Each year a few long-period comets, with orbital periods typically a million years, arrive in the planetary system. One or two percent of these incoming comets are thrown into so-called Halley-type orbits, with orbital periods of 20-200 years. There should be about 5,000 such objects over 10km across, and perhaps 50,000 over a kilometre across. This is hundreds of times more than are actually observed, however, a huge discrepancy. The problem was pointed out some years ago by Mark Bailey and Vacheslav Emel'yanenko and raises the question: where have all the comets gone?
To resolve this paradox it has been proposed that the Halley-type comets disintegrate to dust after two or three passages close to the Sun. However Bill Napier, and Chandra and Janaki Wickramasinghe at Cardiff University, have shown that this explanation doesn't work: the dust so created would yield many strong annual meteor streams without parent comets, and these are not observed. Further, the Sun would be surrounded by a bright spherical cloud of dust, and this too is not seen.
The explanation may lie in a very strong darkening of a comet's surface after it becomes inactive. Bill Napier and colleagues have shown from classical radiation scattering theory that a porous carbonaceous surface, of the sort which may develop on a comet after it has lost its volatile ices through solar heating, has a vanishingly small reflectivity, becoming much blacker than coal. Extremely dark patches, approaching those predicted, were detected on Comet Borrelly during a recent flyby. They suggest that the comets have not disintegrated, they are simply too dark to be seen by current near-Earth object search programmes. There may therefore exist a population of fast, multi-kilometre bodies too dark to be seen. Such bodies would constitute a significant additional impact hazard.