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4. The Discovery and Early Study of Shoemaker-Levy 9

        Comet Shoemaker-Levy 9 was discovered photographically
by the husband and wife scientific team of Carolyn S. and Eugene
M. Shoemaker and David H. Levy on March 24, 1993, using the 0.46-m
(18-in.) Schmidt telescope at Palomar Observatory in California.
Its discovery was a serendipitous product of their continuing
search for "near-Earth objects", and the "9" indicates that it was
the ninth short-period comet (period less than 200 years)
discovered by this team. Near-Earth objects are bodies whose
orbits come nearer to the Sun than that of Earth and hence have
some potential for collisions with Earth. The appearance of the
comet was reported as "most unusual"; the object appeared as "a
dense, linear bar about 1 arc minute long" and had a "fainter,
wispy tail". (A circle is divided into 360 degrees, each degree
into 60 minutes, and each minute into 60 seconds. The word "arc"
is added to denote an angular measure rather than time. The
diameter of the Moon is near 30 arc minutes, for example, while
the apparent diameter of Jupiter when closest to Earth is
50 arc seconds.) The comet's brightness was reported as about
magnitude 14, more than a thousand times too faint to be seen with
the naked eye.

The existence of this object was soon confirmed by James V. Scotti
of the Spacewatch program at the University of Arizona, and the
International Astronomical Union's Central Bureau for Astronomical
Telegrams immediately issued "Circular No. 5725" reporting the
discovery as a new comet, giving it the provisional designation of
1993e (the fifth comet discovered or recovered in 1993). Scotti
reported at least five condensations in a "long, narrow train
about 47 arc seconds in length and about 11 arc seconds in width,"
with dust trails extending 4.20 arc minutes to the east and
6.89 arc minutes to the west and tails extending about
1 arc minute from elements of the nuclear train. Bureau director
Brian G. Marsden noted that the comet was some 4 arc minutes from Jupiter and
that its motion suggested that it could be near Jupiter's distance
from the Sun.

By March 27 Marsden had enough positions to attempt to derive
possible orbits. One elliptical solution gave a close approach to
Jupiter in July 1992. Also on March 27, Jane Luu and David Jewitt
took an image with the 2.2-m telescope on Mauna Kea in Hawaii that
showed as many as 17 separate sub-nuclei "strung out like pearls
on a string" 50 arc seconds long, and this was reported in
Circular No. 5730 two days later. Figure 4 shows an early image
taken by Scotti on March 30, 1993. This long exposure (440 seconds
on a CCD detector) brings out the faint detail of the debris
field, though it overexposes the individual nucleus fragments.
Figure 5 is an image from the Hubble Space Telescope (HST), taken
by Harold A. Weaver and collaborators on July 1, 1993 (before the
HST repair mission), that clearly shows at least 15 individual
fragments in one image frame of the train.

In IAU Circular No. 5744, dated April 3, 1993, Marsden used
positions covering a period of 17 days (including two prediscovery
positions from March 15) and was able to report that no orbit of
very long period (near parabolic) was possible. The orbit had to
be an ellipse of rather small eccentricity relative to the Sun and
relatively short period. Since it was not at all obvious where the
center of mass of this new comet lay, most observers were just
reporting the position of what appeared to be the center of the
train. This made an accurate orbit (or orbits) difficult to
determine. Marsden suggested that a very close approach to Jupiter
in 1992 continued to be a distinct possibility, and the orbit he
chose to publish was one with the comet "at least temporarily" in
orbit around Jupiter.

By May 22 Marsden had almost 200 positions of the center of the
train. In Circular No. 5800 he reported on an orbit computed
May 18 by Syuichi Nakano that showed the comet approaching within
120,000 km of Jupiter on July 8, 1992, and approaching again, this
time within 45,000 km of the center of Jupiter, on July 25, 1994.
Marsden noted that this distance was less than the radius of
Jupiter. In other words, the comet, or at least parts of it, could
very well hit Jupiter.

By October 18, 1993, Paul W. Chodas and Donald K. Yeomans were
able to report at the annual American Astronomical Society's
Division of Planetary Sciences meeting that the probability of
impact for the major fragments of Shoemaker-Levy 9 was greater
than 99%. The fragments apparently would hit over a period of
several days, centered on July 21.2, on the night side of Jupiter
at latitude 44 deg. S and longitude 35 deg. past the midnight meridian,
according to available observations. This unfortunately is also
the back side of Jupiter as viewed from Earth. The 1992 approach
to Jupiter that disrupted the comet was calculated to have been at
a distance of 113,000 km from the planet's center and only
42,000 km above its cloud tops. Furthermore, they found that the
comet had been in a rapidly changing orbit around Jupiter for some
time before this, probably for at least several decades. It did
not fragment during earlier approaches to Jupiter, however,
because these were at much greater distances than that of 1992.

After recovery of the comet on December 9, following the period
during which it was too near to the Sun in the sky to observe,
Chodas and Yeomans found that the probability was greater than
99.99% that all the large fragments will hit Jupiter. The
encounter period is now centered on July 19.5, and orbits for
individual fragments are uncertain by about 0.03 days (40 minutes). The
impact site has moved closer to the limb of Jupiter, now near 75 deg.
from the midnight meridian and only a few degrees beyond the dark
limb as seen from Earth, but all pieces still impact on the back
side. The 1992 approach that split the comet is now calculated to
have occurred on July 7.84 and only 25,000 km (15,500 mi.) above
the clouds. These data now cover a much longer time base and are
based upon calculations for individual fragments. They are
unlikely to change significantly in the future. The comet probably
approached Jupiter no nearer than about 9 million km in the orbit
prior to that of 1992.

In a comprehensive paper prepared for The Astronomical Journal,
Zdenek Sekanina, Chodas, and Yeomans report on the details of the
breakup of Shoemaker-Levy 9 as calculated from the positions,
motions, and brightness of the fragments and debris. They used
data from Jewitt, Luu, and Chen taken in Hawaii, Scotti in
Arizona, and Weaver's Hubble Space Telescope (HST) observing team.
For example, the 11 brightest fragments as measured with the HST,
visual (V) magnitude 23.7-24.8 or about 15 million times too faint
to be seen by the naked eye, had the brightness one would expect
from spheres 4.3 down to 2.5 km in diameter, assuming a normal
cometary reflectivity for the fragments (about 4%). Of course the
fragments are not spheres, since tidal disruption tends to occur
in planes perpendicular to the direction of the object causing the
disruption (Jupiter) and since comets generally are not spherical
to begin with. Nevertheless, adding up the sizes of these
11 fragments, the other fragments not precisely measured, and all
of the debris making up the trails and tails, suggests that the
original comet must have been at least 9 km in average diameter,
and it could have been somewhat larger. This was a good-sized
comet, about the same size as Comet Halley.

When comets split, the pieces do not go flying apart at a high
velocity, each to immediately go into its own independent orbit.
The escape velocity from a non-rotating spherical comet 5 km in
radius with a density of 0.5 g/cm^3 (half that of water) is
2.65 m/s (6.5 mph). If suddenly freed of gravity and molecular
bonds, a particle at the equator of that 10-km body, assuming a
rotation period of 12 hours, would depart with a velocity of only
0.72 m/s (1.6 mph) relative to the center of the comet. Some
comets appear to rotate more rapidly than once per half day, while
many, such as Halley, rotate more slowly. In any case the
centrifugal force on unattached pieces of material lying on the
surface of a rotating comet is not normally sufficient to overcome
the gravity holding them there. Pieces do not fly off of the
nucleus "spontaneously". Even when the tidal forces overcome self-
gravity the pieces separate slowly, and they continue to interact
gravitationally. More important, the pieces bang into one another,
changing their velocities and perhaps fragmenting further.

In the case of Shoemaker-Levy 9, Sekanina, Chodas, and Yeomans
estimate that although fragmentation probably began before closest
approach to Jupiter, dynamic independence of the pieces didn't
occur until almost two hours after closest approach.