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
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