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Astronomers find the Oort Cloud of comets may be far less massive than once thought

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Boulder, Colorado -- January 31, 2001 -- Many comets once thought to have been ejected into the Oort Cloud during the early formation of the solar system are now believed to have been pulverized in violent collisions among themselves. The finding, by researchers at Southwest Research Institute® (SwRI®) and the Jet Propulsion Laboratory (JPL), demonstrates that previous models of Oort Cloud formation may have significantly overestimated the mass of the Sun's Oort Cloud, the giant sphere far beyond the planets in which comets are thought to reside.

In work described in the February 1 issue of Nature, Dr. Alan Stern (SwRI) and Dr. Paul Weissman (JPL) showed that, contrary to a long-standing assumption, the ejection of comets to the Oort Cloud during and just after the formation of the giant planets was a violent and highly collisional process. Stern and Weissman's computer models show that most comets and smaller debris present between the outer planets during the so-called "clearing phase" of outer solar system formation were destroyed in mutual collisions before they could be ejected to the Oort Cloud by the strong gravity of the giant planets.

Previous Oort Cloud formation models neglected the effects of these collisions. The new calculations show this to be a poor assumption.

"One implication of these results is that, because collisions introduced additional inefficiencies in Oort Cloud formation, the Cloud may be 10 times less massive than previously thought," says Weissman.

"Another apparent implication," Stern says, "is that the comets in the Oort Cloud could be smaller and quite likely more heavily damaged as a result of these collisions than many had thought. Our results, along with those recently obtained by others, are revealing that both comet and Oort Cloud formation are more complex processes than had previously been suspected. It's a new ball game now."

For more information, contact: Maria Martinez , San Antonio Communications Department, (210) 522-3305, fax: (210) 522-3547, or Hal Levison, Space Studies Department located in Boulder, Colorado, (303) 546-9670.

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