Warning over 'stealth' comets

Comets that are invisible to astronomers could pose a lethal threat from space, scientists said yesterday.

They believe that giant "stealth" comets made up of loose material reflect so little light that they cannot be seen. If the theory is right, the chance of the Earth being hit by a comet big enough to wipe out human civilisation may be higher than experts believe.

Professor Chandra Wickramasinghe, head of the team at Cardiff University's Centre for Astrobiology, which delivered the warning, said: "It's possible we need to think again about mitigating strategies."

The Cardiff scientists found the surfaces of inactive comets composed of loose, organic material develop such small reflectivities that they are invisible.

Fortunately, the American space agency Nasa has a new weapon with the power to "de-cloak" stealth comets, the professor said. The £115m Widefield Infrared Survey Explorer, to be launched in 2008, will scan the sky for cool failed stars called brown dwarfs, clouds of space dust and faraway galaxies. It is up to 500,000 times more sensitive than previous telescopes.

The Cardiff team's research is published in the current issue of the Monthly Notices of the Royal Astronomical Society.

Benny Peiser, an expert on near-Earth objects at Liverpool John Moores University, said there was no evidence of "dark comet" impacts in the past.