Rocky horror show in deep space

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 110 kilometres wide and orbits the Sun every five years.

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 categorised as asteroids, clearly develop comet-like clouds that can last for many months. Others display much shorter outbursts. Icy materials may be occasionally exposed, either by internal geological processes or by an external one, such as an impact.

HST image of asteroid Scheila

The Hubble Space Telescope imaged asteroid Scheila on December 27, 2010, when it was about 350 million kilometres away. The C-shaped cloud of particles and dust tail suggest the asteroid was struck by another object.

Asteroid or comet?

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 (OH) 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 by dual plumes formed as small dust particles excavated by the impact were pushed away from the asteroid by sunlight.

The teams found the observations were best explained by a small asteroid hitting Scheila’s surface at an angle of less than 30 degrees, leaving a crater 300 metres across. The researchers estimate the crash ejected more than 660,000 tons of dust.

Adapted from information issued by NASA’s Goddard Space Flight Centre. Images courtesy NASA / ESA / D. Jewitt (UCLA) / Goddard Space Flight Centre /Conceptual Image Lab.

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