surprise, and even a shock, to most people. Prior to the launch, skepticism had been widespread about the U.S.S.R.'s technical capabilities to develop both a sophisticated scientific satellite and a rocket powerful enough to put it into orbit. Under Korolyov's direction, however, the Soviet Union had been building an intercontinental ballistic missile (ICBM), with engines designed by Glushko, that was capable of delivering a heavy nuclear warhead to American targets. That ICBM, called the R-7 or Semyorka (“Number 7”), was first successfully tested on August 21, 1957, which cleared the way for its use to launch a satellite. Fearing that development of the elaborate scientific satellite intended as the Soviet IGY contribution would keep the U.S.S.R. from being the first into space, Korolyov and his associates, particularly Tikhonravov, designed a much simpler 83.6-kg (184-pound) sphere carrying only two radio transmitters and four antennas. After the success of the R-7 in August, that satellite was rushed into production and became Sputnik 1. A second, larger satellite carrying scientific instruments and the dog Laika, the first living creature in orbit, was launched November 3. The even larger, instrumented spacecraft originally intended to be the first Soviet satellite went into orbit in May 1958 as Sputnik 3. (For additional information on Korolyov's contribution to the Soviet space program, see Energia.)

      After President Eisenhower (Eisenhower, Dwight D.), in May 1955, had committed the United States to an IGY satellite, the army, navy, and air force competed for the assignment. (No civilian organization existed that was capable of developing the launch vehicle needed.) The mission was assigned to the Naval Research Laboratory, rather than to the army's Redstone Arsenal, where Braun worked, so that the work would not interfere with Redstone's higher-priority development of ballistic missiles. The navy project, called Vanguard, would use a new launch vehicle based on modified Viking and Aerobee sounding rockets to orbit a small scientific satellite. Vanguard made slow progress over the subsequent two years, but, after Sputnik's success, the White House pressed to have the satellite launched as quickly as possible. On December 6, 1957, the Vanguard rocket rose only slightly off its launch pad before exploding and sending the satellite not into orbit but onto a Florida beach.

 Braun and his army superiors had not agreed with the decision to assign the satellite mission to the navy. After the launches of the first two Sputniks, they secured permission to attempt their own satellite launch. In anticipation of such a situation, they had kept in touch with JPL and Van Allen and so were able to prepare a satellite quickly. On January 31, 1958, Braun's Jupiter-C launch vehicle, a modified Redstone ballistic missile, carried into orbit Explorer 1, the first U.S. satellite. Designed at JPL, Explorer 1 carried Van Allen's experiment to measure cosmic rays (cosmic ray). The results from this experiment and similar ones aboard other U.S. and Soviet satellites launched that same year revealed that Earth was surrounded by two zones of radiation, now known as the Van Allen radiation belts (Van Allen radiation belt), comprising energetic particles trapped by Earth's magnetic field.

      Initial satellite launches were scientific in character, but U.S. government interest in reconnaissance satellites persisted. In February 1958, President Eisenhower authorized the development, under conditions of great secrecy, of such a spacecraft. The project, which came to be called Corona, would take pictures over the Soviet Union and return them to Earth by dropping the exposed film in a capsule that would be snatched out of the air as it parachuted back from space. After 12 failures, the first successful Corona mission took place on August 18, 1960; the returned film contained images of many previously unknown Soviet airfields and missile sites.

Development of space organizations

      As part of its response to the first Sputnik launches, the United States government debated how best to organize itself for its space activities. At the time, the military services, particularly the air force and the army, hoped that they would have a leading role in space. As an alternative to this rivalry between the services, President Eisenhower in February 1958 created the Defense Advanced Research Projects Agency (DARPA) and assigned it responsibility for all U.S. space projects. Soon afterward, he decided to separate civilian from military space efforts and proposed the creation of a National Aeronautics and Space Administration to manage the civilian segment. After approval by Congress, NASA began operation on October 1, 1958. DARPA was not successful in establishing itself as a military space agency. By 1960, after the army had been obliged to relinquish control of JPL and Braun's rocket team to NASA management, the air force had emerged as the leading military service for space.

      Eisenhower also decided to create a separate organization to manage the secret reconnaissance satellite program. This effort resulted in the National Reconnaissance Office (NRO), jointly directed by the Department of Defense and the Central Intelligence Agency. The very existence of this organization was kept secret until 1992. The NRO operated the initial Corona program until 1972. It continued to manage the development of successor photointelligence satellite systems of increasing technological sophistication and also developed radar-surveillance and electronic-signals-collection satellites. All were operated under conditions of the highest secrecy.

      After it received its mandate to send Americans to the Moon, NASA grew into a large organization. From its headquarters in Washington, D.C., it operated 10 field centres established throughout the United States to carry out research and technology development and to manage the various universities and industrial contractors involved in the U.S. civilian space program. At the peak of the Apollo program, NASA had 34,000 employees; by the end of the 20th century, this labour force had shrunk to 19,000, but NASA remained by far the largest space agency in the world.

      The air force had no separate organization for space until 1982, when the U.S. Air Force (United States Air Force, The) Space Command was created to manage its military space operations, which involved the use of satellites for meteorology, communication, navigation, and early warning of missile attack. The other U.S. military services soon created similar organizations to administer their smaller space activities. In 1985 these organizations were brought under a unified U.S. Space Command, dominated by the air force, which was responsible for 85 percent of military space activities. Research and development efforts related to military space programs were managed by various government laboratories and carried out primarily by American industry.

      In contrast to the United States, the Soviet Union had no separate, publicly acknowledged space agency. For 35 years after Sputnik, various design bureaus—state-controlled organizations that actually conceived and developed aircraft and space systems—had great influence within the Soviet system. (For information on the history of specific Soviet aerospace design bureaus, see Energia, MiG, Sukhoy, and Tupolev.) Rivalry between those bureaus and their heads, who were known as chief designers, was a constant reality and posed an obstacle to a coherent Soviet space program. Space policy decisions were made by the Politburo of the Central Committee of the Communist Party as well as the Soviet government's Council of Ministers. After 1965 the government's Ministry of General Machine Building was assigned responsibility for managing all Soviet space and missile programs; the Ministry of Defense was also quite influential in shaping space efforts. A separate military branch, the Strategic Missile Forces, was in charge of space launchers and strategic missiles. Various institutes of the Soviet Academy of Sciences, particularly the Institute for Space Research (IKI), proposed and managed scientific missions.

      Only after the dissolution of the U.S.S.R did Russia create a civilian organization for space activities. Formed in February 1992, the Russian Space Agency acted as a central focus for the country's space policy and programs. Although it began as a small organization that dealt with international contacts and the setting of space policies, it quickly took on increasing responsibility for the management of nonmilitary space activities and, as an added charge, aviation efforts. It later was renamed the Russian Aviation and Space Agency.

      In 1961, within four years of the launch of the first U.S. and Soviet satellites, the government of France created the French Space Agency (CNES), which grew to become the largest national organization of its kind in Europe. Gradually other European countries formed government or government-sponsored organizations for space, among them the German Aerospace Center (DLR), the British National Space Centre (BNSC), and the Italian Space Agency (ASI). Still others included space as part of their science or technology ministries.

      In 1964 a European Space Research Organisation (ESRO), created at the initiative of European scientists to pool government resources in support of space science, began operations. Ten western European countries and Australia joined the organization. In the same year, a parallel European Launcher Development Organisation (ELDO), which had seven European member states, was established to develop a space launch vehicle for Europe. Whereas ESRO was successful in mounting a series of science missions, many in collaboration with NASA, ELDO failed in attempts to design and launch a European rocket. In 1975 a new European Space Agency (ESA) was formed from ESRO and ELDO to carry out both of their tasks. At the beginning of the 21st century, ESA had 15 member states—Austria, Belgium, Denmark, Finland, France, Germany, Ireland, Italy, Norway, The Netherlands, Portugal, Spain, Sweden, Switzerland, and the United Kingdom. Canada also participated in some ESA projects. With a budget that made it the world's second largest civilian space agency, ESA carried out a comprehensive program in space science, applications, and infrastructure development. In particular, the Ariane series of expendable launch vehicles was developed under ESA auspices, with France taking the leading role. These launchers proved to be extremely reliable, and they gave Europe independent access to space and a leading position in the commercial space launch industry.

      In Japan the University of Tokyo created an Institute of Space and Astronautical Science (ISAS) in 1964. This small group undertook the development of scientific spacecraft and the vehicles needed to launch them, and it launched Japan's first satellite, Ōsumi, in 1970. In 1981 oversight of ISAS was transferred to the Japanese Ministry of Education. In 1969 the Japanese government founded a National Space Development Agency (NASDA), which subsequently undertook a comprehensive program of space technology and satellite development and built a large launch vehicle, called the H-II, for those satellites. In 2001 both ISAS and NASDA came under the control of the Japanese Ministry of Education, Culture, Sports, Science and Technology.

      China's space program evolved largely in secret, under the joint control of the Chinese military and the Commission on Science, Technology, and Industry for the National Defense. After the communist takeover of 1949, Qian Xuesen, who had worked at GALCIT in the 1940s and helped found JPL, returned to China, where he became the guiding figure in the development of Chinese missiles and launch vehicles, both originally derived from a Soviet ICBM. China developed a family of Long March boosters, which are used domestically and serve as competitors in the international commercial space launch market. Its space development has concentrated on applications such as communications satellites and Earth-observation satellites for civilian and military use. In 1998 an independent Chinese Aerospace Corporation was established to oversee most Chinese space-equipment manufacturers.

      China initiated its own human spaceflight program in 1992. The spacecraft, called Shenzhou, that it developed for the effort was modeled on Russia's time-tested Soyuz design (see the section Soyuz (space exploration) below), but it relied heavily on Chinese-developed technologies and manufacturing. Following four years of unmanned spacecraft tests, China launched its first indigenous astronaut, an air force pilot, into orbit on October 15, 2003. In so doing, it became the third country—after the former Soviet Union and the United States—to achieve human spaceflight.

International participation
      A number of international organizations are involved in space activities. The United Nations General Assembly established a Committee on the Peaceful Uses of Outer Space in 1959 to discuss scientific, technical, and legal issues related to international space activities; 61 countries were members of the committee in 2001. The committee has provided the forum for the development of five treaties and a number of declarations of principles related to space activities. The most important of them is the 1967 Outer Space Treaty, which sets forth the general legal principles governing the uses of space. Other parts of the UN system, most notably the International Telecommunications Union (International Telecommunication Union) (ITU), are engaged in space-related concerns. The ITU is responsible for allocation of radio frequencies and orbital locations for various satellites providing public and commercial services.

      At the initiative of the United States, an International Telecommunications Satellite Consortium ( Intelsat) was founded in 1964 to develop and operate a global system of communications satellites. By 1969 the organization had established a system of satellites with global coverage; in the late 1980s it provided services to more than 200 countries and territories. Intelsat membership grew to 144 countries before a decision was made in 1999 to change the ownership of the organization from national governments to the private sector. A similar consortium, the International Maritime Satellite Organization (Inmarsat), was established as an intergovernmental organization in 1979 to supply maritime and other mobile communications services via satellite; it also was later transformed into a privately owned entity. In addition, a number of regional organizations have been created to operate communication and meteorologic satellites.

Involvement of industry
      Building the systems and components needed to carry out both government and commercial space programs has required the participation of private industry, and a number of firms have grown to have substantial space involvement. Often these firms have also been major suppliers of aviation and defense products, a reflection of the common technological foundation for what has become known as the aerospace industry. Other private firms have specialized in providing space-based services in such areas as telecommunications and Earth observation.

The first human spaceflights
      During the 1950s, space planners in both the Soviet Union and the United States anticipated the launching of a human being into orbit as soon as the required launch vehicle and spacecraft could be developed and tested. Much of the initial thinking focused on some form of piloted space plane, which, after being launched atop a rocket, could maneuver in orbit and then return to Earth, gliding to a horizontal landing.

  In the United States, the air force developed a rocket-powered experimental aircraft, the X-15, which, after being dropped from an in-flight B-52 bomber, could reach altitudes as high as 108 km (67 miles), the edge of outer space. Nevertheless, the X-15 could not achieve the velocity and altitude needed for orbital flight. That was the mission of Dyna-Soar, another air force project. Dyna-Soar was to be a piloted reusable delta-winged vehicle that would be launched into orbit by a modified Titan (Titan rocket) ICBM and could carry out either bombing or reconnaissance missions over the Soviet Union or intercept a Soviet satellite in orbit. Although a full-scale vehicle was built and six people were chosen to train as Dyna-Soar crew, the project was canceled in 1963.

      Rather than base their human spaceflight programs on space planes, the Soviet Union and the United States, in their desire to put people into space as quickly as possible, opted for a less technically demanding ballistic approach. A person would ride in a capsulelike spacecraft atop a rocket to achieve orbit. At the end of the flight, another rocket (called a retro-rocket) would slow down the spacecraft enough for it to fall back to Earth on a ballistic trajectory. To accomplish this feat, the spacecraft would have to survive the intense heat caused by reentering the atmosphere at a high speed and then carry its passenger safely back to Earth's surface.

 Soon after the success of the first Sputniks, Korolyov (Korolyov, Sergey Pavlovich) and his associate Tikhonravov began work on the design of an orbital spacecraft that could be used for two purposes. One was to conduct photoreconnaissance missions and then return the exposed film to Earth. The other was to serve as a vehicle for the first human spaceflight missions, in which a human being would replace the reconnaissance camera. The spacecraft was known as Object K, but it was called Vostok when it was used to carry a human into space. Vostok had two sections—a spherical capsule in which the person would ride and a conical module that contained the instruments needed for its flight. The spacecraft was large for the time, weighing 4.73 metric tons. The crew capsule was completely covered by a thermal coating to protect it during reentry. Vostok was designed so that the human aboard need not touch any control from launch to touchdown; he would be essentially just a passenger. Nor would he land with the spacecraft. Rather, he would be ejected from it at an altitude of 7 km (4.3 miles) and parachute to dry land, while the spacecraft landed nearby with its own parachutes.

      After a series of five test flights carrying dogs and human dummies, the first person lifted into space in Vostok 1 atop a modified R-7 rocket on April 12, 1961, from the Soviet launch site at the Baikonur Cosmodrome in Kazakhstan. The passenger, Yury Gagarin (Gagarin, Yury Alekseyevich), who was by that time being called a cosmonaut, was a 27-year-old Russian test pilot. After firing of the retro-rocket 78 minutes into the mission, the crew capsule separated from the instrument module—although not without problems—and Gagarin parachuted to a soft landing 108 minutes after his launch. He had reported during the mission “I feel fine” and showed no ill effects from his one-orbit trip around the globe.

      There were five additional one-person Vostok missions. In August 1961, Gherman Titov (Titov, Gherman Stepanovich) at age 25 (still the youngest person ever to fly in space) completed 17 orbits of Earth in Vostok 2. He became ill with space sickness (the equivalent of motion sickness on Earth) during the flight, an incident that caused a one-year delay in Vostok flights while Soviet physicians investigated the possibility that humans could not survive for extended times in the space environment. In August 1962, two Vostoks, 3 and 4, were orbited at the same time and came within 6.5 km (4 miles) of one another. This dual mission was repeated in June 1963; aboard the Vostok 6 spacecraft was Valentina Tereshkova (Tereshkova, Valentina), the first woman to fly in space.

      The initial U.S. effort to launch a human into space was known as Project Mercury (Mercury). It was carried out by NASA, which had been given that responsibility over air force objections. NASA engineers, led by Robert Gilruth and Maxime Faget (Faget, Max), designed a small cone-shaped capsule for the mission. Compared with the nearly five-metric-ton Vostok, it weighed 1.94 metric tons. Unlike the Soviet approach, in which a cosmonaut was orbited on the first human spaceflight, NASA planned several suborbital test flights in which an astronaut would be in space for only a few minutes of his 15-minute up-and-down ride. Only after the Mercury equipment was checked and the effects of suborbital flight on the human body were measured would the United States commit to an orbital flight attempt. The Mercury capsule would parachute with its passenger all the way back to Earth's surface, to land in the ocean and be recovered by navy ships. Also in contrast to Vostok, the Mercury capsule was designed to allow the astronaut to control some aspects of its flight while in space.

 The United States used chimpanzees (chimpanzee), rather than dogs, as test subjects prior to human flights. In what was intended to be the final test flight before a human launch, the chimpanzee Ham rode a suborbital trajectory on January 31, 1961, using a Redstone rocket developed by Braun's team. Because the flight had experienced minor problems, Braun insisted on one more test flight with an unoccupied dummy spacecraft. If instead, as originally scheduled, that March 1961 flight had carried an astronaut, the United States would have been first with a human in space, although not in orbit. Alan B. Shepard, Jr. (Shepard, Alan B., Jr.), made the first manned Mercury flight atop a Redstone rocket on May 5, 1961. A second suborbital Mercury mission, carrying Virgil I. Grissom (Grissom, Virgil I.), followed in July.

 John H. Glenn, Jr. (Glenn, John H., Jr.), became the first American astronaut to orbit Earth in his three-orbit mission on February 20, 1962. His Mercury spacecraft was launched by a modified air force Atlas (Atlas rocket) ICBM. Three more one-man Mercury orbital flights, carrying astronauts M. Scott Carpenter (Carpenter, M. Scott), Walter M. Schirra, Jr. (Schirra, Walter M., Jr.), and L. Gordon Cooper, Jr. (Cooper, L. Gordon, Jr.), were conducted, the last being a 22-orbit mission in May 1963.

Gemini and Voskhod
      In 1961 President Kennedy announced that the United States would send people to the Moon “before this decade is out.” In order to test many of the techniques that would be needed to carry out a lunar mission, particularly rendezvousing and docking two objects in space, the United States in late 1961 decided to develop a two-person spacecraft called Gemini. The Gemini spacecraft was much more complex than the rudimentary Mercury capsule and, at 3.81 metric tons, was twice as heavy. Another converted air force ICBM, a Titan (Titan rocket) II, was used to launch the Gemini spacecraft.

  The first manned Gemini mission lifted into space in March 1965; nine more missions followed, the last in November 1966. On the second mission in June 1965, Edward H. White II (White, Edward H., II) became the first American astronaut to operate outside a spacecraft. His 20-minute space walk—also known as extravehicular activity (EVA)—was without incident. Although problems developed on many of the Gemini flights, the program demonstrated that people could live and work in space for as long as 14 days, more than the time needed for a round trip to the Moon. It also showed that astronauts could carry out rendezvous in space and could make useful observations of Earth, both visually and photographically.

      As plans in the United States for multiple-astronaut missions became known, the Soviet Union worked to maintain its lead in the space race by modifying the Vostok spacecraft so that it could carry as many as three persons. Korolyov could accomplish this only by having the crew fly without wearing spacesuits. The redesigned spacecraft was known as Voskhod. There were two Voskhod missions, one with three people aboard in October 1964 and another with a two-man crew in March 1965. On the second mission, cosmonaut Aleksey Leonov (Leonov, Aleksey Arkhipovich) became the first human to leave an orbiting spacecraft, less than three months before White. His 12-minute EVA was full of problems, and his reentry of the Voskhod spacecraft was particularly difficult.

 Korolyov and his associates began work in 1962 on a second-generation spacecraft, to be called Soyuz. It was to be a much more complex vehicle than Vostok, holding as many as three people in an orbital crew compartment, with a separate module for crew reentry and a third section containing spacecraft equipment and rocket engines for in-orbit and reentry maneuvers. Soyuz was to be capable not only of flights in Earth orbit but also, in modified versions, of flights around the Moon and even a lunar landing.

      The first launch of Soyuz, with a single cosmonaut, Vladimir Komarov (Komarov, Vladimir Mikhaylovich), aboard, took place on April 23, 1967. Once the spacecraft reached orbit, it suffered a number of problems, which prompted ground controllers to bring Komarov back to Earth as soon as possible. After reentry, however, the spacecraft's main parachute did not fully deploy, and the Soyuz hit the ground at high speed. Komarov became the first person to perish during a spaceflight, and the accident dealt a major blow to Soviet hopes of orbiting or landing on the Moon before the United States.

      After the problems with the Soyuz design were diagnosed and remedied, various models of the spacecraft served as the means of access to space for the Soviet, and then Russian, program of human spaceflight for more than 30 years. At the start of the 21st century, a version of Soyuz was used as the crew rescue vehicle—the lifeboat—for the early phase of construction and occupancy of the International Space Station.

The race to the Moon

The American commitment
      In the immediate aftermath of Gagarin's orbital flight, President Kennedy was advised by his vice president, Lyndon B. Johnson, of Braun's belief that the Soviet Union, using Korolyov's existing R-7 launcher, could well succeed in sending a multiperson spacecraft into Earth orbit and perhaps even around the Moon before the United States. The first competition that the United States had a good chance of winning would be that of a manned lunar landing, because it would require each country to develop a new, more powerful rocket. On those technical grounds and because a lunar landing would be a very visible demonstration of American strength, Kennedy announced on May 25, 1961, that the United States would commit itself to a lunar landing before 1970. At that time, only one American human spaceflight, Shepard's 15-minute suborbital journey, had been made.

      In response to Kennedy's decision, the United States carried out a warlike, but peaceful, mobilization of financial and human resources. NASA's budget was increased almost 500 percent in three years, and at its peak the lunar landing program involved more than 34,000 NASA employees and 375,000 employees of industrial and university contractors.

 By the end of 1962, the basic elements of what was called Project Apollo (Apollo program) were in place. The launch vehicle would be a powerful Saturn V rocket, 110.6 metres (363 feet) tall and power-driven by five huge engines generating a total of 33,000 kilonewtons (7.5 million pounds) of lifting power at takeoff—100 times the takeoff thrust of the Redstone rocket that had launched Shepard. After an intense debate, NASA chose a spacecraft configuration for Apollo that could be sent up in one launch, rather than a larger spacecraft that would need to be assembled in a series of rendezvous in Earth orbit. The Apollo spacecraft would have three sections. A Command Module would house the three-person crew on liftoff and landing and during the trip to and from the Moon. A Service Module would carry various equipment and the rocket engine needed to guide the spacecraft into lunar orbit and then send it back to Earth. A Lunar Module, comprising a descent stage and an ascent stage, would carry two people from lunar orbit to the Moon's surface and back to the Command Module. The ability of the Lunar Module's ascent stage to rendezvous and dock in lunar orbit with the Command Module after takeoff from the Moon was critical to the success of the mission. NASA also created a large new launch facility on Merritt Island, near Cape Canaveral (Canaveral, Cape), Florida, as the Apollo spaceport.

The Soviet response
      While committing the United States to winning the Moon race, President Kennedy also made several attempts in the early 1960s to convince the Soviet leadership that a cooperative lunar landing program between their two countries would be a better alternative. No positive reply from the Soviet Union was forthcoming, however. In fact, between 1961 and 1963, there was still vigorous debate within the Soviet Union over the wisdom of undertaking a lunar program, and no final decision had been made on the question.

      Meanwhile, the separate design bureaus headed by Korolyov and his rival Vladimir Chelomey (Chelomey, Vladimir Nikolayevich) competed fiercely for a lunar mission assignment, either a flight around the Moon or an actual landing. Finally, in August 1964, Korolyov received the lunar landing assignment, and soon afterward Chelomey was given responsibility for planning a circumlunar flight to be carried out before the 50th anniversary of the Bolshevik Revolution, which would take place in October 1967. In 1965 Soviet leaders decided to combine the efforts of the two rivals for the circumlunar mission, using a version of Korolyov's Soyuz spacecraft and a new rocket, the UR-500 (also called the Proton), designed by Chelomey.

      The rocket that Korolyov designed for the lunar landing effort was called the N1. Like the Saturn V, it was huge, standing 112.8 metres (370 feet) tall and having a planned takeoff thrust of 44,500 kilonewtons (10 million pounds). Instead of a few large rocket engines in its first stage, however, the N1 had 30 smaller engines. These were developed by Nikolay Kuznetsov, an aircraft-engine chief designer who had little experience with rocket engines, rather than the more capable Glushko. Korolyov and Glushko, already personal adversaries for many years, had disagreed on the proper fuel for the N1, and they finally decided that they could no longer work together. Consequently, Korolyov turned to Kuznetsov, who chose the small-engine approach.

      Indecision, inefficiencies, inadequate budgets, and personal and organizational rivalries in the Soviet system thus posed major obstacles to success in the race to the Moon. To these was added the unexpected death of Korolyov, age 59, during surgery on January 14, 1966. This was a serious setback to the Soviet space program. Korolyov had been a charismatic leader and organizer. His successor, Vasily Mishin, attempted to maintain the program's momentum, but he was not the effective manager or politically sophisticated operator that Korolyov had been.

Interim developments
 In the United States, Apollo moved forward as a high-priority program. A major setback occurred on January 27, 1967, when astronauts Grissom, White, and Roger Chaffee (Chaffee, Roger B.) were killed after their Apollo 1 Command Module caught fire during a ground test. The first manned Apollo mission, designated Apollo 7 and intended to test the redesigned Command Module, was launched into Earth orbit on October 11, 1968. The launcher used was a Saturn IB, a less-powerful rocket than the Saturn V needed to reach the Moon. The mission's success cleared the way for a bold step—the first launch of a crew atop a Saturn V to the lunar vicinity. On December 21, 1968, the Apollo 8 Command and Service modules were put on a trajectory that sent them into orbit around the Moon on Christmas Eve, December 24. The three astronauts—Frank Borman (Borman, Frank), James A. Lovell, Jr. (Lovell, James A(rthur), Jr.), and William A. Anders (Anders, William A.)—sent back close-up images of the lunar surface, read from the biblical book of Genesis, and brought back vivid colour photographs of a blue planet Earth rising over the desolate lunar landscape. By the end of the mission, it was clear that the first lunar landing was only months away.

      One reason for conducting the Apollo 8 mission was to allow NASA to test most of the systems needed for a lunar landing attempt while waiting to carry out a manned trial in Earth orbit of the Lunar Module, whose development was behind schedule. Another was the concern that the Soviet Union would beat the United States in sending people to the lunar vicinity. A circumlunar mission indeed had been part of Soviet plans, but the Soyuz 1 accident had made the October 1967 deadline infeasible. During 1968 a number of test flights of a circumlunar mission were made, using the Proton launcher and a version of the Soyuz spacecraft designated Zond. In September Zond 5 carried a biological payload, including two tortoises, around the Moon and safely back to Earth, but two months later the Zond 6 spacecraft depressurized and then crashed on landing, ending any hope for a quick follow-on launch with a human crew. Plans to send cosmonauts around the Moon in a Zond spacecraft were postponed indefinitely in March 1969, but two more scientifically successful unmanned circumlunar missions, Zond 7 and Zond 8, were carried out in 1969 and 1970, respectively.

      The Soviet lunar landing program went forward rather fitfully after 1964. The missions were intended to employ the N1 launch vehicle and another variation of the Soyuz spacecraft, designated L3, that included a lunar landing module designed for one cosmonaut. Although an L3 spacecraft was constructed and three cosmonauts trained for its use, the N1 rocket was never successfully launched. After four failed attempts between 1969 and 1972—including a spectacular launch-pad explosion in July 1969—the N1 program was finally canceled in May 1974, and Soviet hopes for human missions to the Moon thus ended.

The Apollo lunar landings and Apollo-Soyuz
    By contrast with the Soviet lunar landing efforts, during 1969 all went well for the Apollo program. In March the Apollo 9 crew sucessfully tested the Lunar Module in Earth orbit, and in May the Apollo 10 crew carried out a full dress rehearsal for the landing, coming within 15,200 metres (50,000 feet) of the lunar surface. On July 16, 1969, astronauts Armstrong (Armstrong, Neil), Aldrin (Aldrin, Edwin Eugene, Jr.), and Michael Collins (Collins, Michael) set off on the Apollo 11 mission, the first lunar landing attempt. While Collins remained in lunar orbit in the Command Module, Armstrong piloted the Lunar Module, nicknamed Eagle, away from boulders on the lunar surface and to a successful landing on a flat lava plain called the Sea of Tranquillity at 4:18 PM U.S. Eastern Daylight Time on July 20. He reported to mission control, “Houston. Tranquillity Base here. The Eagle has landed.” Six and a half hours later, Armstrong, soon followed by Aldrin, left the Lunar Module and took the first human step on the surface of another celestial body. As he did so, he noted, “That's one small step for [a] man, one giant leap for mankind.” (In the excitement of the moment, Armstrong skipped the “a” in the statement he had prepared.) Concluding 2.5 hours of activity on the lunar surface, the two men returned to the Lunar Module with 21.7 kg (47.8 pounds) of lunar samples. Twelve hours later, they blasted off the Moon in the Lunar Module's ascent stage and rejoined Collins in the Command Module. The crew returned to Earth on July 24, splashing down in the Pacific Ocean.

   The successful Apollo 12 mission followed in November 1969. The Apollo 13 mission, launched in April 1970, experienced an explosion of the oxygen tank in its Service Module on the outbound trip to the Moon. The crew survived this accident only through the improvised use of the Lunar Module as living quarters in order to preserve the remaining capabilities of the Command Module for reentering Earth's atmosphere after they had returned from their circumlunar journey. Four more Apollo missions followed. On each of the final three, the crew had a small cartlike rover that allowed them to travel several kilometres from their landing site. The final mission, Apollo 17, which was conducted in December 1972, included geologist Harrison Schmitt, the only trained scientist to set foot on the Moon.

  The United States had won the race to the Moon, but that race had been motivated primarily by political considerations. After the early 1970s there was no interest within the U.S. government for the next three decades in additional lunar exploration or in sending people to Mars or any other distant destination. No human has traveled beyond near-Earth orbit since Apollo 17.

      An Apollo spacecraft was used for the last time in 1975. Three years earlier, as a sign of improved U.S.-Soviet relations, the two countries had agreed to carry out a joint mission in which an Apollo spacecraft carrying three astronauts would dock in orbit with a Soyuz vehicle having two cosmonauts aboard. The Apollo-Soyuz Test Project, which took place in July 1975, featured a “handshake in space” between Apollo commander Thomas P. Stafford (Stafford, Thomas P.) and Soyuz commander Aleksey Leonov (Leonov, Aleksey Arkhipovich).

Orbiting space platforms
Space stations (space station)
 By 1969, even though the U.S.S.R. was still moving forward with its lunar landing program, it had begun to shift its emphasis in human spaceflight to the development of Earth-orbiting stations in which cosmonaut crews could carry out extended observations and experiments on missions that lasted weeks or months rather than a few days. The first Soviet space station, called Salyut 1, was launched April 19, 1971. The first crew to occupy the station—Georgy Dobrovolsky (Dobrovolsky, Georgy Timofeyevich), Viktor Patsayev (Patsayev, Viktor Ivanovich), and Vladislav Volkov (Volkov, Vladislav Nikolayevich)—spent 23 days aboard carrying out scientific studies but perished when their Soyuz spacecraft depressurized during reentry.

  With similar objectives for a long-term manned platform in space, the United States converted the third stage of a Saturn V rocket into an orbital workshop for solar and biomedical studies. This first U.S. space station, called Skylab, was launched May 14, 1973. Over a period of eight and a half months, three three-person crews using Apollo spacecraft for transport spent time aboard Skylab, with the final crew staying for 84 days. Skylab was abandoned in February 1974 and reentered Earth's atmosphere in July 1979, with some portions of the station surviving reentry and landing in Australia.

 Whereas, because of budgetary cuts, the United States did not launch a planned second Skylab, the Soviet Union orbited and successfully occupied five more Salyut stations in a program that continued through the mid-1980s. Two of these stations had a military reconnaissance mission, but the others were devoted to scientific studies, particularly biomedical research. The Soviet Union also launched guest cosmonauts from allied countries for short stays aboard Salyut 6 and 7.

      These early stations were a reflection of a long-held belief among space visionaries, dating back to Tsiolkovsky at the start of the 20th century, that living and working in space, first in Earth orbit and then on the Moon, Mars, and other locations, were an important part of the human future. It also was thought that increasingly complex orbital outposts would be the first steps in a long-term process of space development and colonization. The early focus of the United States and the U.S.S.R. on sending people to the Moon for political reasons deviated from this vision, which has since returned to dominate space thinking.

   International space endurance recordsThe Soviet Union followed its Salyut station series with the February 1986 launch of the core element of the modular Mir space station. Additional modules carrying scientific equipment and expanding the living space were attached to Mir in subsequent years. In 1994–95 Valery Polyakov (Polyakov, Valery Vladimirovich), a medical doctor, spent 438 continuous days aboard the station. (For a list of human endurance records in space, see the table (International space endurance records).) More than 100 different people from 12 countries visited Mir, including seven American astronauts in the 1995–98 period. The station, which was initially scheduled to operate for only five years, supported human habitation until mid-2000 (continuously between 1989 and 1999), although it experienced a number of accidents and other serious problems. In March 2001 it made a controlled atmospheric reentry, with the surviving pieces falling into the Pacific Ocean.

  Space stations, from 1971The United States did not follow up on Skylab until 1984, when President Ronald Reagan (Reagan, Ronald W.) approved a space station program and invited U.S. allies to participate. By 1988, 11 countries—Canada, Japan, and 9 countries from Europe—had decided to join what was known as Space Station Freedom. Progress in developing the station was slow, however, and in 1993 newly elected President Bill Clinton (Clinton, Bill) ordered a sweeping redesign of the program. The United States and its existing partners invited Russia, which had inherited most of the Soviet Union's space efforts after the U.S.S.R.'s collapse in 1991, to participate in the multinational program, renamed the International Space Station (ISS). Three additional countries joined during the 1990s and thereby made the 16-country project the largest-ever cooperative technological undertaking. The first two elements of the ISS were launched and connected in space in late 1998, and several modules and other equipment were subsequently added. Assembly and initial operation of the facility were scheduled to take place over much of the first decade of the 21st century. (For a summary of space stations launched since 1971, see table (Space stations, from 1971).)

      After the success of the Apollo 11 mission, NASA proposed an ambitious plan that included human flights to Mars, a series of large space stations to be developed during the 1970s, and a new, reusable space transportation system to send people and supplies to those stations. This plan was quickly rejected, as there was no interest in major post-Apollo space programs among the political leadership or the general public. In 1972 NASA received presidential approval to develop a partially reusable transport vehicle called a space shuttle. This vehicle was intended to carry people and as much as 29,500 kg (65,000 pounds) of cargo into low Earth orbit at low cost. On the basis of those expectations, the United States planned to use the shuttle as its sole launch vehicle once it entered operation and to operate a shuttle fleet with a launch rate as high as 60 per year. In the absence of a space station, plans also called for having the shuttle serve double duty as a space platform to conduct in-orbit research for periods as long as two weeks. To that end, Europe pledged to contribute a pressurized laboratory, known as Spacelab, that would be carried in the shuttle's payload bay.

 The space shuttle design had three major components. A reusable winged orbiter would carry crew and cargo and be able to glide to a landing on a conventional runway at the end of its mission. A large external tank would carry the liquid-oxygen and liquid-hydrogen propellants for the orbiter's three powerful engines. The tank would be used only during the first eight minutes of flight; once the fuel was exhausted, it would be discarded and burn up on reentry. Two solid-fuel rockets would assist in accelerating the vehicle during the first two minutes of flight; they would then be detached and parachute into the ocean, where they would be recovered for future use. A fleet of four operational orbiters, named Columbia, Challenger, Atlantis, and Discovery, was built in order to allow multiple shuttle flights each year. Facilities in Florida originally constructed for the Apollo program were remodeled for shuttle use.

      After several years of technical and budgetary delays to the program, the first space shuttle flight took place on April 12, 1981; aboard were astronauts John W. Young (Young, John W.), a veteran of the Gemini and Apollo programs, and Robert Laurel Crippen (Crippen, Robert Laurel). With additional shuttle flights, it became evident that projections of the vehicle's operational costs and performance had been extremely optimistic. Major refurbishment was required between each launch; the highest flight rate achieved was in 1985, when the shuttle was launched nine times. Each launch cost hundreds of millions of dollars, rather than the tens of millions that had been promised in 1972. Although the space shuttle was a remarkable technological achievement as a first-generation reusable launch vehicle, the plans to use it as the only launcher for American payloads proved to be a major policy mistake.

 The optimism surrounding the space shuttle program was publicly shattered on January 28, 1986, when the Challenger orbiter was destroyed in a catastrophic explosion 73 seconds after liftoff. Its seven-person crew perished; among them was schoolteacher Christa McAuliffe (McAuliffe, Christa Corrigan), on board as the first private citizen in space. The launch had taken place in unusually cold weather, and a sealing ring within a segment joint of one of the solid rocket boosters failed. The solid rocket broke loose and hit the external tank, rupturing it. The flame from the leaking booster ignited the shuttle's fuel, causing the explosion.

      After the accident, the shuttle fleet was grounded until September 1988. A replacement orbiter, Endeavour, was built, but, upon the resumption of flights, the shuttle fleet was operated only with much greater assurances for the safety of its crew. This limited the flight rate to six to eight missions per year; the 100th shuttle flight was not achieved until October 2000.

      Both before and after the Challenger accident, the space shuttle demonstrated impressive capabilities in space operations, including the repair and redeployment of damaged satellites—most striking being the in-orbit repair of the Hubble Space Telescope in 1993. Beginning in 1998, the space shuttle was used to carry components of the ISS into orbit, along with the crews to assemble those components. It also was used to ferry people and supplies to and from the space station, the role for which it was first conceived.

 The shuttle program suffered its second fatal disaster on February 1, 2003, when the orbiter Columbia broke up over Texas at an altitude of about 60 km (40 miles) as it was returning from an orbital mission. All seven crew members died, including Ilan Ramon, the first Israeli astronaut to go into space. The shuttle fleet was once again grounded during the ensuing investigation into the cause of the accident, and critical flights to the ISS were conducted by Russian expendable launch vehicles.

      In the 1980s the Soviet Union also developed a space shuttle, called Buran, and a very powerful rocket, called Energia, to launch it and other heavy payloads. Energia was launched only twice, once in 1987 with a military payload and once the next year carrying Buran on a successful unmanned test flight into orbit and back. Use of the two vehicles was abandoned as the Soviet Union faced increasing economic problems.

Human beings in space: debate and consequences
      By the middle of the first decade of the 21st century, some 500 people, coming from about 30 different countries and comprising about 90 percent men and 10 percent women, had flown in space, although some individuals had made several separate flights. As of that same time, only the United States, Russia, and China had the capability of carrying out human spaceflights.

Risks and benefits
      Human spaceflight is both risky and expensive. From the crash landing of the first manned Soyuz spacecraft in 1967 to the explosion of the shuttle orbiter Columbia in 2003, 18 people died during spaceflights. Providing the systems to support people while in orbit adds significant additional costs to a space mission, and ensuring that the launch, flight, and reentry are carried out as safely as possible also requires highly reliable and thus costly equipment, including both spacecraft and launchers.

 From the start of human spaceflight efforts, some have argued that the benefits of sending humans into space do not justify either the risks or the costs. They contend that robotic missions can produce equal or even greater scientific results with lower expenditures and that human presence in space has no other valid justification. Those who support human spaceflight cite the still unmatched ability of human intelligence, flexibility, and reliability in carrying out certain experiments in orbit, in repairing and maintaining robotic spacecraft and automated instruments in space, and in acting as explorers in initial journeys to other places in the solar system. They also argue that astronauts serve as excellent role models for younger people and act as vicarious representatives of the many who would like to fly in space themselves. In addition is the long-held view that eventually some humans will leave Earth to establish permanent outposts and larger settlements on the Moon, Mars, or other locations.

Selecting people for spaceflights
 Most of the individuals who have gone into space are highly trained astronauts (astronaut) and cosmonauts, the two designations having originated in the United States and the Soviet Union, respectively. (Both taikonaut and yuhangyuan have sometimes been used to describe the astronauts in China's manned space program.) Those governments interested in sending some of their citizens into space select candidates from many applicants, on the basis of their backgrounds and physical and psychological characteristics. The candidates undergo rigorous training before being chosen for an initial spaceflight and then prepare in detail for each mission assigned. Training centres with specialized facilities exist in the United States, at NASA's Johnson Space Center, Houston, Texas; in Russia, at the Yury Gagarin Cosmonauts Training Center (commonly called Star City), outside Moscow; in Germany, at ESA's European Astronaut Centre, Cologne; in Japan, at NASDA's Tsukuba Space Center, near Tokyo; and in China, at Space City, near Beijing.

      Astronauts and cosmonauts who undertake multiple spaceflights traditionally fall into one of two categories. One category consists of pilots, often with military backgrounds, who have had extensive experience in flying high-performance aircraft. They are responsible for piloting space vehicles such as the space shuttle and Soyuz. The other category includes scientists and engineers who are not necessarily pilots. They have primary responsibility for carrying out the scientific and engineering activities scheduled for a particular mission. They are known in the U.S. space program as mission specialists and in the Russian space program as flight engineers. With the development of long-duration space stations (space station) such as Mir and the ISS, the distinction between pilot and nonpilot astronauts and cosmonauts has become less clear, because all members of a space station crew carry out station operations and experiments.

      A third category of individuals who have gone into space are called variously payload specialists or guest cosmonauts. These individuals include scientists and engineers who accompany their experiments into orbit; individuals selected to go into space for political reasons, such as members of the U.S. Congress or persons from countries allied with the Soviet Union or the United States; and a few nontechnical people—for example, the rare journalist or teacher or the private individual willing to pay substantial amounts of money for a spaceflight. These people are intensively trained for their particular flight but usually go into space only once. At some future time, the costs and risks of human spaceflight may become low enough to accommodate the business of space tourism, in which many people would be able to experience spaceflight. Until then, access to space will be restricted to a comparatively small number of people.

Biomedical, psychological, and sociological aspects
      Human beings have evolved to live in the environment of Earth's surface. The space environment—with its very low level of gravity, lack of atmosphere, wide temperature variations, and often high levels of ionizing radiation from the Sun, from particles trapped in the Van Allen radiation belts, and from cosmic rays—is an unnatural place for humans. An understanding of the effects on the human body of spaceflight, particularly long-duration flights away from Earth to destinations such as Mars, is incomplete.

      A majority of those going into space experience space sickness (see motion sickness), which may cause vomiting, nausea, and stomach discomfort, among other symptoms. The condition is thought to arise from a contradiction experienced in the brain between external information coming from the eyes and internal information coming from the balance organs in the inner ear, which are normally stimulated continually by gravity. Space sickness usually disappears within two or three days as the brain adapts to the space environment, although symptoms may reappear temporarily when the space traveler returns to Earth's gravity.

      The virtual absence of gravity causes loss of tissue mass in the calf and thigh muscles (muscle), which are used on Earth's surface to counter the effect of gravity. Muscles that are less involved with gravity, such as those used to bend the legs or arms, are less affected. Some loss of muscle mass in the heart has been observed in astronauts on long-duration missions. In the absence of gravity, blood that normally pools in the body's lower extremities initially shifts to the upper regions. As a result, the face appears puffy, the person experiences sinus congestion and headaches, and blood production decreases as the body attempts to compensate. In addition, in the space environment, some weight-bearing bones in the body atrophy.

 Although the changes in muscle, bone, and blood production do not pose problems for astronauts in space, they do so on their return to Earth. For example, in normal gravity, a person with decreased bone mass runs a greater risk of breaking a bone during normal strenuous activity. Countermeasures, particularly various forms of exercise while in space, have been developed to prevent these effects from causing health problems later on Earth. Even so, people recovering from long-duration flights require varying amounts of time to readjust to Earth conditions. Light-headedness usually disappears within one or two days; lack of balance and symptoms of motion sickness, in three to five days; anemia, in one to two weeks; muscle atrophy, in three to five weeks; and bone atrophy, in one to three years or more.

      Except for the Apollo trips to the Moon, all human spaceflights have taken place in near-Earth orbit. In this location, Earth's magnetic field shields humans from potentially dangerous exposure to ionizing radiation from recurrent major disturbances on the Sun and interplanetary cosmic rays. The Apollo missions, which were all less than two weeks long, were timed to avoid exposure to anticipated high levels of solar radiation. If, however, humans were sent on journeys to Mars or other destinations that would take months or even years, such measures would be inadequate. Exposure to high levels of solar radiation or cosmic rays could cause potentially fatal tumours and other health problems (see radiation injury). Space engineers will need to devise adequate radiation shielding for interplanetary manned spacecraft and will require accurate predictions of radiation damage to the body to ensure that risks remain within acceptable limits. Biomedical advances are also necessary to develop methods for the early detection and mitigation of radiation damage. Nevertheless, the effects of radiation may remain a major obstacle to long human voyages in space.

      In addition to the biomedical issues associated with human spaceflight are a number of psychological and sociological issues, particularly for long-duration missions aboard a space station or to distant destinations. To be in space is to be in an extreme and isolated environment. Mission planners will have to consider issues relating to crew size and composition—particularly if the crews are mixtures of men and women and come from several nations with different cultures—if interpersonal conflicts are to be avoided and effective teamwork achieved.

Science in space
      In the decades following the first Sputnik and Explorer satellites, the ability to put their instruments into outer space gave scientists the opportunity to acquire new information about the natural universe, information that in many cases would have been unobtainable any other way. Space science added a new dimension to the quest for knowledge, complementing and extending what had been gained from centuries of theoretical speculations and ground-based observations.

      After Gagarin's 1961 flight, space missions involving human crews carried out a range of significant research, from on-site geologic investigations on the Moon to a wide variety of observations and experiments aboard orbiting spacecraft. In particular, the presence in space of humans as experimenters and, in some cases, as experimental subjects facilitated studies in biomedicine and materials science. Nevertheless, most space science was, and continues to be, performed by robotic spacecraft in Earth orbit or on missions to various bodies in the solar system. In general, such missions are far less expensive than those involving humans and can carry sophisticated automated instruments to gather a wide variety of relevant data.

      In addition to the United States and the Soviet Union, several other countries achieved the capability of developing and operating scientific spacecraft and thus carrying out their own space science missions. They include Japan, Canada, and a number of European countries such as the United Kingdom, France, Italy, and Germany, acting alone and through cooperative organizations involving other European countries. Furthermore, many other countries became involved in space activities through the participation of their scientists in specific missions. Bilateral or multilateral cooperation between various countries in carrying out space science missions grew to be the usual way of proceeding.

      Scientific research in space can be divided into five general areas: (1) solar and space physics, including study of the magnetic and electromagnetic fields in space and the various energetic particles also present, with particular attention to their interactions with Earth, (2) exploration of the planets, moons, asteroids, comets, meteoroids, and dust in the solar system, (3) study of the origin, evolution, and current state of the varied objects in the universe beyond the solar system, (4) research on nonliving and living materials, including humans, in the very low gravity levels of the space environment, and (5) study of Earth from space.

Solar and space physics
 The first scientific discovery made with instruments orbiting in space was the existence of the Van Allen radiation belts (Van Allen radiation belt), by Explorer 1 and other spacecraft in 1958. Subsequent space missions investigated Earth's magnetosphere, the surrounding region of space in which the planet's magnetic field exerts a controlling effect (see Earth: The magnetic field and magnetosphere (Earth)). Of particular and ongoing interest has been the interaction of the flux of charged particles emitted by the Sun, called the solar wind, with the magnetosphere. Early space science investigations showed, for example, that luminous atmospheric displays known as auroras (aurora) are the result of this interaction, and scientists came to understand that the magnetosphere is an extremely complex phenomenon.

      The focus of inquiry in space physics was later extended to understanding the characteristics of the Sun, both as an average star and as the primary source of energy for the rest of the solar system, and to exploring space between the Sun and Earth and other planets (see interplanetary medium). The magnetospheres of other planets, particularly Jupiter with its strong magnetic field, also came under study. Scientists sought a better understanding of the internal dynamics and overall behaviour of the Sun, the underlying causes of variations in solar activity, and the way in which those variations propagate through space and ultimately affect Earth's magnetosphere and upper atmosphere. The concept of space weather was advanced to describe the changing conditions in the Sun-Earth region of the solar system. Variations in space weather can cause geomagnetic storms that interfere with the operation of satellites and even systems on the ground such as power grids.

 To carry out the investigations required for addressing these scientific questions, the United States, Europe, the Soviet Union, and Japan developed a variety of space missions, often in a coordinated fashion. In the United States, early studies of the Sun were undertaken by a series of Orbiting Solar Observatory satellites (launched 1962–75) and the astronaut crews of the Skylab space station in 1973–74, using that facility's Apollo Telescope Mount. These were followed by the Solar Maximum Mission satellite (launched 1980). ESA developed the Ulysses mission (1990) to explore the Sun's polar regions. Solar-terrestrial interactions were the focus of many of the Explorer series of spacecraft (1958–75) and the Orbiting Geophysical Observatory satellites (1964–69). In the 1980s NASA, ESA, and Japan's Institute of Space and Astronautical Science undertook a cooperative venture to develop a comprehensive series of space missions, named the International Solar-Terrestrial Physics Program, that would be aimed at full investigation of the Sun-Earth connection. This program was responsible for the U.S. Wind (1994) and Polar (1996) spacecraft, the European Solar and Heliospheric Observatory (SOHO; 1995) and Cluster (2000) missions, and the Japanese Geotail satellite (1992).

Solar system exploration
 From the start of space activity, scientists recognized that spacecraft could gather scientifically valuable data about the various planets, moons, and smaller bodies in the solar system. Both the United States and the U.S.S.R. attempted to send robotic missions to the Moon in the late 1950s. The first four U.S. Pioneer spacecraft, Pioneer 0–3, launched in 1958, were not successful in returning data about the Moon. The fifth mission, Pioneer 4 (1959), was the first U.S. spacecraft to escape Earth's gravitational pull; it flew by the Moon at twice the planned distance but returned some useful data. Three Soviet missions, Luna 1–3, explored the vicinity of the Moon in 1959, confirming that it had no appreciable magnetic field and sending back the first-ever images of its far side. Luna 1 was the first spacecraft to fly past the Moon, beating Pioneer 4 by two months; Luna 2, in making a hard landing on the lunar suface, was the first spacecraft to strike another celestial object. Later Luna spacecraft soft-landed on the Moon, and some gathered soil samples and returned them to Earth.

   In the 1960s the United States became the first country to send a spacecraft to the vicinity of other planets; Mariner 2 flew by Venus in December 1962, and Mariner 4 flew past Mars in July 1965. Among significant accomplishments of planetary missions in succeeding decades were the U.S. Viking landings on Mars in 1976 and the Soviet Venera explorations of the atmosphere and surface of Venus from the mid-1960s to the mid-1980s. In the years since, the United States has continued an active program of solar system exploration, as did the Soviet Union until its dissolution in 1991. Japan launched missions to the Moon, Mars, and Halley's Comet. Europe's first independent solar system mission, Giotto, also flew by Halley. After the turn of the 21st century, it sent missions to the Moon and Mars and an orbiter-lander to a comet.

   Early on, scientists planned to conduct solar system exploration in three stages: initial reconnaissance from spacecraft flying by a planet, comet, or asteroid; detailed surveillance from a spacecraft orbiting the object; and on-site research after landing on the object or, in the case of the giant gas planets, by sending a probe into its atmosphere. By the start of the 21st century, all three of those stages had been carried out for the Moon, Venus, Mars, Jupiter, and a near-Earth asteroid. Several Soviet and U.S. robotic spacecraft have landed on Venus and the Moon, and the United States has landed spacecraft on the surface of Mars. A long-term, detailed surveillance of Jupiter and its moons began in 1995 when the U.S. Galileo spacecraft took up orbit around the planet, at the same time releasing a probe into the turbulent Jovian atmosphere. In 2001 the U.S. Near Earth Asteroid Rendezvous (Near Earth Asteroid Rendezvous Shoemaker) (NEAR) spacecraft landed on the asteroid Eros and transmitted information from its surface for more than two weeks. Among the rocky inner planets, only Mercury has remained relatively neglected. In the first half century of space exploration, Mercury was visited just once—the U.S. Mariner 10 probe made three flybys of the planet in 1974–75. In 2004 the U.S. Messenger spacecraft was launched to Mercury for a series of flybys beginning in 2008 and establishment of an orbit around the planet in 2011.

      As of 2005, the exploration of the other giant gas planets— Saturn, Uranus, and Neptune—remained at the first or second stage. In a series of U.S. missions launched in the 1970s, Pioneer 10 flew by Jupiter, whereas Pioneer 11 and Voyager 1 and 2 flew by both Jupiter and Saturn. Voyager 2 then went on to travel past Uranus and Neptune. The U.S. Cassini (Cassini-Huygens) spacecraft, launched in 1997, began a long-term surveillance mission in the Saturnian system in 2004; the following year its European-built Huygens probe descended to the surface of Titan, Saturn's largest moon. Thus, every significant body in the solar system except the dwarf planet Pluto and its largest moon, Charon, has been visited at least once by a spacecraft.

      These exploratory missions sought information on the origin and evolution of the solar system and on the various objects that it comprises, including chemical composition; surface topography; data on magnetic fields, atmospheres, and volcanic activity; and—particularly for Mars and perhaps eventually for Jupiter's moon Europa and Saturn's moon Titan—evidence of water or other liquids in the present or past and perhaps even of extraterrestrial life in some form.

      What has been learned to date confirms that Earth and the rest of the solar system formed at about the same time from the same cloud of gas and dust surrounding the Sun. The four outer giant gas planets are roughly similar in size and chemical composition, but each has a set of moons that differ widely in their characteristics, and in some ways they and their satellites resemble miniature solar systems. The four rocky inner planets had a common origin but followed very different evolutionary paths and today have very different surfaces, atmospheres, and internal activity. Ongoing comparative study of the evolution of Venus, Mars, and Earth could provide important insights into Earth's future and its continued ability to support life.

 The question of whether life has ever existed elsewhere in the solar system continues to intrigue both scientists and the general public. The United States sent two Viking spacecraft to land on the surface of Mars in 1976. Each contained three experiments intended to search for traces of organic material that might indicate the presence of past or present life-forms (extraterrestrial life); none of the experiments produced positive results. Twenty years later, a team of scientists studying a meteorite of Martian origin found in Antarctica announced the discovery of possible microscopic fossils resulting from past organic life. Their claim was not universally accepted, but it led to an accelerated program of Martian exploration focused on the search for evidence of the action of liquid water, thought necessary for life to have evolved. A major goal of this program is to return samples of the Martian surface to Earth for laboratory analysis.

 The Galileo mission provided images and other data related to Jupiter's moon Europa that suggest the presence of a liquid water ocean beneath its icy crust. Future missions will seek to confirm the existence of this ocean and search for evidence of organic or biological processes in it.

Exploring the universe (Cosmos)
      Until the dawn of spaceflight, astronomers were limited in their ability to observe objects beyond the solar system to those portions of the electromagnetic spectrum that can penetrate Earth's atmosphere. These portions include the visible region, parts of the ultraviolet region, and most of the radio-frequency region. The ability to place instruments on a spacecraft operating above the atmosphere (see satellite observatory) opened the possibility of observing the universe in all regions of the spectrum. Even operating in the visible region, a space-based observatory could avoid the problems caused by atmospheric turbulence and airglow.

 Beginning in the 1960s, a number of countries launched satellites (satellite observatory) to explore cosmic phenomena in the gamma-ray, X-ray, ultraviolet, visible, and infrared regions. More recently, space-based radio astronomy has been pursued. In the last decades of the 20th century, the United States embarked on the development of a series of long-duration orbital facilities collectively called the Great Observatories. They include the Hubble Space Telescope, launched in 1990 for observations in the visible and ultraviolet regions; the Compton Gamma Ray Observatory, launched in 1991; the Chandra X-ray Observatory, launched in 1999; and the Spitzer Space Telescope, launched in 2003. Europe and Japan have also been active in space-based astronomy and astrophysics.

      The results of these space investigations have made major contributions to an understanding of the origin, evolution, and likely future of the universe, galaxies, stars, and planetary systems. For example, the U.S. Cosmic Background Explorer (COBE) satellite, launched in 1989, mapped the microwave background radiation left over from the early universe, providing strong support for the theory that the universe was created in a primordial explosion, known as the big bang (see big-bang model). The striking images of cosmic objects obtained by the Hubble Space Telescope not only have added significantly to scientific knowledge but also have shaped the public's perception of the cosmos, perhaps as significantly as did the astronomer Galileo's observations of the Moon and Jupiter nearly four centuries earlier. Working as complements to ground-based observatories of increasing sensitivity, space-based observatories have helped create a revolution in modern astronomy.

Microgravity research
 A spacecraft orbiting Earth is essentially in a continual state of free fall (free-fall). All objects associated with the spacecraft, including any crew and other contents, are accelerating—i.e., falling freely—at the same rate in Earth's gravitational field (see Earth: Basic planetary data (Earth)). As a result, these objects do not “feel” the presence of Earth's gravity but instead experience a state of weightlessness, or zero gravity. True zero gravity, however, is experienced only at the centre of mass of a freely falling object. With increasing distance from the centre of mass, the influence of gravity increases in both directions perpendicular to the object's flight path. These constant but tiny accelerations make necessary the use of the term microgravity to describe the space environment. (It is possible to create a similar absence of gravity's effects only briefly on Earth or in an aircraft.) Human activity or the operation of equipment in a spacecraft causes vibrations that impart additional accelerations and so raise gravity levels, which can make it difficult to carry out highly sensitive experiments under sufficiently low microgravity conditions. Although spacecraft designers cannot totally eliminate gravitational effects, they hope to reduce them in some parts of the International Space Station to one microgravity—one-millionth of Earth's gravity—by isolating those areas from vibrations and other disturbances as much as possible.

      The opportunity to carry out experiments in the absence of gravity has interested scientists from the beginning of activities in orbit. In addition to concern about the effects of the weightlessness on humans sent into space (see above Biomedical, psychological, and sociological aspects (space exploration)), scientists are interested in its effects on the reproductive and developmental cycles of plants and animals other than humans. The overall goal is to use space-based research to add to the general understanding of a wide range of biological processes.

      Life-sciences experiments were carried out on the Skylab, Salyut, and Mir space stations (space station) and will constitute a significant portion of work aboard the ISS. Such research also was conducted on space shuttle missions, particularly within the Spacelab facility. In addition, the Soviet Union and the United States launched a number of robotic satellites dedicated to life-sciences research. Together these experiments have involved a wide range of nonhuman organisms, from bacteria, plants, and invertebrate animals to fish, birds, frogs, turtles, and mammals such as rats and monkeys. Human crew members also have served as experimental subjects for research on such topics as the functioning of the neurological system and the process of aging. In October 1998, U.S. senator and former Mercury astronaut John H. Glenn, Jr. (Glenn, John H., Jr.), at age 77 returned to space on a shuttle mission dedicated to life-sciences research, which included studies of similarities between the aging process and the body's response to weightlessness. The hope is that the results of biomedical experiments conducted in microgravity can be used to improve human health and well-being on Earth.

 The microgravity environment also offers unique conditions for experiments designed to explore the behaviour of materials. Among the areas of inquiry are biotechnology, combustion science, fluid physics, fundamental physics, and materials science. Experiments in the microgravity environment on various materials, including metals, alloys, electronic and photonic materials, composites, colloids, glasses and ceramics, and polymers, have resulted in a greater understanding of the role of gravity in similar laboratory and manufacturing processes on Earth. The microgravity environment offers the potential for producing biological materials, including highly ordered protein crystals for crystallographic analysis and even materials resembling human tissues, that are difficult or impossible to make on Earth. Although microgravity research is still largely at the basic level, scientists and engineers hope that additional work—another major focus for the ISS—will lead to practical knowledge of great usefulness to manufacturing processes on Earth.

Observing Earth
 Satellites, space stations, and space shuttle missions have provided a new perspective for scientists to collect data about Earth itself. In addition to practical applications (see below Space applications (space exploration)), Earth observation from space has made significant contributions to fundamental knowledge. An early and continuing example is the use of satellites to make various geodetic measurements, which has allowed precise determinations of Earth's shape, internal structure, and rotational motion and the tidal and other periodic motions of the oceans. Fields as diverse as archaeology, seismology, and oceanography likewise have benefited from observations and measurements made from orbit.

      Scientists have begun to use observations from space as part of comprehensive efforts in fields such as oceanography and ecology to understand and model the causes, processes, and effects of global climate change, including the influence of human activities. The goal is to obtain comprehensive sets of data over meaningful time spans about key physical, chemical, and biological processes that are shaping the planet's future. This is a coordinated international effort, in which the United States, Europe, and Japan are providing satellites to obtain the needed observations.

Space applications
 Space visionaries in the early 20th century recognized that putting satellites into orbit could furnish direct and tangible benefits to people on Earth. For example, Arthur C. Clarke (Clarke, Sir Arthur C.) in 1945 described a way in which three satellites in orbit about 35,800 km (22,250 miles) above the Equator could relay communications around the globe. In this orbit, called a geostationary orbit, the satellites would have an orbital period equal to Earth's rotational period and thus appear from the ground to be stationary in the sky. (For additional information on satellite orbits, see spaceflight: Earth orbit (spaceflight).) A report for the U.S. Army Air Forces in 1946 by Project RAND (the predecessor of the RAND Corporation) identified the benefits of being able to observe Earth from space, which included distinguishing the impact sites of bombs dropped by U.S. aircraft and improved weather forecasting.

      Space development, the practical application of the capabilities of spacecraft and of the data collected from space, has evolved in parallel with space exploration. There are two general categories of space applications. One provides benefits that are considered public goods—i.e., that cannot easily be marketed to individual purchasers—and thus are usually provided by governments, using public funds. Examples of public-good space applications include meteorology; navigation, position location, and timing; and military and national security uses. The other category of applications provides goods or services that can be sold to purchasers at a profit. These applications are the basis for the commercial development of space by the private sector. Examples of existing commercial space applications include various forms of telecommunications via satellites, remote sensing of Earth's surface, and commercial space transportation. Other applications, such as space tourism, space-based power generation, the manufacture of high-value materials in a microgravity environment, and the commercial development of extraterrestrial resources, may appear in the future.

      Many space applications have both civilian and military uses, and thus similar systems have been developed by both sectors. How to manage and use these dual-purpose systems effectively is a continuing policy issue.

      Meteorologists initially thought that satellites (weather satellite) would be used primarily to observe cloud patterns and thus provide warnings of impending storms. They did not expect space observations to be central to improved weather forecasting overall. Nevertheless, as the technology of space-based instrumentation became more sophisticated, satellites were called upon to provide three-dimensional profiles of additional variables in the atmosphere, including temperature, moisture content, and wind speed. These data have become critical to modern weather forecasting.

      Meteorological satellites are placed in one of two different kinds of orbit. Satellites in geostationary orbit provide continuous images of cloud patterns over large areas of Earth's surface. From changes in those patterns, meteorologists can deduce wind speeds and locate developing storms. Satellites in lower orbits aligned in a north-south direction, called polar orbits, can obtain more detailed data about changing atmospheric conditions. They also provide repetitive global coverage as Earth rotates beneath their orbit. In the United States, military and civilian agencies each have developed independent polar-orbiting meteorological satellite systems; China, Europe, and the Soviet Union also have deployed their own polar-orbiting satellites. The United States, Europe, the Soviet Union, India, and Japan have orbited geostationary meteorological satellites.

      Although the research and development activity needed to produce meteorological satellites has been carried out by various space agencies, control over satellite operation usually has been handed over to organizations with general responsibility for weather forecasting. In the United States the National Oceanographic and Atmospheric Administration (NOAA) operates geostationary and polar-orbiting satellites for short- and long-term forecasting; the Department of Defense (DOD) also has developed similar satellites for military use. Operation of U.S. civilian and military polar satellite programs have been combined under joint NOAA-DOD management. In Europe an intergovernmental organization called Eumetsat was created in 1986 to operate Europe's meteorological satellites and provide their observations to national weather services. Agencies around the world cooperate in the exchange of data from their satellites. Meteorological satellites are an excellent example of both the ability of space systems to provide extremely valuable benefits to humanity and the need for international cooperation to maximize those benefits.

Positioning, navigation, and timing
 In 1957 scientists tracking the first satellite, Sputnik 1, found that they could plot the satellite's orbit very precisely by analyzing the Doppler shift (see Doppler effect) in the frequency of its transmitted signal with respect to a fixed location on Earth. They understood that if this process could be reversed—i.e., if the orbits of several satellites were precisely known—it would be possible to identify one's location on Earth by using information from those satellites.

      This realization, coupled with the need to establish the position of submarines carrying ballistic missiles, led the United States and the Soviet Union each to develop satellite-based navigation systems in the 1960s and early '70s. Those systems, however, did not provide highly accurate information and were unwieldy to use. The two countries then developed second-generation products—the U.S. Navstar Global Positioning System (GPS) (GPS) and the Soviet Global Navigation Satellite System (GLONASS)—that did much to solve the problems of their predecessors. The original purpose of the systems was the support of military activities, and, at the start of the 21st century, they continued to operate under military control.

      GPS requires a minimum of 24 satellites, with four satellites distributed in each of six orbits. Deployment of the full complement of satellites was completed in 1994 and included provision for continual replenishment and updating and the maintenance of several spare satellites in orbit. Each satellite carries four atomic clocks accurate to one nanosecond. Because the satellites' orbits are maintained very precisely by ground controllers and the time signals from each satellite are highly accurate, users with a GPS receiver can determine their distance from each of a minimum of four satellites and, from this information, pinpoint their exact location in three dimensions with an accuracy of approximately 18 metres (59 feet) horizontally and 28 metres (92 feet) vertically. GLONASS, which became operational in 1996, functions on the same general principles as GPS. A fully deployed system would consist of 24 satellites distributed in three orbits. Because of Russia's economic difficulties, however, GLONASS has not been well maintained, and replacement satellite deployment has been slow.

      Notwithstanding the military origin of GPS and GLONASS, civilian users have proliferated. They range from wilderness campers, farmers, golfers, and recreational sailors to surveyors, car-rental firms, bus and truck fleets, and the world's airlines. The timing information from GPS satellites is also used by the Internet and other computer networks to manage the flow of information. Users have found ways to increase the accuracy of position location to a few centimetres by combining GPS signals with ground-based enhancements, and affordable GPS receivers make the system widely accessible. The United States regards GPS as a global utility to be offered free of charge to all users, and it has stated its intent to maintain and upgrade the system into the indefinite future. Concern has been expressed, however, that important worldwide civilian activities such as air traffic control should not depend on a system controlled by one country's military forces. In response, Europe began in the late 1990s to develop its own navigation satellite system, called Galileo, to be operated under civilian control.

Military and national security uses of space
      Those countries and organizations with armed forces deployed abroad were quick to recognize the great usefulness of space-based systems in military operations. The United States, the Soviet Union, the United Kingdom, the North Atlantic Treaty Organization (NATO), and, to a lesser degree, other European countries and China have deployed increasingly sophisticated space systems—including satellites for communications, meteorology, and positioning and navigation—that are dedicated to military uses. In addition, the United States and the Soviet Union have developed satellites to provide early warning of hostile missile launches. Many of these satellites have been designed to meet unique military requirements, such as the ability to operate in a wartime environment, when an opponent may try to interfere with their functioning.

      To date, military space systems have served primarily to enhance the effectiveness of ground-, air-, and sea-based military forces. Commanders rely on satellites to communicate with troops on the front lines, and, in extreme circumstances, national authorities could use them to issue the commands to launch nuclear weapons. Meteorological satellites assist in planning air strikes, and positioning satellites are used to guide weapons to their targets with high accuracy.

      Despite the substantial military use of space, no country has deployed a space system capable of attacking a satellite in orbit or of delivering a weapon to a target on Earth. Nevertheless, as more countries acquire military space capabilities and as regional and local conflicts persist around the world, it is not clear whether space will continue to be treated as a weapons-free sanctuary.

 In addition to recognizing the value of space systems in warfare, national leaders in the United States and the Soviet Union realized early on that the ability to gather information about surface-based activities such as weapons development and deployment and troop movements would assist them in planning their own national security activities. As a result, both countries deployed a variety of space systems for collecting intelligence. They include reconnaissance satellites that provide high-resolution images of Earth's surface in close to real time for use in identifying threatening activities, planning military operations, and monitoring arms-control agreements. Other satellites collect electronic signals such as telephone, radio, and Internet messages and other emissions, which can be used to determine the type of activities that are taking place in a particular location. Most national-security space activity is carried out in a highly secret manner. As the value to national security of such satellite systems has become evident, other countries, such as France, China, India, and Israel, have developed similar capabilities, and still others have begun planning their own systems.

Satellite telecommunications
 Although some early space experiments explored the use of large orbiting satellites as passive reflectors of signals from point to point on Earth, most work in the late 1950s and early '60s focused on the technology by which a signal sent from the ground would be received by satellite, electronically processed, and relayed to another ground station. American Telephone and Telegraph, recognizing the commercial potential of satellite communications (satellite communication), in 1962 paid NASA to launch its first Telstar satellite. Because that satellite, which operated in a fairly low orbit, was in range of any one receiving antenna for only a few minutes, a large network of such satellites would have been necessary for an operational system. Engineers from the American firm Hughes Aircraft, led by Harold Rosen, developed a design for a satellite that would operate in geostationary orbit. Aided by research support from NASA, the first successful geostationary satellite, Syncom 2, was launched in 1963; it demonstrated the feasibility of the Hughes concept prior to commercial use.

      The United States also took the lead in creating the organizational framework for communications satellites. Establishment of the Communications Satellite Corporation ( Comsat) was authorized in 1962 to operate American communications satellites, and two years later an international agency, the International Telecommunications Satellite Organization ( Intelsat), was formed at the proposal of the United States to develop a global network. Comsat, the original manager of Intelsat, decided to base the Intelsat network on geostationary satellites. The first commercial communications satellite, Intelsat 1, also known as Early Bird, was launched in 1965. Intelsat completed its initial global network with the stationing of a satellite over the Indian Ocean in mid-1969, in time to televise the first Moon landing around the world.

      The original use of communications satellites was to relay voice, video, and data from one relatively large antenna to a second, distant one, from which the communication then would be distributed over terrestrial networks. This point-to-point application introduced international communications to many new areas of the world, and in the 1970s it also was employed domestically within a number of countries, especially the United States. As undersea fibre-optic cables improved in carrying capacity and signal quality, they became competitive with communications satellites; the latter responded with comparable technological advances that allowed these space-based systems to meet the challenge. A number of companies in the United States and Europe manufacture communications satellites and vie for customers on a global basis.

      Other space-based communications applications have appeared, the most prominent being the broadcast of signals, primarily television programming, directly to small antennas serving individual households. A similar emerging use is the broadcast of audio programming to small antennas in locations ranging from rural villages in the developing world to individual automobiles. International private satellite networks have emerged as rivals to the government-owned Intelsat, which as of 2001 was itself being transformed into a private-sector organization.

      Yet another service that has been devised for satellites is communication with and between mobile users. In 1979 the International Maritime Satellite Organization (Inmarsat) was formed to relay messages to ships at sea. Beginning in the late 1990s, with the growth of personal mobile communications such as cellular telephone services, several attempts were made to establish satellite-based systems for this purpose. Typically employing constellations of many satellites in low Earth orbit, they experienced difficulty competing with ground-based cellular systems. At the start of the 21st century, the outlook for their economic viability was not good, but other satellite-based personal communication systems, including some based on geostationary satellites, were under development.

      The first commercial space application was satellite communications, and that remained the most successful one. One estimate of revenues associated with the industry for the year 2000 included $16 billion for satellite manufacturing, $18 billion for the associated ground systems, and $40 billion from the users of satellite revenues, for a total of $74 billion. In the same year, there were some 235 commercial geostationary communications satellites around the world, operated by almost 60 different owners.

Remote sensing
      Remote sensing is a term applied to the use of satellites to observe various characteristics of Earth's land and water surfaces in order to obtain information valuable in mapping, mineral exploration, land-use planning, resource management, and other activities. Remote sensing is carried out from orbit with multispectral sensors; i.e., observations are made in several discrete regions of the electromagnetic spectrum that include visible light and usually other wavelengths. From multispectral imagery, analysts are able to derive information on such varied areas of interest as crop condition and type, pollution patterns, and sea conditions.

 Because many applications of remote sensing have a public-good character, a commercial remote-sensing industry has been slow to develop. In addition, the secrecy surrounding intelligence-gathering satellites during the Cold War era set stringent limits on the capabilities that could be offered on a commercial basis. The United States launched the first remote-sensing satellite, NASA's Landsat 1 (originally called Earth Resources Technology Satellite), in 1972. The goals of the Landsat program, which by 1999 had included six successful satellites, were to demonstrate the value of multispectral observation and to prepare the system for transfer to private operators. Despite two decades of attempts at such a transfer, Landsat remained a U.S. government program at the start of the 21st century. In 1986 France launched the first of its SPOT remote-sensing satellites and created a marketing organization, Spot Image, to promote use of its imagery. Both Landsat's and SPOT's multispectral images offered a moderate ground resolution of 10–30 metres (about 33–100 feet). Japan and India also launched multispectral remote-sensing satellites.

      In the 1990s, with the end of the Cold War, some of the technology used in reconnaissance satellites was declassified. This allowed several American firms to begin developing high-ground-resolution (less than one metre [3.3 feet]) commercial remote-sensing satellites. The first commercial high-resolution satellite, called Ikonos 1, was launched by the Space Imaging Company in 1999. Among major customers for high-resolution imagery are governments that lack their own reconnaissance satellites. The global availability of imagery previously available only to the leaders of a few countries is troubling to some observers, who express concern that it could lead to increased military activity. Others suggest that this widespread availability will contribute to a more stable world.

      Remote sensing from space has yet to develop into a viable commercial business. Nevertheless, as users become more familiar with the benefits of combining space-derived data with other sources of geographic information, the possibility of commercial success could improve.

Commercial space transportation
      The prosperity of the communications satellite business was accompanied by a willingness of the private sector to pay substantial sums for the launch of its satellites. As growth in the business slowed at the turn of the 21st century, so did growth in commercial space transportation, because there were few other commercial spacecraft needing access to orbit. Initially, most commercial communications satellites went into space on U.S.-government-operated vehicles. When the space shuttle was declared operational in 1982, it became the sole American launch vehicle providing such services. After the 1986 Challenger accident, however, the shuttle was prohibited from launching commercial payloads. This created an opportunity for the private sector to employ existing expendable launch vehicles such as the Delta, Atlas (Atlas rocket), and Titan (Titan rocket) as commercial launchers. In the 1990s, an American commercial space transportation industry emerged. Whereas the Titan was not a commercial success, the other two vehicles found commercial customers for a number of years.

      Europe followed a different path to commercial space transport. After deciding in the early 1970s to develop the Ariane launcher, it created under French leadership a marketing organization called Arianespace to seek commercial launch contracts for the vehicle. In the mid-1980s, both the U.S.S.R. and China initiated efforts to attract commercial customers for their launch vehicles. As the industry developed in the 1990s, the United States initiated joint ventures with Russia and Ukraine to market their launchers; Europe also created a similar alliance with Russia. China continued to market its Long March series of launch vehicles for commercial use, and other countries hoped to market their indigenous launchers on a commercial basis.

      In the first years of the 21th century, only 20 commercial launches were conducted per year on average, although the launch capacity worldwide was significantly greater.

New commercial applications
      Space advocates have identified a number of possible opportunities for the future commercial use of space. For their economic feasibility, many depend on lowering the cost of transportation to space, an objective that has eluded both governments and private entrepreneurs. Access to low Earth orbit continues to cost thousands of dollars per kilogram of payload—a significant barrier to further space development.

      The International Space Station originally was expected to be the scene of significant commercially funded research and other activity as its laboratories began to operate. This was projected to include both industry-funded microgravity research in ISS laboratories and less-conventional undertakings such as hosting fare-paying passengers, filming movies on the facility, and allowing commercial endorsements of goods used aboard the station. Commercial success for the ISS was predicted to lead to the development of new, privately financed facilities in low Earth orbit, including research, manufacturing, and residential outposts, and perhaps to privately financed transportation systems for access to those facilities. Because of delays in completing the station—particularly after the grounding of the shuttle fleet following the Columbia accident in 2003—such commercial use did not emerge. It seemed unlikely that there would be significant commercial use of the ISS after its completion in 2010 or later.

      Another potential commercial application is the transport of fare-paying passengers into space, known as space tourism. Various surveys have suggested a willingness among many in the general public to spend considerable sums for the opportunity to experience space travel. Although a very limited number of wealthy individuals have purchased trips into Earth orbit at a very high price, large-scale development of the space tourism market will not be possible until less-expensive, highly reliable transportation systems are developed.

      One variant of space tourism is to take fare-paying passengers to the edge of space—generally set at 100 km (62 miles) altitude—for brief suborbital flights that offer a few minutes of weightlessness and a broad view of Earth. In 2004, in response to a prize competition initiated in the late 1990s, a privately funded spacecraft, named SpaceShipOne, became the first of its kind to carry human beings on such flights. This achievement could herald the beginning of a commercial suborbital travel business. Nevertheless, the speed reached by SpaceShipOne was just over three times the speed of sound, roughly one-seventh of the speed required to enter a practical low-Earth orbit.

      As an alternative to existing sources of energy, suggestions have been made for space-based systems that capture large amounts of solar energy and transmit it in the form of microwaves or laser beams to Earth. Achieving this objective would require the deployment of a number of large structures in space and the development of an environmentally acceptable form of energy transmission to create a cost-effective competitor to Earth-based energy-supply systems.

 Resources available on the Moon and other bodies of the solar system represent additional potential objectives for commercial development. For example, over billions of years the solar wind has deposited large amounts of the isotope helium-3 in the soil of the lunar surface. Scientists and engineers have suggested that helium-3 could be extracted and transported to Earth, where it is rare, for use in nuclear fusion reactors. In addition, there is evidence to suggest that the Moon's polar regions contain ice, which could supply a manned lunar outpost with drinking water, breathable oxygen, and hydrogen for spacecraft fuel. Significant quantities of potentially valuable resources such as water, carbon, and nitrogen may also exist on some asteroids (asteroid), and space mining of those resources has been proposed.

Issues for the future
      Space exploration and development have been stimulated by a complex mixture of motivations, including scientific inquiry, intense competition between national governments and ideologies, and commercial profit. Underlying them has been a vision of the outward movement of humans from Earth, ultimately leading to permanent settlements in space or on other celestial bodies. In reality, however, as of the start of the 21st century, only 27 people have traveled beyond Earth orbit, all of them Apollo astronauts during the primarily politically inspired race to the Moon. Whether, and under what conditions, human exploration and settlement of the solar system will resume is a major issue for the future.

      Scientists will continue to seek answers to leading questions about the physical and biological universe through the deployment of increasingly advanced instruments on orbiting satellites and space probes. The principal space-faring countries appear willing to continue their substantial support for space science. The availability of government funding will set the pace of scientific progress.

      The various applications of space capability hold the greatest promise for significant change. If other commercial ventures equal or surpass the success of the satellite communications sector, space could become a major centre of business activity. If governments decide to expand the activity in space of their armed forces, space could become another major military theatre—like the land, sea, and air on Earth—for waging war and deploying weapons. If observing Earth from space becomes crucial for effective planetary management, an assortment of increasingly varied and specialized observation satellites could be launched. Thus, outer space could become a much busier area of human activity in the 21st century than in the first four decades of endeavour there. At some point, it even may become necessary to establish a space traffic-control system analogous to traffic-control systems on Earth.

      The development of space as an arena for multiple government and private activities will pose significant policy and legal challenges. The legal framework for space activities is based on the 1967 Outer Space Treaty and four subsequent United Nations treaties implementing its provisions. These agreements were negotiated at a time when governments were the principal players in space and commercial space activities were in their infancy. Whether they form an adequate and appropriate framework for current and future space activities requires review.

      The Outer Space Treaty prohibits the deployment of weapons of mass destruction in outer space and on celestial bodies. Other treaties have limited some military activities in space, but there is no general framework regulating the military uses of space. The wisdom of developing space weapons—or, alternatively, of limiting their development and keeping space a weapons-free environment—is an issue for discussion and debate.

      To date, the benefits of space exploration and development have accrued mainly to those countries that have financed space activities. The contributions of space to the economic and social development of large regions of Earth have been limited. The Outer Space Treaty identifies space as “the common heritage of mankind.” How to ensure that the benefits of this common heritage are more equitably distributed will be a continuing challenge.

John M. Logsdon

Additional Reading

Broad coverage of space activities can be found in Fernand Verger, Isabelle Sourbès-Verger, and Raymond Ghirardi, The Cambridge Encyclopedia of Space: Missions, Applications, and Exploration (2003). An overall history of space exploration is William E. Burrows, This New Ocean: The Story of the First Space Age (1998). Walter A. McDougall, The Heavens and the Earth: A Political History of the Space Age (1985, reissued 1997), traces the U.S.-Soviet rivalry that led to the space race and comments on its impact on the two countries' societies. Earlier historical discussions include Willy Ley, Rockets, Missiles, and Men in Space, newly rev. and expanded ed. (1968); and Wernher von Braun, Frederick I. Ordway III, and David Dooling, Space Travel: A History, 4th ed. (1985). Frank H. Winter, Rockets into Space (1990), provides an account of the development of rocketry.Speculative discussions of the promises of space exploration include Arthur C. Clarke (compiler and ed.), The Coming of the Space Age: Famous Accounts of Man's Probing of the Universe (1967, reissued 1970); Harry L. Shipman, Humans in Space: 21st Century Frontiers (1989); Carl Sagan, Pale Blue Dot: A Vision of the Human Future in Space (1994, reissued 1997); and Robert Zubrin and Richard Wagner, The Case for Mars: The Plan to Settle the Red Planet and Why We Must (1996). Carl Sagan, Cosmos (1980, reissued 1995), based on Sagan's television series of the same name (1980), discusses the universe and the place of life within it.

History of space programs
Many early American astronauts have written of their experiences. The best of these works is Michael Collins, Carrying the Fire: An Astronaut's Journeys (1974, reissued 2001). An account of the Apollo program that is focused on astronauts is Andrew Chaikin, A Man on the Moon: The Voyages of the Apollo Astronauts (1994, reissued in 3 vol., 1999); this book served as the basis for a video series, From the Earth to the Moon (1998), produced by Tom Hanks. A failed Apollo mission is the subject of the theatrical film Apollo 13 (1995). The best account of Apollo from the perspective of its managers and engineers is Charles Murray and Catherine Bly Cox, Apollo: The Race to the Moon (1989). A noted author offers his impressions of Apollo in Norman Mailer, Of a Fire on the Moon (1970, reissued 1985; also published as A Fire on the Moon, 1970). Tom Wolfe, The Right Stuff, new ed. (1983, reissued 1997), provides an account of the early days of U.S. human spaceflight; the book was turned into a 1983 motion picture of the same name. John M. Logsdon, The Decision to Go to the Moon: Project Apollo and the National Interest (1970, reissued 1976), traces the political underpinnings of the Apollo program. An extensive account of the Soviet space program during the race to the Moon is Asif A. Siddiqi, Challenge to Apollo: The Soviet Union and the Space Race, 1945–1974 (2000).The origins of U.S. post-Apollo spaceflight programs are discussed in T.A. Heppenheimer, The Space Shuttle Decision: NASA's Search for a Reusable Space Vehicle (1999); and Howard E. McCurdy, The Space Station Decision: Incremental Politics and Technological Choice (1990). Events leading to the 1986 Challenger accident are detailed in Joseph J. Trento, Prescription for Disaster (1987). A selective view of U.S.-Russian cooperation in human spaceflight is found in Bryan Burrough, Dragonfly: NASA and the Crisis Aboard Mir (1998, reissued 2000).Available in addition to the works cited above are published studies, sponsored by the NASA History Program, of almost every one of the agency's space programs. Original documents tracing the history of the U.S. space program are reprinted in John M. Logsdon et al. (eds.), Exploring the Unknown: Selected Documents in the History of the U.S. Civil Space Program (1995– ).A comprehensive discussion of European space activities up to 1987 is provided in J. Krige, A. Russo, and L. Sebesta, A History of the European Space Agency 1958–1987, 2 vol. (2000). Roger M. Bonnet and Vittorio Manno, International Cooperation in Space: The Example of the European Space Agency (1994), elaborates on international space activities from a European perspective. Other national space efforts are described in Brian Harvey, The Chinese Space Programme: From Conception to Future Capabilities (1998), The Japanese and Indian Space Programmes: Two Roads into Space (2000), and Russia in Space: The Failed Frontier? (2001).

Applications of space exploration
Discussions of various space science efforts include Homer E. Newell, Beyond the Atmosphere: Early Years of Space Science (1980); Bruce Murray, Journey into Space: The First Three Decades of Space Exploration (1989); and William E. Burrows, Exploring Space: Voyages in the Solar System and Beyond (1990); the last two works deal with U.S. missions to explore the solar system. Also pertinent is Robert W. Smith, The Space Telescope: A Study of NASA, Science, Technology, and Politics (1989, reissued 1993).The origins of reconnaissance satellite programs are covered in Dwayne A. Day, John M. Logsdon, and Brian Latell (eds.), Eye in the Sky: The Story of the Corona Spy Satellites (1998). Subsequent spy satellite programs are discussed in Jeffrey T. Richelson, America's Secret Eyes in Space: The U.S. Keyhole Spy Satellite Program (1990); and William E. Burrows, Deep Black: Space Espionage and National Security (1986, reissued 1988). Early debates over the military use of space are described in Paul B. Stares, The Militarization of Space: U.S. Policy, 1945–1984 (1985); and more recent debates on this issue are summarized in Peter L. Hays et al., Spacepower for a New Millennium: Space and U.S. National Security (2000).Heather E. Hudson, Communication Satellites: Their Development and Impact (1990), gives a synopsis of progress in communications satellites. Controversies surrounding the development of Earth observation satellites are followed in Pamela E. Mack, Viewing the Earth: The Social Construction of the Landsat Satellite System (1990).

Periodicals providing extensive coverage of space issues include the weekly publications Aviation Week & Space Technology and Space News. Jane's Space Directory (annual) also provides up-to-date information on various space activities.John M. Logsdon

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