By David Tytell Sky and Telescope Magazine
to recent studies, 99 percent of all Oort cloud comets, like Comet
Hale-Bopp seen here, break apart into debris. Sky & Telescope's
Dennis di Cicco obtained this view on the morning of March 17,
1997 from a dark-sky site south of Boston, Massachusetts. He used
an 8-inch f/1.5 Schmidt camera.
in two flavors: those that originate from the Kuiper Belt about
30 to 60 astronomical units away, and the nearly isotropic comets
(NICs) that come from the far more distant Oort Cloud more than
10,000 a.u. away. Oort Cloud comets fly through the inner solar
system at a rate of about a dozen per year. However, what becomes
of these icy bodies once they stop producing their easy-to-spot
gas comas and tails has remained somewhat of a mystery.
Cloud comets eventually go dormant, the solar system should be
brimming with burnt-out objects that have run out of the frozen
gases that make them glow. Comets from the Kuiper Belt do often
settle into asteroid-like cinders. But whether Oort Cloud objects
undergo a similar fate was uncertain. If Oort Cloud comets evolve
to cinders, search programs such as the Lincoln Laboratory Near
Earth Asteroid Research project (LINEAR) should have uncovered
truckloads of the spent bodies. But they simply aren't there.
A new study
published in last week's Science assigns a grim fate to these
dormant passersby. According to Harold F. Levison (Southwest Research
Institute, Boulder) and his colleagues, 99 percent of the Oort
cloud bodies that come to our solar system disrupt and dissolve
to the conclusion after running computer models where he created
thousands of comets and watched their evolution. Given the known
influx rate of comets and the number of predicted detections,
the conclusion is that nearly all just vanish.
Why they disrupt
remains somewhat of a mystery. One hypothesis proposed by Levison
in his Science paper is that the comets undergo a thermal shock
and internal-gas pressure buildup when they enter our relatively
warm solar system having come from the coldest reaches of interstellar
space. Another possibility is that the Oort Cloud objects are
simply structurally weaker than Kuiper Belt comets.