The Dawn spacecraft entered orbit around the main belt asteroid Vesta just a few weeks ago, and images are coming back in dribs and drabs. NASA just released this fantastic one, taken by Dawn's wide-angle camera:
[Click to protoplanetate.]
Pretty cool, eh? Vesta is about 500 km (300 miles) across, so you're seeing about half the rock from top to bottom here. The most obvious thing are the two ginormous craters. Note the scale bar; the bottom crater is about 70 km across, and the top one about 50. The fact that they nearly overlap, and are clearly the two biggest features for a big area around them, makes me think it was a double impact. Many asteroids are binary, so two objects a few kilometers in size and orbiting each other 50 or more kilometers apart would do the trick*. [Update: Emily Lakdawalla agrees.]
The bottom crater is weird; the bottom right edge looks like it's collapsed a bit, marring the near perfect circle of the rim. That feature itself looks like an arc of a circle; might there have been a third rock that hit? That seems unlikely, and I have a hard time believing even a piece of the rim of a previously existing crater would've survived the impact!
Also, look around the two craters. See how far away from them, the surface is saturated with smaller impact craters? Near the big two, though, there are fewer. It's a sure bet the impacts threw out a lot of debris which blanketed the area. The escape velocity of Vesta is a meager 350 meters/sec (about 750 mph); a lot of the stuff blown out on impact would've been moving faster than that! So some would've escaped the asteroid entirely, but some would've settled down over hundreds of square kilometers of area around the site.
Both crater floors have a filled-in appearance. No doubt a lot of energy of the impact went into melting the surface, which flowed inward. Some of that might be slumped material from the crater edges, too. My knowledge of how craters form is limited, especially on asteroids. But I'd love to see high-resolution images of this! That would answer a lot of these questions straight away.
And of course, we'll be seeing those soon. Dawn is slowly moving down toward Vesta, gradually lowering its height until it settles into its final orbit. At that point, we'll be getting really high-res shots, and maybe a few enigmas will be solved... only to be replaced, no doubt, by ten times as many. Fun!
[Edited to add: there is that biggish third crater at the top, of course, but I'm not sure it's related to the other two. It's much smaller, for one thing, and there are several other craters that size nearby that appear unrelated. It has a softer rim, implying greater age due to erosion (meteorite impacts), and isn't aligned with them -- though the sharp curvature of the asteroid makes that difficult to verify; in some images they look more colinear (that is, aligned).]
Image credit: Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
* At first the size of the impactors was a guess, but then I poked around and found this crater diameter calculator. I put in values for Vesta, and found that the impactors would've been roughly 5 km across. My instincts, sometimes, are good.