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.