Faint dust plumes bookend asteroid 596 Scheila, which is overexposed in this composite image. Visible and ultraviolet images from Swift's UVOT (circled) are merged with a Digital Sky Survey image of the same region. The UVOT images were acquired on Dec. 15, 2010, when the asteroid was about 232 million miles from Earth. Credit: NASA/Swift/DSS/D. Bodewits (UMD)
Hubble then took a look at the asteroid and spotted two plumes, a bright dust plume to the north of the asteroid and a fainter one to the south. Astronomers surmized that the two plumes were most likely caused by a collision with a smaller asteroid striking Scheila's surface at an angle of "less than 30 degrees, leaving a crater 1,000 feet [300 meters] across," according to the press release.
"The Hubble data are most simply explained by the impact, at 11,000 mph, of a previously unknown asteroid about 100 feet in diameter," said Hubble team leader David Jewitt at the University of California in Los Angeles.
The collision would have blasted 660,000 tons of dust into space (nearly twice the mass of the Empire State Building), producing the comet-like coma surrounding the asteroid.
"The dust cloud around Scheila could be 10,000 times as massive as the one ejected from comet 9P/Tempel 1 during NASA's Deep Impact mission," said co-author Michael Kelley, also at the University of Maryland. "Collisions allow us to peek inside comets and asteroids. Ejecta kicked up by Deep Impact contained lots of ice, and the absence of ice in Scheila's interior shows that it's entirely unlike comets."
Only last year, Hubble was used to take a closer look at a bizarre "X"-shaped formation in the asteroid belt. Again, it was assumed that the shape may have been caused by a main-belt comet or an asteroid-comet hybrid. In actuality, the shape was caused by an energetic collision between two asteroids, kicking up a tail of debris being swept back by solar wind pressure.
It is estimated that, on average, astronomers should be able to detect one asteroid collision per year, so the Scheila collision appears to confirm this.
Note: It's worth checking out the NASA video accompanying this news, it includes a rather familiar Star Wars scene...