[meteorite-list] Comets Get A Snowball's Chance

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:54:02 2004
Message-ID: <200202071638.IAA28382_at_zagami.jpl.nasa.gov>

http://www.guardian.co.uk/science/story/0,3605,645982,00.html

Comets get a snowball's chance

A stream of comets is falling into the sun, writes Duncan Steel

By Duncan Steel
The Guardian (United Kingdom)
February 7, 2002

Theory and observation go hand in hand in science. Until recently we were
limited to a theoretical evaluation of just what is a snowball's chance in hell. But
now we have experimental verification. As expected, the chance is essentially
zilch.

Comets are snowballs. Dirty ones, with rock and organic chemicals mixed in, but
snowballs none the less. And hell, one imagines, is much like the sun's surface.
With a temperature near 6,000 C, it's more than red-hot.

Comets have a major obstacle to pass before they can plunge into the sun.
Surrounding our local star is the corona, the multimillion-degree solar
atmosphere that can only be seen during an eclipse, which occurs rarely. Satellite
studies of the corona have enabled astronomers to see a multi tude of comets
make their death plunge.

To better understand the solar atmosphere and its effect on the terrestrial
environment we need continuous monitoring.

The solution is obvious, though costly. Produce an artificial eclipse using a
satellite-borne telescope and a black disk to occult the sun. Several instruments
of this type have flown, one of the most successful being Soho (the solar and
heliospheric observatory), a joint project of ESA and Nasa.

In the Soho image pictured, the large dark area straddling the centre results from
the obscuring disk. The white circle inside it indicates the size of the sun. Several
bright regions are seen around the disk's periphery, showing outward gas flows
in the corona.

But there is also an obvious bright streak at the lower right. This is a small
comet. Successive satellite images, obtained in October, showed it to be falling
into the solar furnace. Almost 400 comets have been identified using Soho.
Although it is operated by professional teams, they are mostly interested in solar
physics. This means that many comets pass unnoticed and remain in archives for
years before amateurs identify possible comets.

Champion among these is Michael Oates, a member of the Manchester
Astronomical Society. He has found well over a hundred comets in Soho images.
Although Soho detects some comets near the sun that will turn around and pass
outwards again most are seen in their death throes. Many are so small, not much
bigger than a house, that they hardly deserve to be called comets. They produce
a detectable trail because as the intense solar radiation causes their constituent
ice to evaporate, dust is released, which scatters sunlight.

These tiny comets, often called sungrazers, are interesting because the majority
seem to follow almost the same path. That is, they appear to be fragments of a
much larger comet that broke apart millennia ago.

We saw the same sort of thing happen, on a smaller scale, a few years ago.
Comet Shoemaker-Levy 9 was found in orbit around Jupiter, apparently having
broken apart in 1992, just before its discovery, when it flew too close to the
planet. By the time of the collision with Jupiter in July 1994, its 20-odd major
fragments had separated sufficiently for the fireworks to continue for more than
a week.

With sungrazer comets, the spreading is much greater. In the late 19th century,
Heinrich Kreutz, working in Kiel, Germany, noted the orbital similarity of several
bright comets observed in the preceding decades. They became known as the
Kreutz group, but until Soho entered operation at the end of 1995 it was never
suspected how many individual members the complex might contain. There are
certainly tens of thousands to be found, and the parent of the swarm must have
been a behemoth, more than a hundred kilometres in size and so over a
thousand times the mass of Comet Halley.

The history of the Kreutz parent breakup is gradually being pieced together. To
explain the fragment dispersion, a time scale of at least 10,000 years is required,
but there have been subsequent disintegration events. In 372 BC the Greek
astronomer Ephorus saw a bright comet break asunder. One of its two
daughters seems to have been observed again about every 350 years since, the
other taking 700 years.

Each time subsidiary splits have taken place, the stream of debris has built up.
But why do comets split? Some fall apart for no apparent reason.
Shoemaker-Levy 9 was torn into pieces by the tidal force of Jupiter's gravity
when it strayed too close. But what causes them to fall apart is apparently
thermal stress. As each split occurs, more of the ice is exposed, and the
fragments get smaller until on the next visit they vaporise. Giant comets such as
the Kreutz progenitor exist, and when they split, they leave myriad smaller
bodies zipping through space, each potentially lethal.

Although the Kreutz comets have orbits crossing that of the Earth, their
orientation is such that they consistently miss us. Just as well, else these
snowballs would be giving us hell.
Received on Thu 07 Feb 2002 11:38:42 AM PST