[meteorite-list] Rare meteor shower to shed light on dangerous comets

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Wed, 08 Aug 2007 18:48:48 -0400
Message-ID: <gthkb310pcd8h3prpbh9mh89fecflkbpag_at_4ax.com>

http://space.newscientist.com/article/dn12443-rare-meteor-shower-to-shed-light-on-dangerous-comets.html

Rare meteor shower to shed light on dangerous comets

    * 17:07 08 August 2007
    * NewScientist.com news service
    * Stephen Battersby

A rare meteor shower predicted to hit Earth on 1 September should give
astronomers only their second chance to study an ancient comet's crust. It could
also help them develop a warning system against an otherwise insidious threat ?
a comet aimed at Earth from the dark fringes of the solar system.

September's shower, called the alpha Aurigids, has only been seen three times
before, in 1935, 1986 and 1994. The reason for this elusiveness is the shower's
unusual origin.

Most meteor showers are caused by short-period comets, dirty iceballs that loop
around the inner solar system on orbits lasting less than 200 years, shedding
debris each time they approach the Sun's heat. This debris builds up into a
broad band along the comet's orbit. Every year, when we pass through, it burns
up in the atmosphere and appears as shooting stars.

The Aurigids come from a comet that takes 2000 years to orbit the Sun. With such
infrequent visits, Comet Kiess can't build up a broad dust band; it only
generates a narrow trail of debris each time.

The showers happen when Earth passes through one of these dust trails in
particular, which was thrown off by the comet in 83 BC. "It is only a very
narrow trail, and it is only once in a while that it crosses Earth's path," says
Peter Jenniskens of NASA's Ames Research Center in Moffett Field, California,
US.

He thinks the gravity of Jupiter and Saturn controls the path of the dust trail,
waving it around like a garden hose, occasionally aiming it at Earth. Along with
his colleague J?r?mie Vaubaillon at Caltech, US, Jenniskens has calculated that
the hose should be pointed at us again this year.
Hard crust

Several teams of astronomers will be watching the shower, both from the ground
and from two aircraft following the Earth's shadow.

They are hoping to see fragments of the ancient crust of Comet Kiess. For 4.5
billion years before some gravitational accident nudged it towards the inner
solar system, Kiess was drifting among a vast swarm of icy bodies called the
Oort cloud lying far beyond the planets.

All that time, high-energy particles called cosmic rays bombarded the comet, and
astronomers suspect that created a hard crust by blasting out some of its more
volatile substances.

Only once before have astronomers knowingly seen a shower from a long-period
comet, when Jenniskens predicted an appearance of the alpha Monocerotids in
1995. They penetrated unusually far into the atmosphere, suggesting that they
were made of relatively tough material, perhaps from such a cosmic-ray-produced
crust.

This time, astronomers will be looking at the spectral signature of evaporating
meteors to test this theory. "Now we are better prepared, we can do more
in-depth studies to understand the properties of the material," Jenniskens told
New Scientist.
Contribute observations

He also wants to know whether meteor showers such as this could warn of
planetary peril. At present, astronomers can only spot a long-period comet a few
years before it arrives in the inner solar system, leaving little time to
deflect it if it were pointed right at Earth.

But if it had visited the inner solar system before, the resulting meteor shower
might be used to trace the comet's orbit and get a much earlier warning. The
size and number of Aurigid meteors will tell the researchers how debris has
spread along the orbit and how these showers evolve.

They are keen for amateurs to contribute their observations. "We're interested
to know what is the brightest, biggest Aurigid," says Jenniskens. "Somebody is
going to capture that, and it's probably not going to be us."

The best view of the meteors will be from the west coast of North America,
before dawn on 1 September. Based on past showers, there should be up to 200
bright meteors visible per hour, and they may have an unusual blue-green colour.

The shower probably won't return for at least 50 years, according to Jenniskens'
calculations. "It's a once in a lifetime event."
Received on Wed 08 Aug 2007 06:48:48 PM PDT