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That's no comet! The asteroid debris tail in January 2010 (Image credit: NASA/ESA)

Between Mars and Jupiter lies the Asteroid Belt, a swarm  of millions of shards of rock, metal and ice.  Somewhere out here, between 5-10 February 2009, a small asteroid, perhaps 5m across, violently slammed into another, larger (120 m), asteroid at a closing speed of about 18 000 km/h. As much energy as released in this cosmic crash as the detonation of a small nuclear bomb. The smaller asteroid was vaporised, blasting material from the larger one which spewed debris in space. No one on Earth noticed this cataclysm until January 2010, almost a year later, when the LINEAR survey discovered something odd, initially thought to be a comet.

Astronomers thought they just had witnessed a fresh collision between two asteroids when images taken by the Hubble Space Telescope and ESA’s Rosetta spacecraft revealed  a comet-like trail of dusty material with an unusual X-shaped object at its head. For details of observations at this time, see Crash of the Asteroids by Sinead McNicholl in the March 2010 issue of Astronotes.

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Colliding asteroid P/2010 A2 as seen by Rosetta (Image credit: ESA)

This tail contains enough dust to make a ball of rock 20 m wide, so most of it was excavated from the bigger body by the explosive impact. Astronomers expected to see the streaming tail of debris rapidly expand but after five months of Hubble observations of this enigmatic phenomenon, they had instead witnessed the plume grow slowly and gradually as radiation pressure from the Sun blew the debris behind the remnant asteroid.  Eventually the astronomers  came to the realisation that they had actually missed the suspected smash-up by a year.

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These HST images, taken from January to May 2010 with Wide Field Camera 3, reveal a point-like object about 120 m wide, with a long, flowing dust tail behind an X-shaped pattern of a kind that had never been seen before. The observations also show that the object retained its X-shape even as the debris field slowly expanded. Particle sizes in the tail are estimated to vary from about 1 mm to 2.5 cm in diameter. (Image credit: NASA/ESA)

Such violent and destructive encounters between asteroids are probably common, perhaps even happening annually. Collisions like this would be a source of the fine dust scattered across interplanetary space. Observations of the surviving asteroid, designated P/2010 A2, enable astronomers to refine their predictions about often these collisions will occur and the amount of dusty detritus they produce.

Both asteroids were probably no strangers to collisions. All present day asteroids are probably relics from ancient impacts between larger asteroids. Over billions of years mutual collisions of asteroids is believed to lead to a gradual reduction in asteroid sizes in a process  known as collisional grinding.  This is thought to be one of the main processes by which asteroids are destroyed.

Advances in instruments have led to a renaissance in the study of small Solar System bodies in recent decades, revealing these tiny denizens of the interplanetary void to be dynamic and fascinating objects, perhaps even worthy of human exploration.


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