Asteroid-bound Spacecraft Cuts Engines, Coasts Awhile

NASA's Dawn spacecraft shut down its ion propulsion system yesterday as scheduled, NASA announced.

The spacecraft is now gliding toward a Mars flyby in February. Then it will sail on to the asteroid Vesta.

"Dawn has completed the thrusting it needs to use Mars for a gravity assist to help get us to Vesta," said Marc Rayman, Dawn's chief engineer, of NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Dawn will now coast in its orbit around the sun for the next half a year before we again fire up the ion propulsion system to continue our journey to the asteroid belt."

The high-tech engines (there are three) might be fired up in January to provide final orbital adjustments prior to its encounter with the Mars. In June, the engines will be turned on to propel Dawn toward asteroid Vesta and later the dwarf planet Ceres.

The whole journey will be 3 billion miles (4.9 billion kilometers) and take 8 years. Dawn launched in 2007.

The ion engines use light thrust that gradually speeds the craft over long periods of time. The setup generates more than 24 hours of thrusting while consuming about .26 kilograms (about 9 ounces) of the spacecraft's xenon fuel supply -- less than the contents of a can of soda. Over the course of the mission, the engines will fire cumulatively for about 50,000 hours -- more than any previous craft.

Dawn will begin its exploration of asteroid Vesta in 2011 and the dwarf planet Ceres in 2015. It will measure shape, surface topography, tectonic history, elemental and mineral composition, and will seek out water-bearing minerals. In addition, the spacecraft's orbit around each space rock will reveal the celestial bodies' masses and gravity fields.

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