Fragments split off from the average comet at least once every century, according to two astronomers who have examined images of nearly 50 such bodies. This means that cometary debris is sprayed throughout the inner Solar System, where it may threaten the Earth.
Jun Chen and David Jewitt of the University of Hawaii in Honolulu examined images of comets taken with charge-coupled devices between 1986 and 1993. CCDs detect light electronically and are much more sensitive than photographic plates.
In order not to bias their sample, Chen and Jewitt excluded the most famous split comet, Shoemaker-Levy 9, which will hit Jupiter this July (see 'Live crash from Jupiter', New Scientist, 5 March). It was this comet's odd appearance on photographs that led astronomers to the discovery that it had split into many pieces.
Chen and Jewitt instead looked at 49 comets that had been observed for other reasons. The CCD images revealed that three of the comets, or 6 per cent of the total sample, had broken apart. In all three, a smaller nucleus lay near the main one. Two of the bodies - Comet Chernykh and Comet Wilson - were already known to be split; for the third object, Comet Ciffreo, the discovery of the break-up was new.
If a comet splits in two, the fragments drift apart as time passes. Chen and Jewitt say that about six years after the break-up, the two objects will lie so far apart that a single CCD image would not have revealed both. From this and the percentage of comets that were split, the astronomers estimate that a typical comet breaks up at least once every 100 years.
The high splitting rate implies that comets do not usually break into two equal halves; if they did, even a large comet would soon vanish. Instead, say Chen and Jewitt, the smaller nucleus must typically have less than a thousandth the mass of the main body.
So where do all these small fragments go? Almost two years ago, David Rabinowitz, now at the Carnegie Institution of Washington, discovered that far more small asteroids than expected pass near Earth (New Scientist, Science, 31 October 1992). Chen and Jewitt suggest that some of them may be debris from split comets.
Chen and Jewitt's work has other important implications. Comets are primitive objects that preserve information about the origin of the Solar System. Exposure to sunlight alters their surfaces, but if astronomers can observe a comet after it splits they will see fresh material from its interior. The more often that comets split, the more opportunities there should be to study such primitive material.
Chen and Jewitt will report their work in a future issue of the journal Icarus.
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