Speaking of asteroid collisions, in January 2010 the automated skywatching telescope LINEAR spotted what looked like a comet orbiting the Sun in the inner asteroid belt, just beyond the orbit of Mars. It looked decidedly odd, so Hubble was pointed at it... and what it saw was so bizarre it caused a big stir in the astronomical community: the aftermath of a violent collision between two asteroids in space!
is unprecendented: an X-shaped streak of light with a dramatic 50,000-kilometer-long tail sweeping away. Apparently, an asteroid roughly 150 meters or so across - which you can see as a point of light at the upper left tip of the X - was hit at high speed by a smaller rock only a few meters across. The smaller object was vaporized by the energy of collision, which would've had the same yield as an atomic explosion!
The tail is from sand-grain to pebble-sized debris from the explosion moving away due to pressure of sunlight, which acts like a very gentle wind on the particles. The other line of the X is probably from a piece of rubble ejected off the main rock, leaving its own trail of debris behind. Judging from changes in the debris shape over time, the collision probably happened in February or March 2009
, but the asteroid was up during the day at that time and was unobservable. It took several months to discover it, and a few more to understand what this strange object was telling us.
Collisions like this are estimated to happen roughly once per year in the asteroid belt, but the distance makes them very hard to observe. Hopefully, as more survey telescopes come online, we'll see more of these spectacular events. Get the higher-res version here.Image credit: NASA, ESA, and D. Jewitt (UCLA)