Man, Vesta is weird.
It's a 500 km (300 mile) wide asteroid, the second biggest, so its gravity should be strong enough to crush it into a sphere. But it's not a ball; it's lumpy and stretched out and, weirdest of all, has an enormous circular depression at its south pole which flattens that entire hemisphere of the little world.
Here's a recent image taken by the Dawn spacecraft, looking down on Vesta's strange southern region:
[Click to envestanate.]
You can tell it's not round! It's like looking up at a pancake held over your head. The Sun angle is such that the left hand side of the rock is in darkness, which actually helps with giving the image some relief and perspective.
My first assumption is that something really big hit Vesta a long time ago, carving out that basin. Central peaks are common in events like that, smack dab in the middle of the crater (like the gorgeous mountain in the Moon's Tycho crater), and we see one here, too -- that dark spot in the middle, more obvious in an earlier image from Dawn. It's huge, many kilometers across, but like the asteroid itself it's weird: it's really round and smooth, not jagged and sharp like most central peaks. I was at first surprised to find out some scientists are wondering if the basin was from an impact (the usual suspect) or if it were due to some internal process... but then, looking again at the basin and mound, they're strange enough that maybe sometimes you need to think a little differently.
In a few months, Dawn will drop in from its current 2700 km orbit down to just a few hundred kilometers. When it does, the sharpness of these images will improve dramatically. I hope we get enough information to solve some of the questions about this rock (some detailed commentary on the geology of Vesta has been posted at the DLR website). We have samples of Vesta thanks to meteorites we think came from collisions to the asteroid, so having images and other data to investigate it up close and tie in what we know from the samples will improve our knowledge of this world immensely.
Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA