On July 7, NASA is scheduled to launch the spacecraft on a four-year journey to the asteroid belt. Once there, Dawn will do some asteroid-hopping, going into orbit around Vesta in 2011 and Ceres in 2015. Dawn will be the first spacecraft to orbit two targets. At least 100,000 asteroids inhabit the asteroid belt, a reservoir of leftover material from the formation of our solar-system planets 4.6 billion years ago.
Dawn also will be the first satellite to tour a dwarf planet. The International Astronomical Union named Ceres one of three dwarf planets in 2006. Ceres is round like planets in our solar system, but it does not clear debris out of its orbit as our planets do.
To date, NASA has flown by only a few small asteroids ( up to a few dozen kilometers in size). Those bodies typically look like oddly shaped giant rocks and are fairly uniform in composition.
At almost 1000 km across, Ceres is the largest object in the asteroid belt and is roughly spherical. Vesta is about half that size, but is one of the most heterogeneous objects in the belt with a differentiated crust and mantle.