There is really cool geology being explored on large, oddly shaped asteroids. The Near Earth Asteroid Rendezvous (NEAR) mission reached 433 Eros in 2000, and one of the exciting results was the discovery of features called "ponds".
253 Mathilde is the largest asteroid that has ever been visited by a spacecraft. It's held that distinction for more than twelve years, but next year it'll be upstaged by the considerably larger 21 Lutetia, which Rosetta will fly by on July 10.
Let's Change the World
Become a member of The Planetary Society and together we will create the future of space exploration.