Significance
 ============
 The impact of comet Shoemaker-Levy 9 onto Jupiter represented the
 first time in human history that people have discovered a body in
 the sky and been able to predict its impact on a planet more than
 seconds in advance.  The impact delivered more energy to Jupiter
 than the largest nuclear warheads ever built, and up to a
 significant percentage of the energy delivered by the impact which
 is generally thought to have caused the extinction of the dinosaurs
 on Earth, roughly 65 million years ago.
 
 
 History
 =======
 The comet, the ninth short-period comet discovered by Gene and
 Carolyn Shoemaker and David Levy, was first identified on a
 photograph taken on the night of 24 March 1993 with the 0.4-meter
 Schmidt telescope at Mt. Palomar.  On the original image it appeared
 'squashed'.  Subsequent photographs at a larger scale taken by Jim
 Scotti with the Spacewatch telescope on Kitt Peak showed that the
 comet was split into many separate fragments.  Before the end of
 March it was realized that the comet had made a very close approach
 to Jupiter in mid-1992 and at the beginning of April, after
 sufficient observations had been made to determine the orbit more
 reliably, Brian Marsden found that the comet is in orbit around
 Jupiter.  By late May it appeared that the comet was likely to
 impact Jupiter in 1994.  Since then, the comet has been the subject
 of intensive study.  Searches of archival photographs have
 identified pre-discovery images of the comet from earlier in March
 1993 but searches for even earlier images have been unsuccessful.
 
 
 Cometary Orbit
 ==============
 According to the most recent computations, the comet passed less
 than 1/3 of a Jovian radius above the clouds of Jupiter late on 7
 July 1992 (UT).  The individual fragments separated from each other
 1 1/2 hours after closest approach to Jupiter and they are all in
 orbit around Jupiter with an orbital period of about two years.
 Calculations of the orbit prior to 7 July 1992 are very uncertain
 but it seems very likely that the comet was previously in orbit
 around Jupiter for two decades or more.  Because the orbit takes the
 comet nearly 1/3 of an astronomical unit (30 million miles) from
 Jupiter, the sun causes significant changes in the orbit.  Thus,
 when the comet again came close to Jupiter in 1994 it actually
 impacted the planet, moving almost due northward at 60 km/sec aimed
 at a point only halfway from the center of Jupiter to the visible
 clouds.
 
 The 23 identified fragments all hit Jupiter in the southern
 hemisphere, at latitudes near 45 S, between 16 and 22 July 1994,
 approaching the atmosphere at an angle roughly 45 deg from the
 vertical.  The times of the impacts were known to within a few hours
 but observations in early 1994 significantly improved the precision
 of the predictions.  The impacts happened on the back side of
 Jupiter as seen from Earth in an area that was also in darkness.
 This area was close to the limb of Jupiter and was carried by
 Jupiter's rotation to the front, illuminated side less than half an
 hour after the impact.  The grains ahead of and behind the comet
 impacted Jupiter over a period of four months, centered on the time
 of the impacts of the major fragments.  The grains in the tail of
 the comet passed behind Jupiter and remain in orbit around the
 planet.
 
 
 The Nature of the Comet
 =======================
 The exact number of large fragments is not certain since the best
 images show hints that some of the larger fragments were multiple.
 At least 23 major fragments were identified.  No observations are
 capable of resolving the individual fragments to show the solid
 nuclei.  Images with the Hubble Space Telescope suggested that there
 were discrete, solid nuclei in each of the largest fragments which,
 although not spatially resolved, produce a single, bright pixel that
 stood out above the surrounding coma of grains.  Reasonable
 assumptions about the spatial distribution of the grains and about
 the reflectivity of the nuclei implied sizes of 2 to 4 km (diameter)
 for each of the 11 brightest nuclei.  Because of the uncertainties
 in these assumptions, the actual sizes were very uncertain.
 
 No outgassing was detected from the comet.   The dust distribution
 suggests that the material ahead of and behind the major fragments
 in the orbit were likely large particles from the size of sand up to
 boulders.  The particles in the tail are very small, not much larger
 than the wavelength of light.  The brightnesses of the major
 fragments were observed to change by factors up to 1.7 between March
 and July 1993, although some became brighter while others became
 fainter.
 
 Summary of impact times, impact locations, and impact geometries
 ----------------------------------------------------------------
 Published estimates of the impact times and locations of the
 fragments of SL9 are given below (Table 5 in [CHODAS&YEOMANS1996]).
 Impact was defined to occur at the 100mbar level of Jupiter's
 atmosphere.  The impact for all fragments except J and M are based
 on independent orbit solutions given by [CHODAS&YEOMANS1996].  The
 estimates for the 'lost' fragments J and M were obtained by applying
 the tidal disruption model to the orbit for fragment Ql and matching
 the astrometry of these two fragments relative to Ql.  The third
 column of the table contains the final pre-impact prediction for
 each of the fragments as distributed electronically by the UMd
 e-mail exploder.   The fourth column lists the final best estimates,
 which were inferred directly from impact phenomena for 16 fragments,
 and computed from the orbit solutions for the rest.  All times are
 as viewed from the Earth, and therefore include the light travel
 time.  The impact time uncertainties are rough estimates which
 indicate a confidence level in the accepted time; they are not
 formal 1-sigma uncertainties.  The impact latitude is jovicentric,
 while the longitude is System III, measured westwards on the
 planet.  The meridian angle is the jovicentric longitude of the
 impact point measured from the midnight meridian towards the morning
 terminator.  At the latitude of the impacts, the limb as viewed from
 the Earth was at meridian angle 76 deg, and the terminator was at
 meridian angle 87 deg.  The final column gives the angular distance
 of the impacts behind the limb.
 
 Event    Impact Time (UTC)          Impact Location Merid. Ang. Dist.
     -----------------------------   --------------- Angle Behind Limb
    Date Predicted  Accepted   +/-    Lat.     Lon.
   (July) h m s      h m s     (s)   (deg)    (deg)   (deg)   (deg)
 
 A   16  19:59:40   20:10:40    60   -43.35    184    65.40   7.7
 B   17  02:54:13   02:50:00   180   -43.22     67    63.92   8.8
 C   17  07:02:14   07:10:50    60   -43.47    222    66.14   7.1
 D   17  11:47:00   11:52:30    60   -43.53     33    66.16   7.1
 E   17  15:05:31   15:11:40   120   -43.54    153    66.40   6.9
 F   18  00:29:21   00:35:45   300   -43.68    135    65.30   7.7
 G   18  07:28:32   07:33:33     3   -43.66     26    67.09   6.4
 H   18  19:25:53   19:31:59     1   -43.79     99    67.47   6.1
 J   19  02:40      01:35     3600   -43.75   ~316    68.05  ~6
 K   19  10:18:32   10:24:17     2   -43.86    278    68.32   5.5
 L   19  22:08:53   22:16:49     1   -43.96    348    68.86   5.1
 M   20  05:45      06:00      600   -43.93   ~264    69.25  ~5
 N   20  10:20:02   10:29:20     2   -44.31     71    68.68   5.1
 P2  20  15:16:20   15:21:11   300   -44.69    249    67.58   5.8
 P1  20  16:30      16:32:35   800   -45.02   ~293    65.96   6.9
 Q2  20  19:47:11   19:44:00    60   -44.32     46    69.26   4.7
 Q1  20  20:04:09   20:13:53     1   -44.00     63    69.85   4.3
 R   21  05:28:50   05:34:57    10   -44.10     42    70.21   4.1
 S   21  15:12:49   15:16:30    60   -44.22     33    70.34   4.0
 T   21  18:03:45   18:09:56   300   -45.01    141    67.73   5.7
 U   21  21:48:30   22:00:02   300   -44.48    278    69.54   4.5
 V   22  04:16:53   04:23:20    60   -44.47    149    69.96   4.2
 W   22  17:59:45   08:06:16     1   -44.13    283    71.19   3.4