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Chance of a Cometary Impact Re-assessed

The chances of the Earth suffering a collision with a cometary body may be higher than previously thought, according to new research by astronomers Bill Napier and Chandra Wickramasinghe. If so, international programmes designed to detect a large class of potentially threatening objects, namely near-Earth asteroids, as well as strategies to mitigate the worst effects of collisions, may be in need of urgent review.

This is the disturbing conclusion reached by the astronomers in a paper which is to be published shortly in the Monthly Notices of the Royal Astronomical Society. Their argument is based on the known rate at which comets enter the inner solar system from the Oort cloud, a nearly spherical swarm of some 100 billion comets that surrounds the solar system out to a distance almost halfway to the Sun's nearest neighbouring star.

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. Surfaces reflecting less than 0.1 percent of the incident sunlight could be formed when a comet made up of a mixture of organic grains and ices approaches the sun and sublimates, leaving an outer layer of loosely connected organic material.

Bill Napier via the Armagh Observatory, College Hill, Armagh, BT61 9DG. Tel.: 028-3752-2928; FAX: 028-3752-7174; wmnarm.ac.uk; Website: star.arm.ac.uk/staff/billn.html.

Chandra Wickramasinghe at the Centre for Astrobiology, Cardiff University. Tel.: 029-2087-4201; FAX: 029-2087-6425; Wickramasinghecardiff.ac.uk; Website: www.astrobiology.cf.ac.uk.

Download full paper (PDF format).

See also: NEO Impact Hazard


Last Revised: 2004 October 15th
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