Thousands of unseen dark comets may be on collision course with Earth, warn experts

via The Daily Mail


Thousands of invisible 'dark' comets may be posing an unseen threat to Earth, according to two British astronomers.

A 'dark' comet is one which has shed its bright water ice, leaving an inner organic crust that reflects only a small fraction of light.

Because it does not glow, a dark comet on collision course with the Earth could easily escape detection until disaster looms.

Professor Bill Napier, from the University of Cardiff, told New Scientist magazine: 'There is a case to be made that dark, dormant comets are a significant but largely unseen hazard.'

He and fellow astronomer Dr David Asher, from Armagh Observatory in Northern Ireland, warn that many 'periodic' comets which take less than 200 years to complete an orbit of the Sun may be 'dark'.

The rate that bright comets pass through the Solar System suggests there should be 3,000 of them flying around, yet only 25 have been detected. Most may have remained hidden because they are too dark to see, say the astronomers.

In 1983 the comet IRAS-Araki-Alcock passed by the Earth at a distance of five million kilometres, the closest approach of any comet for 200 years.

With just 1 per cent of its surface active and reflecting light, it was only spotted two weeks ahead of its arrival, said Dr Napier.

Comet Borrelly, visited by American space agency Nasa's Deep Space One probe in 2001 was found to be covered in dark patches.

US expert Clark Chapman, from the Southwest Research Institute in Boulder, Colorado, said dark comets 'would absorb sunlight very well' and should therefore be possible to detect from the heat they emit.