Astronomers mostly spend their time looking at light streaming from events that happened thousands and even millions of years ago. But this year they got to look at something almost as it was happening -- two asteroids that slammed into each other in the asteroid belt between Mars and Jupiter in our own solar system.

The object, dubbed P/2010 A2, was originally defined as a comet when it was first spotted on Jan. 6, 2010 But the more UCLA astronomer David Jewitt looked, the less it seemed like a comet. Its 'tail' was made up of chunks of rock too big for a comet and it glowed. And it seemed to be in a more circular orbit around the sun, rather than the long ellipses that comets usually have.

He put out a call to astronomers around the world to train their cameras on it. The call was answered by the Hubble space telescope. In Germany, Colin Snodgrass at the Max Planck Institute for Solar System Research in Katlenburg-Lindau, also started watching the "comet," with a camera on the European Space Agency's Rosetta spacecraft, which happened to be heading towards the asteroid belt.

Comparing notes and photos, they confirmed not only that the 'comet' wasn't a comet at all, but the remnants of a crash, but also that it happened on Feb. 10, 2009.

Their paper appears this week in the journal Nature.

Seeing thing in almost real time is exciting, but it also gives astronomers very useful data. For example, they had thought that the dust that pervades our solar system might be made up of the leftovers of similar asteroid collisions. But looking at P/2010 A2 and the tiny cloud of chunks it produced, they'll have to go back to the drawing board to figure out where it came from.

By Elizabeth Weise