- WASHINGTON (Reuters) -
Astronomers
have delivered a little piece of good news -- we are much less likely to
get wiped out by a big asteroid than previously thought.
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- The odds are only about 1 in 5,000 that an asteroid big
enough to wipe out civilisation will hit the Earth in the next 100 years,
a team at Princeton University reported -- far lower than previous
estimates
of 1 in 1,500.
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- Research on asteroids that have hit the Earth in the
past -- like the one that wiped out the dinosaurs 65 million years ago
-- shows that a collision with a large asteroid half a mile or 1 km in
diameter could kill a quarter of the world's population.
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- Research has also shown that such enormous asteroids
strike the planet regularly -- every 100 million years or so. Astronomers,
understandably concerned, have been looking around to see how many of those
asteroids might be out there.
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- Using data from the Sloan Digital Sky Survey, the
Princeton
team, headed by researcher Zeljko Ivezic, estimated the solar system
contained
about 700,000 asteroids that size -- about one-third the number in earlier
estimates that put the number of big asteroids at about 2 million.
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- That in turn suggested there was a 1-in-1,500 chance
one would hit Earth in the next century.
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- "Our estimate for the chance of a big impact
contains
some of the same uncertainties as previous estimates, but it is clear that
we should feel somewhat safer than we did before we had the Sloan survey
data," Ivezic said in a statement.
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- Several teams of astronomers are taking part in the Sloan
survey, which is mapping one-quarter of the sky using the telescope at
Apache Point Observatory in New Mexico.
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- Its main purpose is to look at objects outside Earth's
galaxy, but it is also cataloguing smaller and closer objects such as
asteroids.
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- Writing in the November issue of the Astronomical
Journal,
Ivezic said they were able to assess more accurately the size of known
asteroids.
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- Asteroids with a surface of carbon are dark, like lumps
of coal, while rocky asteroids are much brighter. To a casual observer,
a small, rocky asteroid looks as bright as -- and as big as -- a larger,
rocky one.
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- "You don't know precisely the size of an object
you are looking at unless you know what type it is," Ivezic said.
He said the Sloan survey looks at the colour of objects, so astronomers
can distinguish between carbon and rock.
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- They looked at 10,000 asteroids, and based on those,
the Princeton team was able to extrapolate and estimate that the asteroid
belt contained about 700,000 that were bigger than 1 kilometre -- about
half a mile -- in diameter.
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