By Dr. Tony Phillips
October 2010: Astronomers using NASA's Hubble Space Telescope have captured rare images of a suspected asteroid collision. The snapshots show a bizarre X-shaped object at the head of a comet-like trail of material. Their findings will be published in the Oct. 14th issue of Nature.
In January 2010, astronomers began using Hubble to track the object, named "P/2010 A2," for five months. At first they thought they had witnessed a fresh asteroid collision, but were surprised to learn the collision occurred in early 2009.
"We expected the debris field to expand dramatically, like shrapnel flying from a hand grenade," says astronomer David Jewitt of the University of California in Los Angeles, who is a leader of the Hubble observations. "But what happened was quite the opposite. We found that the object is expanding very, very slowly."
This suggested an older collision than the team had anticipated.
P/2010 A2 is located in the asteroid belt, a reservoir of millions of rocky bodies between the orbits of Mars and Jupiter. Astronomers estimate that modest-sized asteroids in the main belt smash into each other about once a year. Catching asteroids in the act of colliding, however, is difficult because large impacts are rare while small ones, such as the one that produced P/2010 A2, are exceedingly faint. The Lincoln Near-Earth Research (LINEAR) Program Sky Survey first spotted the object's comet-like tail in January 2010, and indeed some astronomers thought it might be a comet. But only Hubble discerned the X pattern, offering unequivocal evidence that something stranger than a comet outgassing had occurred.
The Hubble images, taken from January to May 2010 with the telescope's Wide Field Camera 3, reveal a point-like object about 400 feet wide, with a long, flowing dust tail behind a never-before-seen X pattern. The tail contains enough dust to make a ball 65 feet wide, most of it blown out of the bigger body by the impact-caused explosion. The 400-foot-wide object in the Hubble image is the remnant of a slightly larger precursor body.
Astronomers think a smaller rock, perhaps 10 to 15 feet wide, slammed into the larger one. The pair probably collided at high speed, about 11,000 mph, which smashed and vaporized the small asteroid and stripped material from the larger one. Jewitt estimates that the violent encounter happened in February or March 2009 and was as powerful as the detonation of a small atomic bomb.
Astronomers still do not have a good explanation for the X shape. The crisscrossed filaments at the head of the tail suggest that the colliding asteroids were not perfectly symmetrical. Material ejected from the impact, therefore, did not make a symmetrical pattern, a bit like the ragged splash made by throwing a rock into a lake. Particle sizes in the tail are estimated to vary from about 1/25th of an inch to an inch in diameter. Larger particles in the X disperse very slowly and give this structure its longevity.
Astronomers plan to use Hubble again next year to view the object. Jewitt and his colleagues hope to see how far the dust has been swept back by the sun's radiation and how the mysterious X-shaped structure has evolved.
For images, movies, and more information about asteroid encounter P/2010 A2, visit: http://hubblesite.org/news/2010/34
Although the Hubble images give compelling evidence for an asteroid collision, Jewitt says he still does not have enough information to rule out other explanations for the peculiar object. In one such scenario, a small asteroid's rotation increases from solar radiation and loses mass, forming the comet-like tail.
"These observations are important because we need to know where the dust in the solar system comes from, and how much of it comes from colliding asteroids as opposed to 'outgassing' comets," Jewitt said. "We also can apply this knowledge to the dusty debris disks around other stars, because these are thought to be produced by collisions between unseen bodies in the disks. Knowing how the dust was produced will yield clues about those invisible bodies."
Last Updated: 24 January 2011