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AS GOVERNMENTS around the world prepare to spend millions studying the threat
of nearby asteroids hitting the Earth, an astronomer in Northern Ireland is
warning that comets might pose a greater danger. 鈥淲e may be looking for a swarm
of bees while standing on a railway line with the train coming,鈥� says Bill
Napier of the Armagh Observatory.

Icy comets with their tails of gas and dust are much rarer than rocky
asteroids, but they don鈥檛 even have to hit the Earth to do damage. A giant comet
evaporating under the Sun鈥檚 glare would release billions of tonnes of dust into
the path of the Earth, Napier has shown in a new study. If this dust rains down
on Earth, it could blot out the Sun and trigger a new ice age.

Astronomers already know of four objects they believe are giant comets
hundreds of kilometres across. And there may be as many as 2000 more lurking in
the Oort Cloud far beyond Pluto. Such comets visit the inner Solar System so
rarely that the risk of an impact is negligible. But Napier calculates that they
could release millions of tonnes of dust into our atmosphere, which would linger
for as long as 10,000 years, blocking out most of the Sun鈥檚 light and heat.

Astronomers had thought that the amount of dust around the inner planets
remains fairly constant because dust from the break-up of comets and asteroids
is balanced by dust falling into the Sun. But this can be upset by just a single
large comet.

Napier and his colleagues believe that the Earth has already suffered at
least once from the effects of comet dust. Data collected in the 1980s shows an
unexpectedly large amount of minute interplanetary dust particles, each with a
mass of about a nanogram. The excess can be explained if a giant comet broke up
in the inner Solar System around 70,000 years ago鈥攖he onset of the last
ice age. 鈥淚 think we should be looking for cometary dust in polar cores,鈥� says
Napier.

Napier rates the chance of being swamped by comet dust as 1 in 100,000, the
same as a chance of a collision with a near-Earth object. Others are more
doubtful. 鈥淚 don鈥檛 know if we鈥檝e discovered enough comets to do a statistical
analysis,鈥� says Robert McMillan of the University of Arizona鈥檚 Spacewatch
project, which tracks near-Earth objects.

But David Williams of University College London, who served on the British
government鈥檚 Near Earth Objects task force last year, agrees with Napier that
work needs to be done on the risks posed by comets. 鈥淭his area is perhaps one
that鈥檚 opening up now,鈥� he says. 鈥淲e thought it was too controversial for the
谤别辫辞谤迟.鈥�