[meteorite-list] Solving the Case of the Missing Comets

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:01:34 2004
Message-ID: <200206201951.MAA19272_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/solarsystem/comet_missing_020620.html

Solving the Case of the Missing Comets
By Robert Roy Britt
space.com
20 June 2002

Astronomers know where most newly discovered comets come from, a
reservoir on the outskirts of our solar system called the Oort Cloud,
which extends nearly halfway to the next star.

Now and then, one of these distant comets is booted into the inner solar
system and loops around the Sun on its first close pass. Most of these
first-time comets are then kicked clear out beyond the Oort Cloud, never
to return. A smaller number are swallowed by the Sun. The remainder are
set on a new course that will bring them back around the Sun in anywhere
from 20 years to a million years, depending on their new orbits.

Yet in the five decades that scientists have known all this, they have
puzzled over why they don't see hundreds of times more returning comets
than they do. The answer may be that the objects simply disintegrate,
according to a new study.

Dutch astronomer Jan Oort, for whom the reservoir of frozen dirtballs is
named, figured out that of those comets that survive their first trip and
should therefore be locked into recurring passes through the inner solar
system, far too few ever seem to come back. The leading idea to explain
this problem suggests that the comets essentially turn off, possibly
having depleting the volatile gases that make their glowing heads and
tails. They become, in effect, asteroids.

Scientists at the Southwest Research Institute in Boulder, Colorado,
plugged all these factors into a new computer model and found that there
simply aren't enough asteroids being discovered to account for the
prevailing suggestion, however.

"This implies is that what's happening to these things is that they
disintegrate," said Harold Levison, who led the study. "Ninety-nine
percent of them disintegrate." Astronomers saw an example of this last
year when a comet called LINEAR came apart in dramatic fashion.

The new study further found, however, that comets originating in a nearer
reservoir known as the Kuiper Belt do not come apart as frequently.

In a telephone interview, Levison said this is the first evidence
suggesting that the two populations of comets may be composed
differently.

"Here's a population of comets that just go 'poof' and disappear," he
said. "The other doesn't"

Figuring out why could help researchers better understand the dynamics of
the solar system and how planets came to be, since comets are thought to
be pristine records of the formation era. The study will be published in
the June 21 issue of the journal Science.

Mark Bailey of the Armagh Observatory in Northern Ireland writes in an
accompanying analysis in Science that if the interpretation by Levison
and his colleagues is correct, it would indeed be the first hard evidence
for physical differences between the two comet populations, possibly due
to different origins or evolutionary processes. The case is not firm,
though.

"At present, comets remain a puzzle," Bailey said.
Received on Thu 20 Jun 2002 03:51:18 PM PDT