March 11 NASA has selected the proposed Dawn Discovery mission for a concept study leading to a Step 2 proposal. Dawn uses solar electric propulsion to fly to both Vesta and Ceres orbiting each for a period of nine months. Vesta is a dry, differentiated asteroid with a basaltic crust and is the presumed parent body of the HED meteorites.
Ceres appears to be wet and have less distinct features.
It has no known associated meteorites. It has been postulated that Vesta accreted dry and Ceres wet and that the water kept Ceres cool enough to avoid differentiation. Thus these two asteroidal neighbors represent two quite different end members of solar system evolution. The top level science objectives, the measurement suite, and the science team were approved during the Step 1 selection process. Over the next several months the Dawn team will demonstrate to NASA that we can safely achieve those objectives within the constraints of the Discovery program.