'Stealth' Comets Pose
Unseen Threat

The Age - Australia
"Stealth" comets that are invisible to astronomers could pose a lethal new threat from space, it was revealed.
Scientists believe some giant comets composed of loose material reflect so little light they cannot be seen.
If the theory is right, the chances of a the Earth being hit by a kilometre-wide comet big enough to wipe out human civilisation may be higher than experts currently believe.
Invisible comets could force an urgent review of international programs designed to spot near-Earth objects, and ways of averting a collision, it is claimed.
Professor Chandra Wickramasinghe, head of the team at Cardiff University's Centre for Astrobiology that delivered the warning, said: "It's possible that we are missing many of these Earth-threatening objects and we need to think again about mitigating strategies - some of which assume decades or centuries of warning before impact."
The Cardiff scientists found that the surfaces of inactive comets composed of loose, organic material develop such small reflectivities they appear invisible.
Near-Earth objects "may therefore be dominated by a population of fast, kilometres-wide bodies too dark to be seen with current surveys".
Fortunately the American space agency NASA has a new weapon with the power to "de-cloak" stealth comets, said Professor Wickramasinghe.
The 115 million pound Widefield Infrared Survey Explorer, due to be launched in 2008, will scan the entire sky to search for cool failed stars called brown dwarfs, clouds of space dust, and faraway galaxies. Up to 500,000 times more sensitive than previous infrared telescopes, it would be capable of detecting invisible dark comets.
The Cardiff team's research is published in the current issue of the Monthly Notices of the Royal Astronomical Society.
However, Dr Benny Peiser, an expert on near-Earth objects at Liverpool John Moores University, was sceptical.
He said there was no evidence of "dark comet" impacts in the past, and no dark comets had been detected by earlier generation infrared telescopes.
"The fact that none has been found to date suggests to me that they don't exist in large numbers," said Dr Peiser.
"But even if there were to be such a population, we should be able to catalogue these murky NEOs in the near future."
© 2004 AAP



This Site Served by TheHostPros