NASA's Dawnspacecraft officially began its interplanetary eight-year cruise to see thelargest space rocks in the solar system.
Thespacecraft is firing its ion propulsion engines nonstop to travel three billionmiles and reach the asteroid Vesta in 2011 and the dwarf planet Ceres in 2015.
Mission goals include looking at asteroidfeatures such as shape, elemental and mineral composition, and gravity fields.
An earliercheckout phase of the mission tested Dawn's ion drive at different throttlelevels. The ion drive shoots charged particles out at 90,000 miles per hour to slowlyaccelerate the spacecraft, and draws on 937 pounds of xenon propellant as fuel.
The Dawnmission launched on Sept. 27 from Cape Canaveral, Florida.
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