Vesta is the last protoplanet with a differentiated interior. This means that if we were to take a chunk out of the asteroid, we'd see that it has a crust, mantle and core, just like Earth.
This is very exciting and very important, because the Earth was most likely formed from a protoplanet similar to Vesta. There are estimated to have been a few hundred bodies like Vesta, which eventually (over the course of 100 million years) collided due to perturbations in their orbits and formed the dominant 8 planets we know today. (Before you say "what about Pluto?!", hang on! I'll address our lonely little friend in my next post.)
The Dawn mission objective is to visit Vesta (done!) and Ceres, a dwarf planet in the asteroid belt. These two bodies are useful because of some specific characteristics.
- Vesta has highly magnetized rocks. These, along with a differentiated interior, could help explain what starts a planetary dynamo. This is the process that creates a magnetic field, which in turn helps shield a planet from solar winds. (Earth has a very strong magnetic field, while Mars has a very weak one.)
- Ceres is believed to have a layer of surface or sub surface water ice, as well as a thin but permanent atmosphere.
These two bodies present incredible insights into the formation of worlds like Earth. It could help us understand our own origins, and possibly predict what we'll encounter around stars similar to our own.
Sources: NASA Dawn Mission
BBC News, "Dawn probe leaves Asteroid Vesta"