There may be more of them out there than we know:

With about 1 percent of incoming comets ending up on relatively short-period Earth-crossing orbits, it is expected that several thousand dormant comets could be currently posing a potential threat to our planet.

Recent surveys of the Earth’s immediate vicinity should have turned up some 400 such objects, whereas only a handful have so far been found.

The researchers dismiss the current belief that all the “missing” comets have disintegrated into meteor streams. If this had happened, they argue, then we should be seeing a far greater number of meteor showers and a much brighter zodiacal cloud than is observed.

They propose instead that the majority of these comets have become exceedingly black, with such low surface reflectivities that they could not be observed against the blackness of space by optical means.