By far the most spectacular event in the Solar System ever witnessed by the human race was the collision of fragments of comet P/Shoemaker-Levy 9 with Jupiter between July 16-23, 1994 (Figure 1).
The comet was discovered on March 24, 1993 by C.S. and E.M. Shoemaker and D. Levy [1993] and reported to have a squashed appearance. Subsequent photographs at a larger scale taken by Jim Scotti with the Spacewatch telescope on Kitt Peak, Tucson, AZ showed that the comet was split into many separate fragments [ Shoemaker et al., 1993]. By April it was realized that the comet had made a very close approach to Jupiter in mid-1992 [ Marsden, 1993a] and after sufficient observations had been made to determine the orbit more reliably, it was found to be in orbit around Jupiter [ Marsden, 1993b]. By late May it appeared that the comet was likely to impact Jupiter in July, 1994 [ Marsden, 1993c]. Since then, the comet has been the subject of intensive study and theoretical modeling of the effects of the impacts. Searches of archival photographs have identified pre-discovery images from earlier in March, 1993 [ Otomo and Helin 1993] but searches for even earlier images which covered the orbit have been unsuccessful. Astrometric observations of the comet fragments enabled orbit determinations, predictions of the impact times and locations into Jupiter. Observations of the impacts were carried out using the complement of ground-based telescopes all over the world including three spacecraft: Galileo, Ulysses, and Voyager 2, and three Earth-orbiting telescopes: Hubble Space Telescope (HST), International Ultraviolet Explorer (IUE), and Extreme Ultraviolet Violet Explorer (EUVE). The observed brightness of the rising plume of hot gases exceeded anyone's predictions or expectations and the impact scars were observable in the ultraviolet, visible, near-infrared and infrared spectral regions. The observational data collected covered the extreme ultraviolet through radio frequencies and produced a scientifically rich data set that scientists are currently validating, calibrating and interpreting. From the data, the mass and composition of the impacting projectiles will be derived. The destruction of this comet provides insight into the internal structure and chemical composition of the comet nucleus not previously available. Cochran et al., [1994] and Lisse et al., [1994], among others, report significant color differences among the fragments observed prior to impact. Correlations between fragments and the impact scars require separating the contribution of the comet's chemistry from that of Jupiter, a challenging, but probably tractable task.
Upper limits to the fragments' diameters were determined to be
between 2.5-4.3 km from magnitudes measured by Hubble Space
Telescope [ Weaver et al., 1994] and 2-8 km for the
precursor parent body Scotti and Melosh, [1993].
Ahrens et al. [1994a,b] derive diameters of
approximately 2 km for the fragments by comparing observed plume
heights, which reached 
km, with detailed Smoothed
Particle Hydrodynamic modeling. Asphaug and Benz
[1994] claim that the fragments were less than 2 km each as the
entire parent body was approximately 4 km in diameter prior to
break-up. Upon analysis of the wide range of available data some
of the controversy is expected to subside. The internal structure
of the comet is also a subject of current vigorous debate. Whether
or not the fragments were cohesive, single nuclei with significant
strength is debated by both those who model the impact with
hydrodynamic theory [ Taketa et al., 1994] and
those modelling self-gravitational
reaccretion after tidal disruption [ Asphaug and
Benz, 1994].
In addition to deriving the mass and internal state of the comet, this event was significant for the opportunity to observe one of the primary processes modifying bodies in the solar system, collisions. We see the remnants of impact processes throughout the solar system as we observe planetary surfaces. One of the major motivations of study of the asteroids and comets responsible for craters is to understand the impact process and assess the hazard to the planets, including Earth. We were able to witness this impact, collect data from it, and were surprised by the measured intensity of the rising plume of hot gases. The impact energy probably exceeded the energy of any natural or man-made phenomena on Earth by three to four orders of magnitude.