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.