Asteroid 62142 sporting a tail. Photo by Scott Sheppard.

Dec 3, 2014 If it walks like an asteroid but quacks like a comet… what is it?

Sen—Years ago, there was a pretty clear difference between asteroids and comets.

Asteroids were rocky or metallic, basically giant solid chunks of matter, most of which orbit the Sun between Mars and Jupiter. Comets, on the other hand, were similar to asteroids, but had little or no metal in them; instead they’re rock and ice. When they get near the Sun the ice turns into gas, surrounding the solid nucleus with a fuzzy head, and leaving behind a long tail of material.

But, as usual with astronomy, when we look at things more deeply we find puzzling objects that straddle zoological lines. For example, what happens when a comet finally loses all its ice? It looks more like an asteroid. Such dead comets have been found, like (the very nearly-dead) Don Quixote.

And some asteroids have been found sporting comet-like tails. Sometimes this happens after a small impact; the pulverized debris expands around the asteroid, forming a fuzzy head. Other times asteroids have disintegrated, again flinging debris out and making them temporarily masquerade as a comet.

And then there’s 2000 SY 178 (62412), a 5-8 km wide asteroid orbiting in the outskirts of the main asteroid belt. It was discovered in 2000, but just recently was found to have a very comet-like tail flowing out from it. You can see that in the picture above.

It turns out 62412 is part of a family of asteroids, groups that have similar orbital and physical characteristics. In this case it’s the dark Hygiea family of asteroids, which are high in carbon content, but have never shown comet-like properties before. Moreover, they’ve been main belt denizens for billions of years, making it very unlikely that 62412 is a lost comet—comets tend to come from much farther out in the Solar System.

So what’s going on? There are some very interesting clues from the asteroid’s behavior.

One is that it’s rapidly rotating. This can be seen in observations taken over several hours; the asteroid gets brighter and dimmer systematically over a 3.3 hour period. If it’s elongated and spinning, it’s brighter when it’s broadside to us, and fainter when it’s end-on.

That period means it’s spinning pretty rapidly! If it spun much faster, centrifugal forces would make it fly apart. If you were a rock sitting on the surface of the asteroid; you’d feel very little gravity in the first place, but that spin would make it even easier to fly off given even a little kick. This is probably a big factor in the creation of the tail; material from the asteroid can escape easily.

But why is it spinning so rapidly? There’s a mechanism called the YORP effect which can increase the spin of an asteroid, due to the way it absorbs and re-emits sunlight. Or it’s possible the asteroid got smacked by some smaller rock recently, and a grazing impact caused it to spin up. Also, we know that some asteroids, especially ones in the outer main belt, have water ice in them. Not as much as comets, perhaps, but some. If a landslide on the surface or a small impact exposed the ice to sunlight, it would turn into a gas, expand, and push on the asteroid like a rocket. That too could increase the asteroid’s spin.

Interestingly, the tail points backwards along the asteroid’s orbit, away from the direction it orbits the Sun. This means the tail is likely made of bigger, or slower-moving, chunks of rock. Earlier observations showed no sign of a tail, so the tail is definitely a transient effect. It appeared not too long after the asteroid reached the closest point to the Sun in its orbit, so it’s possibly (though not conclusively) related to that.

It’s hard to say exactly what’s going on with this particular rock; more observations should help nail down when the tail appears and disappears, for example, as well as its composition. Only a little over a dozen such active asteroids are known, though there may be as many as 100 in the main belt. As we find more of them, and study them longer, we’ll almost certainly get a better handle on these weird denizens straddling the line of different species of objects.