After a three year, 117 million mile trip out to the asteroid belt between Mars and Jupiter, NASA's Dawn spacecraft has entered orbit around a 300-mile-wide asteroid named Vesta, snapping the clearest picture ever of what some scientists say might actually be a tiny little protoplanet.
You may remember Dawn for its futuristic DS1 xenon ion thrusters, which are the engines that the spacecraft is using to tool around the solar system. They're not very powerful, but they're so efficient that Dawn can get from Earth to Vesta, enter Vesta's orbit, leave Vesta's orbit, and then head over to orbit another (even larger) asteroid called Ceres by 2015. And it'll still have fuel left over.
But we're getting ahead of ourselves. Currently orbiting at about 26,000 miles above Vesta's surface, Dawn will slowly spiral down to a mere 1,700 miles, taking better and better pictures along the way. The spacecraft will also take multispectral measurements to figure out what Vesta is made of, and how it manages to be three times as bright as our moon is.
While Vesta is generally thought of as an asteroid, it's more than just some random hunk of rock. Scientists think that at some point it had a mantle and a liquid core, and it may even have had volcanoes. There's a group of meteorites that we're pretty sure came from Vesta, and they show evidence of igneous processes, which is something that you only get on what people generally think of as planets. Dawn's next stop, Ceres, also falls into this protoplanet or dwarf planet category, but it's much different than Vesta, being icy instead of rocky.
Being able to compare these two objects directly is going to be a pretty big deal for astrogeologists everywhere, and should go a long way towards helping us understand what went on as our solar system was forming.