[meteorite-list] Unseen Comets May Raise Impact Risk for Earth

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Oct 19 13:53:52 2004
Message-ID: <200410191752.KAA00536_at_zagami.jpl.nasa.gov>

http://www.nature.com/news/2004/041018/full/041018-3.html

Unseen comets may raise impact risk for Earth
Mark Peplow
Nature News Update
October 18, 2004

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 Society[1], 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 comet[2].

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

   1. 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).
   2. Kissel J., Krueger F. R., Silen J. & Clark B. C. Science, 304.
      1774 - 1776 (2004).
Received on Tue 19 Oct 2004 01:52:12 PM PDT