Science —

Astronomers spot a six-tailed asteroid

Neither comet nor a mythical beast, it might be an asteroid spinning apart.

If your child looked through a (quite good) telescope and said, “Look- that asteroid has a tail!” you would probably chuckle and explain the difference between asteroids and comets. But if that child had been looking through the Pan-STARRS telescope on August 27, he or she would have been right.

It’s comets, of course, that we associate with tails. These dusty balls of ice occasionally swing into our neighborhood of the Solar System from their usual home beyond the planets. When they get closer to the inner Solar System, a wispy atmosphere of warmed dust and gas is blown into a tail by the Sun. Asteroids, on the other hand, are rocky or metallic bodies mostly found in a belt between the orbits of Mars and Jupiter.

The strange object astronomers spotted in August was located in the inner portion of the asteroid belt but appeared to have a tail of material extending from it. This contradiction is not completely unknown—a handful of such “active asteroids” have been found in the past. Some might contain ice that heats up, turns to gas, and shoots outward, and some might simply be the short-lived aftermath of a collision with a smaller body. Given the limited number of observations, though, little is known.

When astronomers trained the Hubble Space Telescope on this active asteroid a couple weeks later, they discovered that it actually boasted a splayed array of tails. By the time they got their second look with Hubble after another two weeks, the tails had changed noticeably.

From what Hubble could see, the asteroid is probably a rocky one about a half-kilometer across. The material in the tails is probably not more than a few months old, and in the case of two of the tails, it probably only left the surface a few days before the observations.

In this case, the researchers say the tails are unlikely to be sublimating ice given that this type of asteroid would have been metamorphosed at fairly high temperatures—more than hot enough to drive off any ice. And given that some of the tails seem to have formed over a few months, they’re also unlikely to be the result of collisions. The explanation the researchers favor is more like setting some gravel on a merry-go-round and giving it a spin.

Solar radiation can get some asteroids rotating wildly enough to make a figure skater jealous. Since only the portion of the asteroid facing the Sun heats up, the energy it emits back to space is unevenly distributed, providing torque that can set it spinning. If it gets going fast enough, it could crumble and fling particles with enough velocity to overcome the asteroid’s gravity. The researchers write, “Rotational mass-shedding could be intermittent as unstable clumps of material avalanche towards the rotational equator, break and detach from the central body.” That could account for the series of tails in this case.

Astronomers are excited to learn more about this active asteroid, especially if it gives them a chance to observe the rotational breakup process in action. In a NASA press release, lead author David Jewitt from the University of California at Los Angeles also noted, "In astronomy, where you find one, you eventually find a whole bunch more.”

The Astrophysical Journal Letters , 2013. DOI: 10.1088/2041-8205/778/1/L21  (About DOIs).

You must to comment.

Channel Ars Technica