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3. The Fragmentation of Comets
Every body is held together by two forces, its self-
gravitation and its internal strength due to molecular bonding.
With no external forces on it (and no initial rotation) a liquid
body would form a perfect sphere just from self-gravitation (and
from very weak molecular forces -- surface tension). Approaching
another body, the sphere would begin to elongate toward that body.
Finally, when the difference in gravitational force on the near
side and far side of the former sphere exceeded the self-
gravitation, the body would be torn apart. The distance from the
larger body at which this disruption occurs is the so-called Roche
limit, named for the man who first studied the problem. The
differential gravitational effects of the Moon and the Sun are
what raise the tides in Earth's oceans, and such forces are often
referred to as tidal forces.
Solid bodies have intrinsic strength due to their molecular bonds.
Aluminum wire may have a tensile strength of 2.4 x 10^9 dynes/cm^2
(5 million lb./ft.^2) and good steel wire a tensile strength
10 times larger still, which far exceeds the tidal force of
anything short of a black hole. As stated in Section 1, comets
have very low tensile strength, near 1 10^3 dynes/cm^2
(2 lb./ft.^2). They can be pulled apart very easily by tidal force
(or any other substantial force, for that matter). Some 25 comets
have been observed to split over the past two centuries. In other
cases two or more comets have been discovered in nearly the same
orbit, and calculations have indicated that they were once a
single comet. A few of these cases have been obviously
attributable to the tidal forces of Jupiter (Comet Brooks 2 and
Comet Shoemaker-Levy 9) or the Sun (the Kreutz comet family),
while other splittings have to be attributed to less obvious
causes. For example, the loss of material from an active comet,
which tends to occur from a few localized areas, is bound to
weaken it. It may be that a rapidly rotating comet can be weakened
to the point where the centrifugal force is sufficient to cause
large pieces to break off.
The Kreutz family is the name given to many comets which closely
approach the Sun from one direction in space. They always approach
the Sun to within 3 million km or less, and some have actually hit
the Sun. The family was named for Heinrich Kreutz who published
extensive monographs on three of these comets and supported the
idea that they had a common origin, perhaps in a giant comet
observed in 372 B.C. Today the Kreutz family has eight definite,
well-studied members; 16 probable members (that are listed as
probable only because they didn't survive passage within
800,000 km of the Sun to permit further study); and three more
possible members. Extensive work by Brian Marsden suggests that
all of these may have resulted from the splitting of two comets
around 1100 A.D., which in turn may have been the parts of the
great comet of 372 B.C. Those Kreutz fragments which survive their
encounters with the Sun are often found to have split yet again!
The classic Roche limit for a (fluid) body of density 1 g/cm^3
approaching Jupiter is about 119,000 km above the cloud tops of
the planet. It is about 169,000 km for a body having a density of
0.5 g/cm^3. More complete modern theories making different
assumptions result in a somewhat smaller limit. In 1886 Comet
Brooks 2 came within 72,000 km of Jupiter's clouds and split into
two pieces. In July 1992 Comet Shoemaker-Levy 9 came within about
25,000 km of Jupiter's clouds and fragmented into 21 or more large
pieces and an enormous amount of smaller debris down to micron or
submicron size. Details of this last event follow.
Acknowledgments:
This booklet is the product of many scientists, all of
whom have cooperated enthusiastically to bring their best
information about this exciting event to a wider audience. They
have contributed paragraphs, words, diagrams, slides, and
preprints as well as their critiques to this document, which
attempts to present an event that no one is quite sure how to
describe. Sincere thanks go to Mike A'Hearn, Paul Chodas, Gil
Clark, Janet Edberg, Steve Edberg, Jim Friedson, Mo Geller, Martha
Hanner, Cliff Heindl, David Levy, Mordecai-Mark Mac Low, Al
Metzger, Marcia Neugebauer, Glenn Orton, Elizabeth Roettger, Jim
Scotti, David Seal, Zdenek Sekanina, Anita Sohus, Harold Weaver,
Paul Weissman, Bob West, and Don Yeomans -- and to those who might
have been omitted. The choice of material and the faults and flaws
in the document obviously remain the responsibility of the author
alone.
The writing and production of this material was made possible
through the support of Jurgen Rahe and Joe Boyce, Code SL, NASA,
and of Dan McCleese, Jet Propulsion Laboratory (JPL). For help in
the layout and production of this booklet, on a very tight
schedule, additional thanks go to the Design Services Group of the
JPL Documentation Section.
All comments should be addressed to the author:
Ray L. Newburn, Jr.
Jet Propulsion Laboratory, MS 169-237
4800 Oak Grove Dr.
Pasadena, CA 91109-8099