An alternate title for this post could be “When a Comet Might Not Be a Comet”.
On the night of Jan 6, the LINEAR (Lincoln Near Earth Asteroid Research) survey detected a previously unknown comet. This is LINEAR’s 193rd comet discovery with only the space-based SOHO spacecraft has found more comets. Most of their comets are called Comet LINEAR so this one should be more specifically referred to as comet P/2010 A2 (LINEAR).
So what’s so special about this comet? First let’s take a look at image below taken by Bob McMillian (University of Arizona) with the Spacewatch 1.8-m telescope on Kitt Peak.
At first glance this looks like a normal comet with most of the expected characteristics of a comet. Fuzzy appearance, check. Tail (though not all comets have tails, tails are a sign of cometary activity), check.
One characteristic shown in the image above is odd. Many early observers of this comet have commented on its lack of an obvious bright center in the coma, what is termed its central condensation. Comets are due to dust and gas outgassing from an ice-rich rocky nucleus (like an asteroid but with a large fraction of volatile ices). As a result, the coma should have a relatively sharp bright center in the vicinity of the nucleus. Instead Comet LINEAR’s coma appears sheared tailward with no definite sign of an active nucleus.
As odd as its appearance may be, we have seen this before. It is possible the comet has just experienced a short lived outburst. Just like smoke dissipating after an explosion, the coma and tail we see are remnants of an outburst slowly being blown away by the solar wind. Alternately, the nucleus may have just disintegrated with the remnants again being dispersed by the solar wind. As last week’s SOHO comet showed, the disintegration of comets are not rare events.
Up to now all of the signs point towards this object being a comet, albeit one that is acting a little strange. That is until you look at its orbit. With a perihelion distance of 1.87 AU, an aphelion of 2.62 AU and a semi-major axis of 2.25 AU, P/2010 A2 is on a very un-comet-like orbit. The orbit more closely resembles the orbit of a Flora family asteroid of the inner Main Belt. Though a few asteroids in the Main Belt have been observed to occasionally show cometary activity (the so called Main Belt Comets), these objects have all been located in the outer Main Belt and are volatile-rich carbonaceous asteroids. The inner Main Belt, and the Flora family especially, is dominated by stoney asteroids with few, if any, carbonaceous objects.
In summary, we have an object that looks like a comet orbiting where no volatile-rich object should be. So what might be happening?
- Comet LINEAR is a rare carbonaceous asteroid located in the inner Main Belt that has just experienced a recent episode of cometary activity. This is exciting because it may mean that there are more volatile-rich asteroids in the inner Main Belt and closer to Earth.
- Comet LINEAR really is just a “boring” stoney asteroid, but it recently was impacted by another smaller asteroid and the cometary appearance we see is the result of this collision. If so this would be our first imaged asteroid impact.
Images like the one above will allow astronomers to model the dust in the coma and tail. A quick inspection of the above image and a few others I’ve seen show the tail to be made of large particles trailing the comet along its orbit. There may be some evidence of a much shorter and fainter tail stretching to the SE which is consistent with the direction expected for a normal anti-solar tail of small dust particles being blown back by the solar wind.
Just as forensic scientists can reconstruct a crime scene well after the fact from evidence left behind, past dust activity can be reconstructed from the morphology of a comet’s coma and tail. Evidence of a single dust producing event may suggest an asteroid impact, while evidence of continuous activity points towards a volatile-rich asteroid.
I’m sure there will be much more to this object’s story in the weeks ahead…
[Note added later by editor: I forgot to add that the comet is faint at a magnitude of 18-19. You will need a CCD camera on a moderate sized telescope to image this comet. It is definitely too faint to be seen with out optical and digital imaging aid.]