Ceres and Vesta

Dawn will investigate two different and complementary protoplanets, Ceres and Vesta, that have remained intact since their formation. Dawn will measure their mass, shape, volume and spin rate with both imagery, laser altimetry and gravity. Dawn records the protoplanets' remanent magnetization, and elemental and mineral composition to determine their thermal history and evolution and provides context for meteorites (asteroid samples already in hand). Dawn images Ceres and Vesta's surfaces to determine their bombardment and tectonic history, uses gravity, spin state and magnetic data to limit the size of any metallic core, and infrared and gamma ray spectrometry to search for water-bearing minerals.

What we already know about Ceres...

Ceres is the first asteroid discovered and the largest known asteroid, measuring approximately 950 km (590 miles) and lies in the Asteroid Belt between Mars and Jupiter (see asteroids), at an average distance from the sun of 3.5 A.U, revolving around the Sun in 4.6 terrestrial years.
Ceres was first discovered by Giuseppe Piazzi of the Palermo Observatory on Jan. 1, 1801 and named after the Roman goddess of agriculture. Additional observations by Piazzi were cut short due to illness. Carl Friedrich Gauss, at the age of 24, was able to solve a system of 17 linear equations to determine Ceres' orbit and to allow it to be rediscovered, a remarkable feat for this time. As a result within one year of its initial discovery, both Heinrich Olbers and Franz von Zach were able to relocate Ceres.

...and about Vesta