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NASA's Swift and Hubble probe asteroid collision debris

This is the first time scientists have been able to catch an asteroid fragment just weeks after the smash-up, long before the evidence fades away.
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 a smaller asteroid struck Scheila.

“Collisions between asteroids create rock fragments, from fine dust to huge boulders, that impact planets and their moons,” said Dennis Bodewits from the University of Maryland in College Park. “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 (110 kilometers) across and orbits the Sun every 5 years.

“The Hubble data are most simply explained by the impact, at 11,000 mph (17,700 km/h), of a previously unknown asteroid about 100 feet (30 meters) in diameter,” said David Jewitt from 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.

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 months. Others display shorter outbursts. Icy materials may be exposed occasionally either by internal geological processes or by an external one, such as an impact.

On December 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 November 11 and December 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, shows 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 December 27, 2010, and January 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°, leaving a crater 1,000 feet (305 meters) 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 Michael Kelley from 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.”

Asteroid-Scheila
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 December 15, 2010, when the asteroid was about 232 million miles (373 million kilometers) from Earth. NASA/Swift/DSS/D. Bodewits (UMD)
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 a smaller asteroid struck Scheila.

“Collisions between asteroids create rock fragments, from fine dust to huge boulders, that impact planets and their moons,” said Dennis Bodewits from the University of Maryland in College Park. “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 (110 kilometers) across and orbits the Sun every 5 years.

“The Hubble data are most simply explained by the impact, at 11,000 mph (17,700 km/h), of a previously unknown asteroid about 100 feet (30 meters) in diameter,” said David Jewitt from 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.

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 months. Others display shorter outbursts. Icy materials may be exposed occasionally either by internal geological processes or by an external one, such as an impact.

On December 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 November 11 and December 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, shows 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 December 27, 2010, and January 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°, leaving a crater 1,000 feet (305 meters) 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 Michael Kelley from 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.”

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