If the weather cooperates and there are no technical glitches, the Dawn spacecraft will launch aboard a Delta II about 7:20 AM Eastern Time (4:20 Pacific Time) on Thursday for a long voyage to visit Vesta and Ceres, two asteroids who between them account for about 1/3 of the mass of the rubble that makes up the asteroid belt between Mars and Jupiter. Dawn will reach Vesta in 2011 and spend 10 months there before voyaging on to Ceres, which it will reach in 2015. Ceres is interesting because observations from Hubble indicate it has a thin atmosphere and a thick layer of water ice beneath its surface. If that turns out to be true, Ceres may contain nearly six times as much fresh water as Earth (in frozen form). Ceres is large enough that it is now classified as a “dwarf planet” like Pluto.
The Dawn spacecraft is interesting because it will be using ion propulsion on its long trek (once it leaves Earth orbit), rather than chemical rockets. Dawn is also notable because it will be the first spacecraft to enter into orbit around two different planetary bodies other than the Earth and Moon.
Visit NASA’s Dawn Mission Home Page.