[meteorite-list] NASA's Swift And Hubble Probe Asteroid Collision Debris

[Ron Baalke]

April 28, 2011

Trent J. Perrotto 
Headquarters, Washington 
202-358-0321 
trent.j.perrotto at nasa.gov 

Lynn Chandler 
Goddard Space Flight Center, Greenbelt, Md. 
301-286-2806 
lynn.chandler-1 at nasa.gov 

RELEASE: 11-128

NASA'S SWIFT AND HUBBLE PROBE ASTEROID COLLISION DEBRIS

WASHINGTON -- Late last year, astronomers noticed an asteroid named 
Scheila had unexpectedly brightened, and it was sporting short-lived 
plumes. Data from NASA's Swift satellite and Hubble Space Telescope 
showed these changes likely occurred after Scheila was struck by a 
much smaller asteroid. 

"Collisions between asteroids create rock fragments, from fine dust to 
huge boulders, that impact planets and their moons," said Dennis 
Bodewits, an astronomer at the University of Maryland in College Park 
and lead author of the Swift study. "Yet this is the first time we've 
been able to catch one just weeks after the smash-up, long before the 
evidence fades away." 

Asteroids are rocky fragments thought to be debris from the formation 
and evolution of the solar system approximately 4.6 billion years 
ago. Millions of them orbit the sun between Mars and Jupiter in the 
main asteroid belt. Scheila is approximately 70 miles across and 
orbits the sun every five years. 

"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. Hubble did not see any discrete collision fragments, 
unlike its 2009 observations of P/2010 A2, the first identified 
asteroid collision. 

The studies will appear in the May 20 edition of The Astrophysical 
Journal Letters and are available online. 

Astronomers have known for decades that comets contain icy material 
that erupts when warmed by the sun. They regarded asteroids as 
inactive rocks whose destinies, surfaces, shapes and sizes were 
determined by mutual impacts. However, this simple picture has grown 
more complex over the past few years. 

During certain parts of their orbits, some objects, once categorized 
as asteroids, clearly develop comet-like features that can last for 
many months. Others display much shorter outbursts. Icy materials may 
be exposed occasionally, either by internal geological processes or 
by an external one, such as an impact. 

On Dec. 11, 2010, images from the University of Arizona's Catalina Sky 
Survey, a project of NASA's Near Earth Object Observations Program, 
revealed Scheila to be twice as bright as expected and immersed in a 
faint comet-like glow. Looking through the survey's archived images, 
astronomers inferred the outburst began between Nov. 11 and Dec. 3. 

Three days after the outburst was announced, Swift's 
Ultraviolet/Optical Telescope (UVOT) captured multiple images and a 
spectrum of the asteroid. Ultraviolet sunlight breaks up the gas 
molecules surrounding comets; water, for example, is transformed into 
hydroxyl and hydrogen. But none of the emissions most commonly 
identified in comets, such as hydroxyl or cyanogen, show up in the 
UVOT spectrum. The absence of gas around Scheila led the Swift team 
to reject scenarios where exposed ice accounted for the activity. 

Images show the asteroid was flanked in the north by a bright dust 
plume and in the south by a fainter one. The dual plumes formed as 
small dust particles excavated by the impact were pushed away from 
the asteroid by sunlight. Hubble observed the asteroid's fading dust 
cloud on Dec. 27, 2010, and Jan. 4, 2011. 

The two teams found the observations were best explained by a 
collision with a small asteroid impacting Scheila's surface at an 
angle of less than 30 degrees, leaving a crater 1,000 feet across. 
Laboratory experiments show a more direct strike probably wouldn't 
have produced two distinct dust plumes. The researchers estimated the 
crash ejected more than 660,000 tons of dust -- equivalent to nearly 
twice the mass of the Empire State Building. 

"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," 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." 

NASA's Goddard Space Flight Center in Greenbelt, Md., manages Hubble 
and Swift. Hubble was built and is operated in partnership with the 
European Space Agency. Science operations for both missions include 
contributions from many national and international partners. For more 
information, video and images associated with this release, visit: 

http://www.nasa.gov/topics/universe/features/asteroid-collision.html 
	
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