dawn

—dawnlike, adj.

/dawn/, n.

1. the first appearance of daylight in the morning: Dawn broke over the valley.

2. the beginning or rise of anything; advent: the dawn of civilization.

v.i.

3. to begin to grow light in the morning: The day dawned with a cloudless sky.

4. to begin to open or develop.

5. to begin to be perceived (usually fol. by on): The idea dawned on him.

[bef. 1150; ME dawen (v.), OE dagian, deriv. of daeg DAY; akin to ON daga, MD, MLG dagen, OHG tagen]

Syn. 1. daybreak, sunrise. 5. appear, occur, break.

Ant. 1. sunset.

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▪ United States satellite

      U.S. satellite, designed to orbit the large asteroid Vesta and the dwarf planet Ceres. Dawn was launched Sept. 27, 2007, and will fly past Mars on Feb. 4, 2009, to help reshape its trajectory toward the asteroid belt. Dawn is scheduled to arrive at Vesta on Aug. 14, 2011, and to orbit Vesta until May 22, 2012, when it will leave for Ceres. It will arrive at Ceres on Feb. 1, 2015. Vesta and Ceres exemplify planetary evolution from early in the history of the solar system.

      Dawn is powered by three xenon- ion thrusters that are based on those of the U.S. Deep Space 1 satellite and that continuously produce 92 millinewtons (0.021 pound) of thrust. The xenon thrusters provide the cruise thrust to get the spacecraft from Earth to Ceres and Vesta, but more powerful hydrazine thrusters will be used for orbital insertion and departure.

      The primary science instruments are two identical 1,024 × 1,024-pixel cameras provided by four German agencies and universities. A filter wheel can pass white light or select one of seven bands from the near-ultraviolet to the near-infrared. A series of imaging tests using star fields as targets has demonstrated that the cameras operate as planned.

      The Visible and Infrared Mapping Spectrometer, provided by the Italian National Institute of Astrophysics, is based on an earlier instrument that is on board the European Space Agency satellite Rosetta. This spectrometer will assay minerals and other chemicals on Vesta and Ceres based on what they absorb from incident sunlight. The Gamma Ray/Neutron Spectrometer developed by the U.S. Los Alamos National Laboratory will also assay surface chemistry by measuring radiation from the Sun that is scattered back into space. In particular, it will measure abundances of oxygen, silicon, iron, titanium, magnesium, aluminum, and calcium—all key to the makeup of planetary (planet) bodies—and of trace elements such as uranium and potassium.

Dave Dooling, Jr.

 

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