Hidden Comets may Raise Impact Risk for Earth

Unseen comets may raise impact risk for Earth
Published online: 18 October 2004
http://www.nature.com/news/2004/041018/full/041018-3.html

Mark Peplow
Thousands of dark objects could be hiding in our Solar System.

The Solar System could be teeming with almost invisible comets, according to
some astronomers' calculations. If they are right, such extra comets would
significantly increase the risk of a catastrophic impact with Earth.

These objects have never been observed, but the astronomers argue that 'dark
comets' provide a likely explanation for an astronomical puzzle: we can only
see a tiny fraction of the comets that theory predicts.

Astronomers think that many comets come from the Oort cloud, a field of
billions of icy objects that lies up to 100,000 times farther away from the
Sun than the Earth does and marks the outer boundary of our Solar System.
The icy objects are sometimes driven towards the Sun by gravitational tides
generated by the shifting masses of stars in our Galaxy. When this happens
they become comets, orbiting the Sun every 20 to 200 years on paths that lie
at an angle to the planets' orbits.

Given the size of the Oort cloud, astronomers have calculated that there
should be about 3,000 comets in these orbits, 400 times more than are
actually observed.

The common explanation for this discrepancy is that the comets quickly
disintegrate into smaller lumps after just one or two orbits, says Bill
Napier, a recently retired astronomer who worked at the Armagh Observatory,
Northern Ireland. But his mathematical model now suggests that, if this were
true, the debris should cause many more major meteorite showers on Earth
than we see, perhaps up to 30 every year.

In a paper to be published in the Monthly Notices of the Royal Astronomical
Society1, Napier concludes that the predicted comets are out there after
all; we just cannot see them.

Little fluffy clouds

Napier worked with Chandra Wickramasinghe, an astronomer at Cardiff
University in Wales, to explain the comets' invisibility. Wickramasinghe has
suggested that Sedna, the most distant body identified in our Solar System,
could have an orbiting twin that is dark, fluffy and made of tarry carbon
compounds (see "Sedna 'has invisible moon'").

As Sedna may be a member of the Oort cloud, Napier thinks that other members
of the cloud could be equally dark. Once ejected, the tarry comets would
simply suck up visible light, he says, remaining cloaked in darkness.
"Photons go in, but they don't come out."

"It's an intriguing possibility," says Alan Fitzsimmons, an astrophysicist
at Queen's University of Belfast in Northern Ireland. "But while we have
seen dark objects before, Bill is proposing something much, much darker than
anything we've ever detected."

NASA's Stardust probe, which is bringing back samples of dust from the comet
Wild 2, lends some support to Napier's idea. In June this year it reported
finding lots of tarry carbon compounds spraying from the comet2.

Infrared challenge

The dark comets would present a major challenge to astronomers searching the
skies for objects that might collide with the Earth. "They're so black you
can't see the damn things," says Napier. "These things will just come out of
the dark and hit you with no warning. It looks as if we're dealing with a
substantial impact hazard that people haven't clicked into yet."

However, although they reflect almost no visible light, the dark comets
should give out a tiny glow of heat, visible as infrared radiation. The
infrared Spitzer Space Telescope, which has been operating from Earth orbit
for just over a year, has not seen any dark comets. But this could be
because it focuses on very small, distant parts of the sky, says Napier.

Fitzsimmons disagrees, saying that if these objects existed in the numbers
proposed by Napier, either Spitzer or near-Earth object surveys such as
Spacewatch, based at the University of Arizona in Tucson, would have picked
them up by now.

A new space telescope might provide the answer. Earlier this month, NASA
announced that it would launch an orbiting infrared telescope called the
Wide-field Infrared Survey Explorer (WISE) in 2008, which will map much
wider areas of the sky. Given enough time, it should be able to detect the
dark comets, says Napier.

References
Napier W. M., Wickramasinghe J. T. & Wickramasinghe N. C. Mon. Not. R.
Astron. Soc, published online, doi:10.1111/j.1365-2966.2004.08309 (2004).
Kissel J., Krueger F. R., Silen J. & Clark B. C. Science, 304. 1774 - 1776
(2004). | Article | PubMed | ISI | ChemPort |

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