The comet was discovered by astronomers Carolyn and Eugene M. Shoemaker and David Levy in 1993. Shoemaker–Levy 9 had been captured by Jupiter and was orbiting the planet at the time. It was located on the night of March 24 in a photograph taken with the 46 cm (18 in) Schmidt telescope at the Palomar Observatory in California. It was the first comet observed to be orbiting a planet, and had probably been captured by Jupiter around 20–30 years earlier.
Calculations showed that its unusual fragmented form was due to a previous closer approach to Jupiter in July 1992. At that time, the orbit of Shoemaker–Levy 9 passed within Jupiter's Roche limit, and Jupiter's tidal forces had acted to pull apart the comet. The comet was later observed as a series of fragments ranging up to 2 km (1.2 mi) in diameter. These fragments collided with Jupiter's southern hemisphere between July 16 and 22, 1994 at a speed of approximately 60 km/s (37 mi/s) (Jupiter's escape velocity) or 216,000 km/h (134,000 mph). The prominent scars from the impacts were more easily visible than the Great Red Spot and persisted for many months.
In May 1990 the comet resembled a string of pearls headed for a collision with Jupiter. I was unable to see the collision was able to hear the pieces colliding with the Jovian atmosphere via a radio telescope.
The discovery that the comet was likely to collide with Jupiter caused great excitement within the astronomical community and beyond, as astronomers had never before seen two significant Solar System bodies collide. Intense studies of the comet were undertaken, and as its orbit became more accurately established, the possibility of a collision became a certainty. The collision would provide a unique opportunity for scientists to look inside Jupiter's atmosphere, as the collisions were expected to cause eruptions of material from the layers normally hidden beneath the clouds.
Astronomers estimated that the visible fragments of SL9 ranged in size from a few hundred metres (around 1,000 ft) to two kilometres (1.2 mi) across, suggesting that the original comet may have had a nucleus up to 5 km (3.1 mi) across—somewhat larger than Comet Hyakutake, which became very bright when it passed close to the Earth in 1996. One of the great debates in advance of the impact was whether the effects of the impact of such small bodies would be noticeable from Earth, apart from a flash as they disintegrated like giant meteors. The most optimistic prediction was that large, asymmetric ballistic fireballs would rise above the limb of Jupiter and into sunlight to be visible from Earth. Other suggested effects of the impacts were seismic waves travelling across the planet, an increase in stratospheric haze on the planet due to dust from the impacts, and an increase in the mass of the Jovian ring system. However, given that observing such a collision was completely unprecedented, astronomers were cautious with their predictions of what the event might reveal.
Anticipation grew as the predicted date for the collisions approached, and astronomers trained terrestrial telescopes on Jupiter. Several space observatories did the same, including the Hubble Space Telescope, the ROSAT X-ray-observing satellite, and significantly the Galileo spacecraft, then on its way to a rendezvous with Jupiter scheduled for 1995. Although the impacts took place on the side of Jupiter hidden from Earth, Galileo, then at a distance of 1.6 AU (240 million km; 150 million mi) from the planet, was able to see the impacts as they occurred. Jupiter's rapid rotation brought the impact sites into view for terrestrial observers a few minutes after the collisions
The first impact occurred at 20:13 UTC on July 16, 1994, when fragment A of the nucleus entered Jupiter's southern hemisphere at a speed of about 60 km/s (35 mi/s). Instruments on Galileo detected a fireball that reached a peak temperature of about 24,000 K (23,700 °C; 42,700 °F), compared to the typical Jovian cloudtop temperature of about 130 K (−143 °C; −226 °F), before expanding and cooling rapidly to about 1,500 K (1,230 °C; 2,240 °F) after 40 seconds. The plume from the fireball quickly reached a height of over 3,000 km (1,900 mi). A few minutes after the impact fireball was detected, Galileo measured renewed heating, probably due to ejected material falling back onto the planet. Earth-based observers detected the fireball rising over the limb of the planet shortly after the initial impact.
Despite published predictions, astronomers had not expected to see the fireballs from the impacts and did not have any idea in advance how visible the other atmospheric effects of the impacts would be from Earth. Observers soon saw a huge dark spot after the first impact. The spot was visible even in very small telescopes, and was about 6,000 km (3,700 mi) (one Earth radius) across. This and subsequent dark spots were thought to have been caused by debris from the impacts, and were markedly asymmetric, forming crescent shapes in front of the direction of impact.
Over the next six days, 21 distinct impacts were observed, with the largest coming on July 18 at 07:33 UTC when fragment G struck Jupiter. This impact created a giant dark spot over 12,000 km (7,500 mi) across, and was estimated to have released an energy equivalent to 6,000,000 megatons of TNT (600 times the world's nuclear arsenal). Two impacts 12 hours apart on July 19 created impact marks of similar size to that caused by fragment G, and impacts continued until July 22, when fragment W struck the planet.