Vesta is the asteroid belt's second largest object, but figuring out what it is has driven astronomers crazy. Is it an asteroid? A planet, perhaps a protoplanet? Whatever the answer, Vesta might just be the solar system's most important object.
With a violent history, a watery origin, and a diameter of 256 kilometers, 2 Pallas is one seriously badass asteroid. In fact, new images of it from the Hubble Telescope have led researchers to dub it a protoplanet.