Most comets disintegrate after their
first few passages through the inner solar system, say scientists at
Southwest Research Institute (SwRI). A new study has revealed that 99
percent of the objects from the cloud of comets at the edge of the solar
system, known as the Oort cloud, break apart sometime after they enter the
inner solar system. The findings appear in the current issue of the journal

For several decades, astronomers have wondered about the fate of comets once
they stop producing their beautiful and easily seen tails. Some have
speculated that these objects are still there but have become dormant —
that is, they have lost the material that allows them to generate tails —
making them much harder to detect. Others have suggested that comets
disintegrate, leaving no visible trace.

Led by Dr. Harold F. Levison, a staff scientist at the SwRI Boulder office,
the team compared computer models with observations to determine the fate of
the missing comets. The team created thousands of fictitious new comets,
tracked the comets as they entered the solar system from the Oort cloud, and
calculated their evolution based on the gravitational influences of the sun,
planets, and Milky Way. By following comet trajectories until they were
ejected from the solar system, hit a planet, or struck the sun, team members
estimated the number of dormant comets that should have been observed if all
active comets had become dormant. This number is 100 times larger than what
is actually seen. From this, team members deduced that 99 percent of these
objects vanish.

“These objects are simply not where we expect them to be,” says Levison.
“The only explanation that I can think of is that they go ‘poof.'”

Interestingly, comets originating in the Kuiper Belt, a cometary source just
beyond Neptune, do not disrupt nearly as often as those originating in the
Oort cloud. Both comet classes are believed to be composed of similar
mixtures of ice and rock, but their different disruption behaviors could
reflect the chemical or physical characteristics of their formation areas.
Another theory is that the inconsistency between the classes could be
related to evolutionary processes. Most Oort cloud comets move quickly from
distant orbits to orbits that closely approach the sun, while Kuiper Belt
objects move slowly through the planetary regions. This could suggest that
different thermal histories lead to different disruption rates.

“It is possible that Oort cloud comets disrupt because of strong thermal
gradients or volatile pressure buildup, while Kuiper Belt objects survive
because they are warmed more slowly,” says Levison.

NASA provided funding for the program. The paper “The Mass Disruption of
Oort Cloud Comets,” by Levison, Dr. Alessandro Morbidelli (Observatoire de
la Cote d’Azur), Dr. Luke Dones (SwRI), Dr. Robert Jedicke (University of
Arizona), Dr. Paul A. Wiegert (Queen’s University, Ontario), and Dr. William
F. Bottke (SwRI) appears in the June 21 issue of Science.

SwRI is an independent, nonprofit, applied research and development
organization based in San Antonio, Texas, with more than 2,700 employees and
an annual research volume of more than $319 million.