- "Stealth" comets that are invisible to astronomers
could pose a lethal new threat from space, it was revealed.
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- Scientists believe some giant comets composed of loose
material reflect so little light they cannot be seen.
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- 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.
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- 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.
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- 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."
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- The Cardiff scientists found that the surfaces of inactive
comets composed of loose, organic material develop such small reflectivities
they appear invisible.
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- Near-Earth objects "may therefore be dominated by
a population of fast, kilometres-wide bodies too dark to be seen with current
surveys".
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- Fortunately the American space agency NASA has a new
weapon with the power to "de-cloak" stealth comets, said Professor
Wickramasinghe.
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- 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.
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- The Cardiff team's research is published in the current
issue of the Monthly Notices of the Royal Astronomical Society.
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- However, Dr Benny Peiser, an expert on near-Earth objects
at Liverpool John Moores University, was sceptical.
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- 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.
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- "The fact that none has been found to date suggests
to me that they don't exist in large numbers," said Dr Peiser.
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- "But even if there were to be such a population,
we should be able to catalogue these murky NEOs in the near future."
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- © 2004 AAP http://www.theage.com.au/news/Breaking-News/Invisible-comets
-pose-a-new-threat/2004/11/02/1099262819202.html?oneclick=true
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