[meteorite-list] Without Even Hitting Earth, A Comet Could Be As Lethal As An Asteroid

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:42 2004
Message-ID: <200103231848.KAA10746_at_zagami.jpl.nasa.gov>


Sting In The Tail
Eugenie Samuel
New Scientist
March 24, 2001

Without even hitting Earth, a comet could be as lethal as an asteroid.

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. "We may be
looking for a swarm of bees while standing on a railway line with the train
coming," 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't even have to hit the Earth to do damage. A giant
comet evaporating under the Sun's 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

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's 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--the onset
of the last ice age. "I think we should be looking for cometary dust in
polar cores," 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. "I don't know if we've discovered enough comets to do a
statistical analysis," says Robert McMillan of the University of Arizona's
Spacewatch project, which tracks near-Earth objects.

But David Williams of University College London, who served on the British
government's Near Earth Objects task force last year, agrees with Napier
that work needs to be done on the risks posed by comets. "This area is
perhaps one that's opening up now," he says. "We thought it was too
controversial for the report."

More at: Monthly Notices of the Royal Astronomical Society (vol 321, p 463)
Received on Fri 23 Mar 2001 01:48:44 PM PST

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