On January 6 an asteroid-spotting telescope at White Sands Missile Range in New Mexico detected a new and unusual object in the night sky. Towing a streaky debris tail, the object was classified as a comet, although its orbit belied a different origin. Visible comets generally have elongated orbits that carry them into Earth's neighborhood from the colder outer reaches of the solar system, but the newfound body had a neat, nearly circular orbit in the Asteroid Belt [the green ring in the video below], between the orbits of Mars and Jupiter.

Within weeks, a group of astronomers had secured time on the Hubble Space Telescope to get a better look at the curious object, dubbed P/2010 A2, which appeared not to be a comet at all but a previously undiscovered asteroid that had somehow spewed its own debris into a comet-mimicking tail.

Now two groups have used those Hubble photographs, as well as observations from ground-based telescopes and the European Space Agency's comet-chasing Rosetta spacecraft, to confirm that P/2010 A2 is indeed an asteroid that was disrupted, quite possibly by a collision with a smaller asteroid. The disruption appears to have occurred in early 2009, which is remarkably recent in terms of the evolution of the solar system. The two groups reported their findings in the October 14 issue of Nature. (Scientific American is part of Nature Publishing Group.)

"I knew that this was an object the likes of which we hadn't seen before," says David Jewitt, a co-author of one of the new papers and an astronomer at the University of California, Los Angeles. "This is the first time we've seen an asteroid in the act of disrupting." Using Hubble, Jewitt and his colleagues watched the nucleus and tail of P/2010 A2 evolve over several months, from January to May 2010. Tracking the tail's changing position with respect to the nucleus, the researchers estimated that the disruption of the parent asteroid must have happened in February or March 2009.

Jewitt's group concluded that the impact of a small asteroid, just meters across, into the 120-meter nucleus of P/2010 A2 could excavate enough debris from the asteroid to produce the curious tail. But a less violent phenomenon could also be the culprit: The asteroid may have been spun up by the force of the sun, eventually rotating so fast that it began to shed mass. "Like wind blowing onto a propeller, the solar radiation can exert a torque on an asteroid," Jewitt says. He notes that a collision is his "favorite" scenario but that it is not possible to discriminate conclusively between the two causes based on the observations.

The authors of the other Nature paper on P/2010 A2 also favor the collision scenario. "It's not possible for us to tell whether it was a collision or a spin-up—we simply say collision because a collision is much more likely," says Colin Snodgrass, a postdoctoral astronomer at the Max Planck Institute for Solar System Research in Katlenburg–Lindau, Germany.

And indeed, the spin-up mechanism is "probably not the most likely scenario," says William Bottke, a planetary scientist at the Southwest Research Institute in Boulder, Colo., who did not participate in the new research, adding that a spinning asteroid would likely produce a disk around its equator. "This is more consistent with a small body impacting and making a crater, throwing debris off the asteroid."

Snodgrass and his colleagues used ground-based telescopes as well as a camera on the Rosetta spacecraft, which in March 2010 was headed toward the Asteroid Belt as part of its journey to a planned encounter with a comet in 2014, to look at P/2010 A2 from different angles. With those observations, which Snodgrass likens to taking a three-dimensional stereoscopic image, the group modeled the production of the tail and fine-tuned their impact date estimate to February 10, 2009.

Collisions between objects of this size in the Asteroid Belt, Snodgrass and his colleagues calculated, should occur every 12 years or so, a timeframe that matches the search time logged by LINEAR, the asteroid survey project that discovered P/2010 A2. "We expect them to happen, and we expect them to happen on roughly the timescale that LINEAR has been looking," he says.

Part of the reason that LINEAR did not detect the cometlike P/2010 A2 until nearly a year after the asteroid's disruption is because the event occurred when P/2010 A2 was in the direction of the sun from Earth's vantage point. By January 2010, when LINEAR finally registered the puzzling object, the asteroid was at a much more favorable viewing angle.

With earlier detection, Jewitt notes, astronomers might have been able to determine the true origin of the asteroidal disruption—a high-speed asteroid collision would throw off fast-moving debris that would be visible near the asteroid for a short time before dispersing, whereas a sun-spun asteroid gently shedding particles would not. "It's a bit of bad luck" that P/2010 A2 came apart in the daytime sky, he says. "If we were able to see it initially, then we would be able to detect those fast particles if they were there."

Studying such collisions and disruptions has obvious implications for understanding the dynamics of the Asteroid Belt and can also help identify the sources of the solar system's interplanetary dust and the origins of the planets themselves. "The Asteroid Belt is a natural laboratory for impacts," Bottke says. "You just wait for nature to do its thing, and then you can get a lot of science from it." He notes that were it not for the collisional mergers of primitive rocky bodies such as those now populating the belt, planets such as Earth never would have formed. "By learning about these processes in the Asteroid Belt, we're also learning about what happened at the start of the solar system," Bottke says.