Last January a military telescope detected a bizarre object hurtling around the asteroid belt, leaving a long tail of dust. UCLA planetary scientist David Jewitt took one look and said, “Asteroid collision.” The millions of rocky objects orbiting between Mars and Jupiter probably collide all the time, but this is the first instance in which astronomers have seen direct evidence of an impact.
Jewitt and his colleagues watched the object and its fading, X-shaped tail for five months with the Hubble Space Telescope. The most likely scenario, he says, is that an asteroid just 10 or 20 feet wide struck the larger object, called P/2010 A2, which measures almost 400 feet across. (A less likely alternative is that P/2010 A2 is a solitary asteroid rotating so quickly that it flings off dust, producing the tail.) The speed and location of debris suggest that the crash happened in February or March 2009 at more than 11,000 miles per hour.
The find should help astronomers determine how much dust in the solar system originates in asteroid collisions; such impacts may also create fragments that reach Earth as small meteorites. It could also open a whole new field of study. Now that people know what an asteroid smashup looks like, “I’d be quite surprised if someone doesn’t find another one this year,” Jewitt says.