Objects Impacted At About 11,000 MPH
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
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