Asteroid-Bound Probe Zooms Past Mars

This story was updated at 9:55 a.m. EST.

NASA?s Dawn asteroid probe zoomed past the planet Mars late Tuesday to grab a speed boost aimed at flinging it on toward the largest space rocks in the solar system.

The ion-powered spacecraft used the gravitational pull of Mars to slingshot around the planet and hurtle outward toward its next stop, the asteroid Vesta. The maneuver was expected to boost Dawn?s speed by more than 5,800 mph (9,330 kph) and set the asteroid probe on track to reach Vesta in August 2011.

"Without the gravity assist, our mission would not have been affordable, even with the extraordinary capability of the ion propulsion system," said Marc Rayman, chief engineer for Dawn?s mission at NASA?s Jet Propulsion Laboratory in Pasadena, Calif.

Dawn uses electricity generated by its solar arrays to charge a stream of xenon gas particles and fires the resulting ions out a nozzle to slowly build up speed as it flies through space. Ion engines allow spacecraft to fly farther using less fuel.

Tuesday?s Mars flyby was expected to give Dawn a power boost equivalent to about 230 pounds (104 kg) of xenon fuel. For comparison, Dawn?s current setup allows it to fire its engines for more than 24 hours while consuming about 9 ounces ( 0.26 kg) of fuel.

Dawn is expected to fire its engine for about 50,000 hours - more than any other spacecraft - by the end of its mission. Japan?s asteroid probe Hayabusa, which reignited its xenon engine last month, has fired its ion drive for more than 30,000 hours during its mission to the space rock Itokawa. Hayabusa is slowly returning to Earth to deliver a capsule that may contain samples of the Itokawa asteroid.

Next stop, Vesta

Launched in 2007, Dawn is making its way toward Vesta and Ceres as part of a mission to better understand the formation of the solar system. The 3 billion-mile (4.9 billion-km) trip is expected to take about eight years, with Vesta, the probe?s first target, still more than 2 1/2 years away.

Vesta is a large oblong asteroid with a rocky composition. Ceres, meanwhile, is round and the largest space rock in the asteroid belt. About the size of Texas, it actually qualifies as a small dwarf planet. Dawn carries instruments to hunt for water-bearing minerals, as well as measure the shape, surface topography, tectonic history, elemental and mineral composition of both targets. It is also expected to measure their masses and gravity fields.

But first, Dawn has to actually reach Vesta and Ceres, and to do that it needed the speed boost from Tuesday?s flyby.

The spacecraft made its closest approach past Mars 7:28 p.m. EST (0028 Feb. 18 GMT), when it zoomed within 341 miles (549 km) of the red planet. Mission scientists planned to use the rendezvous to test out Dawn?s cameras and other instruments as a sort of dress rehearsal for the Vesta encounter.

But there is a downside to Dawn?s swing past Mars. Just as the probe nabbed a speed boost from the planet, the encounter slowed Mars by a tiny fraction, mission managers said.

"The flyby will cause Mars to slow in its orbit enough that after one year, its position will be off by about the width of an atom. If you add that up, it will take about 180 million years for Mars to be out of position by one inch (2.5 cm),? Rayman said. ?We appreciate Mars making that sacrifice so Dawn can conduct its exciting mission of discovery in the asteroid belt."

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