leggo e riporto:
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MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109 TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov
Contact: Martha J. Heil, (818) 354-0850
FOR IMMEDIATE RELEASE January 31, 2001
COMET COLLISIONS: ONLY THE STRONG SURVIVED?
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.
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In questi nuovi calcoli la distribuzione delle comete nella nube di
Oort sarebbe ancora sferica, oppure ci si troverebbe coi dati ultimi
delel comete a lunga orbita che sembrerebbero dire che ci sono dei
punti "privilegiati" di provenienza?
Ernesto
Received on Sat Feb 03 2001 - 02:12:27 CET