What happens when you think you have seen a recent collision between two asteroids but find a bizarre X-shaped object at the head of a comet-like trail of material?    You keep looking until you can figure it out.

In January, astronomers thought the new collision they found would have a debris field expanding rapidly, like shrapnel flying from a hand grenade, but when they watched the aftermath using Hubble, they found the opposite; the object was expanding very, very slowly and that it started, not a few days, but nearly a year before the January observations.

"When I saw the Hubble image I knew it was something special," says astronomer Jessica Agarwal, who works for the European Space Agency in the Netherlands. "The nucleus seemed almost detached from the dust cloud and there were intricate structures within the dust."


astronomer David Jewitt of UCLA, leader of the Hubble observations, estimates that modest-sized asteroids actually smash into each other roughly once a year. When the objects collide, they inject dust into interplanetary space. But until now, astronomers have relied on models to make predictions about the frequency of these collisions and the amount of dust they produce.

P/2010 A2
Images from Hubble's Wide Field Camera 3 shows the slow evolution of the debris coming from asteroid P2010/A2, which is thought to be due to a collision with a smaller asteroid.   When astronomers first spotted the unusual X shape, they assumed that they were observing the immediate aftermath of an impact. However, the slower than anticipated rate of change in the appearance of the debris suggests that the collision in fact took place about a year earlier than the first observations.   These seven exposures were taken between January and May 2010 while the asteroid was rapidly receding from Earth. As the asteroid gets further away from us, we are able to see more of its comet-like tail in each subsequent frame. In the top picture, the portion of the tail we see is around 50,000 km long. In the bottom picture, we see around 85,000 km.  For all images, the field of view remains about 75 arcseconds across. The images were taken in visible light (filter F606W) and artificially colored blue.  Credit: NASA, ESA and D. Jewitt (UCLA)







Although the Hubble images give compelling evidence for an asteroid collision, Jewitt says he still does not have enough information to rule out all alternative ideas. In one such scenario, a small asteroid's rotation increases from sunlight pressure and loses mass, forming the comet-like tail.

"Catching colliding asteroids on camera is difficult," Jewitt says, "because large impacts are rare, while small ones, such as the one that produced P/2010 A2, are exceedingly faint." The two asteroids whose remains make up P/2010 A2 were unknown before the smash-up because they were too faint to be noticed. The collision itself was unobservable because it happened when the asteroids were in the same direction as the Sun. About 10 or 11 months later, in January 2010, the Lincoln Near-Earth Research (LINEAR) Program Sky Survey spotted the comet-like tail produced by the collision. But only Hubble resolved the X pattern, offering unequivocal evidence that something stranger than a comet outgassing had occurred.


Astronomers plan to use Hubble again in 2011 to view the remnant asteroid. 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.

Citation: David Jewitt, Harold Weaver, Jessica Agarwal, Max Mutchler, Michal Drahus, 'A recent disruption of the main-belt asteroid P/2010 A2', Nature, 467, 817-819 (13 October 2010) doi:10.1038/nature09456