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Asteroid Scheila awakened by collision
Posted: 02 May 2011

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Last year’s brightening and outbursting of 112 kilometre-wide asteroid Scheila was likely sparked by a collision with a much smaller asteroid, say astronomers studying data from NASA’s Swift and Hubble space telescopes.

“The Hubble data are most simply explained by the impact, at 11,000mph [17,700 km/h] of a previously unknown asteroid about 100 feet [30 metres] in diameter,” says Hubble team leader David Jewitt at the University of California.

Faint dust plumes bookend asteroid (596) Scheila, which is overexposed in this composite. 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. Image: NASA/Swift/DSS/D. Bodewits (UMD)

Last year, the aftermath of the first asteroid-asteroid collision was spotted by Hubble (read our report here), but unlike the observations of P/2010 A2, which turned out to have resulted from an impact many months previously (see here for more), the impact was Scheila was captured just weeks after the collision.

The new results add to the growing body of evidence that, when triggered by an impact, seemingly quiescent asteroids can show characteristics much more like a comet, displaying “tails” of material exposed during the impact.

The Hubble Space Telescope imaged (596) Scheila on Dec. 27, 2010, when the asteroid was about 218 million miles away. Scheila is overexposed in this image to reveal the faint dust features. The asteroid is surrounded by a C-shaped cloud of particles and displays a linear dust tail in this visible-light picture acquired by Hubble's Wide Field Camera 3. Because Hubble tracked the asteroid during the exposure, the star images are trailed. Image: NASA/ESA/D. Jewitt (UCLA).

Interest in asteroid Schiela began on 11 December 2010, when observations conducted by the University of Arizona’s Catalina Sky Survey revealed the asteroid to be twice as bright as previously seen. Back-tracking through archival data suggested that the outburst had begun sometime between 11 November and 3 December. Follow-up observations by Swift’s Ultraviolet/Optical Telescope (UVOT) did not detect any emissions of water or gas typical of icy emissions associated with comets. Instead, the plumes were found to be composed of dust ejected in the impact and subsequently pushed away from the asteroid by sunlight. Hubble observations show that two dust plumes – a bright one to the north of the asteroid and a fainter one to the south – faded between observations at the end of 2010 and early 2011.

The observations, say the two teams, are best explained by a collision of a small asteroid striking Scheila’s surface at an angle of less than 30 degrees to the horizontal, which would explain the presence of two dust plumes; a vertical impact would not have produced the same plume pattern. The astronomers also estimate that the impact carved a crater around 300 metres across, spewing over 660,000 tons of dust into its local environment.

“The dust cloud around Scheila could be 10,000 times as massive as the one ejected from comet 9P/Tempel 1 during NASA’s UMD-led Deep Impact mission,” says co-author Michael Kelley 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.”