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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