Spacewatch: Asteroid collisions

Watch some science fiction movies, and you may think that any interplanetary traveller foolish enough to venture into the asteroid belt would be in for a wild, bumpy ride. The reality is more prosaic. Although our Sun's asteroid belt may hold more than a million asteroids of at least 1km wide, space is big; the chances of hitting one by accident, or having to dodge out of its way, are remote. Those encounters between spacecraft and asteroids that have occured have had to be carefully targeted.

Yet collisions between asteroids must occur. Given low relative speeds, they might stick together; more likely, debris and fragments will spill outwards, creating interplanetary dust and perhaps new asteroids. Just such an event probably created a curious object, P/2010 A2, that was thought to be a comet when it was discovered early this year. Images by Hubble and the Rosetta spacecraft show a tail of dust streaming away from an X-shaped "head", now taken as the spatter pattern from a collision between two small asteroids in about February 2009.

The much larger asteroid Scheila, about 113km wide, and discovered back in 1906 may have been hit by an unseen smaller body. Observations last weekend show that it has just sprouted fuzzy curling tufts that make it look more like a comet. Are the tufts the debris from a collision, or could it be that Scheila was a dormant giant comet all along? And if it is really a comet, did a collision trigger its outburst?