Spaceflight Now: Breaking News

Comet collisions: Only the strong survived?
Posted: February 3, 2001

Comet Hale-Bopp. Photo: Courtesy NASA/JPL
Recurring collisions between comets during the solar system's formation may have ground smaller comets to bits, leaving only big comets larger than 20 kilometers (12 miles) to survive, according to a new model developed by researchers at NASA's Jet Propulsion Laboratory, Pasadena, Calif., and the Southwest Research Institute, Boulder, Colo.

The finding, by Dr. Paul Weissman of JPL and Dr. Alan Stern of Southwest Research Institute, published in the February 1 issue of the journal Nature, demonstrates that previous models may have significantly overestimated the mass of the Oort cloud -- a region far beyond the planets populated by comets flung outward in the solar system's youth.

"We're introducing a new wrinkle in the process of how the Oort cloud formed," said Weissman. One result of the new finding, he said, is that "the cloud may be 10 times less massive than previously thought."

By studying comets of different sizes, the scientists predicted how the comets would collide with each other, and how the collisions would erode the comet's cores, dirty snowballs of dust and ice. Their model showed that comets with nucleus diameters smaller than 20 kilometers (12 miles) would have been destroyed in the early solar system's demolition derby. Previous Oort cloud formation models neglected the effects of these collisions.

Another apparent implication of this violent collisional environment is that the comets in the Oort cloud could be smaller than previously thought, said the scientists. If comets were so eroded that they would never have left the region of the giant planets, then few of them would have survived to be ejected to the Oort cloud. Taking into account their new findings, Weissman estimates that typical comets in the Oort cloud may be about half as large across as compared with current best estimates.

JPL is a division of the California Institute of Technology in Pasadena.