The Puzzle of the Half-Comet, Half-Asteroid

In 1996, astronomers identified an extraordinary object orbiting the Sun between Mars and Jupiter in a region best known for its asteroids. And yet this body, called 133P, defied description: it had the orbit of an asteroid yet emitted dust like a comet.

Clearly, this was a rare object. After centuries of observation, not a single other object in the asteroid belt had burped gas and dust in the same way.

So how could this have got there? According to Henry Hsieh at Queen’s University, Belfast in Northern Ireland, there can be only two explanations. The first is that 133P is a comet that has somehow recently become trapped in an asteroid-like orbit. This would have required a hugely unlikely combination of gravitational kicks from other planets as the comet travelled into the solar system from the Kuiper Belt or Oort cloud.

Hsieh says this is so fantastically unlikely that it is almost certainly a one-off event. So there’s almost no chance that we’d see another comet-like object in this kind of orbit.

The second explanation is that 133P is an asteroid formed partly of ice and that this is being released, perhaps by a collision with another asteroid. If this were the case, there would almost certainly be other asteroids with a similar makeup releasing dust. These we ought to be able to see.

So Hsieh set out to find one, making some 657 observations of 599 asteroids in the asteroid belt. The big news is that he has found one other object, called 176P/LINEAR, which is also emitting dust.

So it looks as if the mystery is solved. That more or less rules out the possibility that 133P is a captured comet. Instead, 133P and 176P are a new class of comet-like asteroids made up partly of ice, which is ejected whenever these objects are struck in the occasional unavoidable collision.

That’s an interesting new addition to the asteroid menagerie. The only question now is what to call these beasts that are half comet and half asteroid. Comsteroids? Asteromets? Hsiehroids?

Ref: arxiv.org/abs/0907.5505: The Hawaii Trails Project: Comet-Hunting in the Main Asteroid Belt