In 2011, the NASA space mission Dawn will enter orbit around the second largest asteroid: Vesta. This rock is about 530 km (320 miles) across, and is just barely big enough for its own gravity to crush itself into a sphere.
Well, almost. It’s not quite spherical, but it’s close. Since it never gets closer than about 200 million km (120 million miles), we don’t have clear pictures of it. But we need to get data before Dawn enters orbit, so that surprises are kept to a minimum. That’s why Hubble was pointed at Vesta in February, returning some of the best images yet:
I know, they’re still fuzzy, but c’mon! You’re looking at something roughly the size of my home state of Colorado from more than 200 million kilometers away!
And there’s stuff to see. Vesta rotates once every 5.34 hours, and the numbers under each picture represent the rotation angle; think of it like the longitude of Vesta we’re looking at. You can see the variation in color and brightness as it spins: those are real features on the surface of this small world.
Also, in the top row, you can see that one side of Vesta looks flattened. That’s real too: it’s the edge of a huge (450 km (270 miles) diameter) impact crater covering a lot of the asteroid. It got hit with something very large, probably about a billion years ago. Debris exploded outward, and some of it even fell to Earth. Using spectroscopic analysis of the meteorites and of Vesta the two could be tied together, and the age of the meteorites tell us when the impact must have occurred. Also, some other asteroids are similar enough to Vesta that we think they were debris from the event as well. Analysis of their orbits also indicate an age of about a billion years.
So actually, we know quite a bit about the asteroid. But images like this nail down things like its spin axis, which is important to know when putting a probe in orbit. As the spacecraft circles the asteroid, the tug it feels from gravity changes as the shape of the asteroid below changes. If we can nail down the spin axis (the north and south poles) we can be more confident about how the probe will behave while it orbits. And we can also compare the spacecraft’s motion with models made with images like this to improve our models, so future spacecraft can benefit too. And a final bonus is that with the spin understood better, we’ll have a better grasp of the lighting on the surface from the Sun, making interpreting the images returned from Dawn easier.
All in all, like the ones from comet Hartley 2, these images are very important for spacecraft visiting other worlds. The close up pictures and data we get are incredible, but they’re helped a lot by the fleet of observatories we keep back home, too.