Where have all the comets gone?
Posted: June 20, 2002

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 Science.

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.

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