9 November 2001

Odds lengthen on chance of an asteroid disaster

By David Derbyshire, Science Correspondent

First the good news: the chances of the human race being wiped out by an
asteroid within the next 100 years are far lower than was previously thought.

The bad news is that there's still a one in 5,000 probability that the
Earth will collide with a space rock before the start of the 22nd century. While
that may seem a remote possibility, it is four times higher than the risk of
dying in a train crash.


An artists impression of an asteroid impacting the earth

The new doomsday prediction comes from researchers at Princeton University
in New Jersey, who found that the solar system contains 700,000 asteroids big
enough to destroy civilisation.

That number is one third the size of earlier estimates. Previously, the
risks of a catastrophic space collision within 100 years were put at one in
1,500.

Dr Zeljko Ivezic, who led the research, said: "Our estimate of 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."

The Sloan survey, based at the Apache Point Observatory in New Mexico, is
mapping one quarter of the sky. Its main aim is to look for objects outside our
galaxy, but it also records objects far closer to home.

The estimate draws on observations of many more asteroids than past
studies, including small, faint ones not available in earlier risk estimates.

The data allowed astronomers to gauge the size of the asteroids more
accurately by analysing their colour and composition. A small, rocky asteroid
looks just as bright as a larger one made from darker carbon.

"You don't know precisely the size of an object you are looking at unless
you know what type it is," said Dr Ivezic.

The asteroid belt contains 700,000 objects larger than three-fifths of a
mile in diameter. That is the minimum size thought to pose a catastrophic risk.

Previous studies could detect only asteroids three miles or larger,
forcing astronomers to estimate the number of smaller objects. The small
asteroids have turned out not to be as plentiful as had been expected.

The risk of dying in a rail accident is about one in 20,000, while the
chance of being killed in a lightning strike is one in 10 million.





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