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1. What is a Comet?

        Comets are small, fragile, irregularly shaped bodies
composed of a mixture of non-volatile grains and frozen gases.
They usually follow highly elongated paths around the Sun. Most
become visible, even in telescopes, only when they get near enough
to the Sun for the Sun's radiation to start subliming the volatile
gases, which in turn blow away small bits of the solid material.
These materials expand into an enormous escaping atmosphere called
the coma, which becomes far bigger than a planet, and they are
forced back into long tails of dust and gas by radiation and
charged particles flowing from the Sun. Comets are cold bodies,
and we see them only because the gases in their comae and tails
fluoresce in sunlight (somewhat akin to a fluorescent light) and
because of sunlight reflected from the solids. Comets are regular
members of the solar system family, gravitationally bound to the
Sun. They are generally believed to be made of material,
originally in the outer part of the solar system, that didn't get
incorporated into the planets -- leftover debris, if you will. It
is the very fact that they are thought to be composed of such
unchanged "primitive" material that makes them extremely
interesting to scientists who wish to learn about conditions
during the earliest period of the solar system.

Comets are very small in size relative to planets. Their average
diameters usually range from 750 m or less to about 20 km.
Recently, evidence has been found for much larger distant comets,
perhaps having diameters of 300 km or more, but these sizes are
still small compared to planets. Planets are usually more or less
spherical in shape, usually bulging slightly at the equator.
Comets are irregular in shape, with their longest dimension often
twice the shortest. (See Appendix A, Table 3.) The best evidence
suggests that comets are very fragile. Their tensile strength (the
stress they can take without being pulled apart) appears to be
only about 1,000 dynes/cm2 (about 2 lb./ft.^2). You could take a
big piece of cometary material and simply pull it in two with your
bare hands, something like a poorly compacted snowball.

Comets, of course, must obey the same universal laws of motion as
do all other bodies. Where the orbits of planets around the Sun
are nearly circular, however, the orbits of comets are quite
elongated. Nearly 100 known comets have periods (the time it takes
them to make one complete trip around the Sun) five to seven Earth
years in length. Their farthest point from the Sun (their
aphelion) is near Jupiter's orbit, with the closest point
(perihelion) being much nearer to Earth. A few comets like Halley
have their aphelions beyond Neptune (which is six times as far
from the Sun as Jupiter). Other comets come from much farther out
yet, and it may take them thousands or even hundreds of thousands
of years to make one complete orbit around the Sun. In all cases,
if a comet approaches near to Jupiter, it is strongly attracted by
the gravitational pull of that giant among planets, and its orbit
is perturbed (changed), sometimes radically. This is part of what
happened to Shoemaker-Levy 9. (See Sections 2 and 4 for more
details.)

The nucleus of a comet, which is its solid, persisting part, has
been called an icy conglomerate, a dirty snowball, and other
colorful but even less accurate descriptions. Certainly a comet
nucleus contains silicates akin to some ordinary Earth rocks in
composition, probably mostly in very small grains and pieces.
Perhaps the grains are "glued" together into larger pieces by the
frozen gases. A nucleus appears to include complex carbon
compounds and perhaps some free carbon, which make it very black
in color. Most notably, at least when young, it contains many
frozen gases, the most common being ordinary water. In the low
pressure conditions of space, water sublimes, that is, it goes
directly from solid to gas -- just like dry ice does on Earth.
Water probably makes up 75-80% of the volatile material in most
comets. Other common ices are carbon monoxide (CO), carbon dioxide
(CO2), methane (CH4), ammonia (NH3), and formaldehyde (H2CO).
Volatiles and solids appear to be fairly well mixed throughout the
nucleus of a new comet approaching the Sun for the first time. As
a comet ages from many trips close to the Sun, there is evidence
that it loses most of its ices, or at least those ices anywhere
near the nucleus surface, and becomes just a very fragile old
"rock" in appearance, indistinguishable at a distance from an
asteroid.

A comet nucleus is small, so its gravitational pull is very weak.
You could run and jump completely off of it (if you could get
traction). The escape velocity is only about 1 m/s (compared to
11 km/s on Earth). As a result, the escaping gases and the small
solid particles (dust) that they drag with them never fall back to
the nucleus surface. Radiation pressure, the pressure of sunlight,
forces the dust particles back into a dust tail in the direction
opposite to the Sun. A comet's tail can be tens of millions of
kilometers in length when seen in the reflected sunlight. The gas
molecules are torn apart by solar ultraviolet light, often losing
electrons and becoming electrically charged fragments or ions. The
ions interact with the wind of charged particles flowing out from
the Sun and are forced back into an ion tail, which again can
extend for millions of kilometers in the direction opposite to the
Sun. These ions can be seen as they fluoresce in sunlight.

Every comet then really has two tails, a dust tail and an ion
tail. If the comet is faint, only one or neither tail may be
detectable, and the comet may appear just as a fuzzy blob of
light, even in a big telescope. The density of material in the
coma and tails is very low, lower than the best vacuum that can be
produced in most laboratories. In 1986 the Giotto spacecraft flew
right through Comet Halley only a few hundred kilometers from the
nucleus. Though the coma and tails of a comet may extend for tens
of millions of kilometers and become easily visible to the naked
eye in Earth's night sky, as Comet West's were in 1976, the entire
phenomenon is the product of a tiny nucleus only a few kilometers
across.

Because comet nuclei are so small, they are quite difficult to
study from Earth. They always appear at most as a point of light
in even the largest telescope, if not lost completely in the glare
of the coma. A great deal was learned when the European Space
Agency, the Soviet Union, and the Japanese sent spacecraft to fly
by Comet Halley in 1986. For the first time, actual images of an
active nucleus were obtained (see Figure 1) and the composition of
the dust and gases flowing from it was directly measured. Early in
the next century the Europeans plan to send a spacecraft called
Rosetta to rendezvous with a comet and watch it closely for a long
period of time. Even this sophisticated mission is not likely to
tell scientists a great deal about the interior structure of
comets, however. Therefore, the opportunity to reconstruct the
events that occurred when Shoemaker-Levy 9 split and to study
those that will occur when the fragments are destroyed in
Jupiter's atmosphere is uniquely important (see Sections 4, 7, and
8).




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