NASA's Dawn spacecraft officially began its interplanetary eight-year cruise to see the largest space rocks in the solar system.

The spacecraft is firing its ion propulsion engines nonstop to travel three billion miles and reach the asteroid Vesta in 2011 and the dwarf planet Ceres in 2015.

Mission goals include looking at asteroid features such as shape, elemental and mineral composition, and gravity fields.

An earlier checkout phase of the mission tested Dawn's ion drive at different throttle levels. The ion drive shoots charged particles out at 90,000 miles per hour to slowly accelerate the spacecraft, and draws on 937 pounds of xenon propellant as fuel.

The Dawn mission launched on Sept. 27 from Cape Canaveral, Florida.