Celestial mechanics quickly worked out its history: The object had passed very close to Jupiter and was fragmented as a result of the tidal forces from the planet. The fragments were then in orbit around the planet and were predicted to hit Jupiter in July 2014.
The deadlines were too short to launch a space mission dedicated to the event, but existing resources were to be used: the Hubble Space Telescope and the Galileo spacecraft en route to Jupiter. Many telescopes and radio telescopes were mobilized to observe the falls.
The impacts were occurring behind the disc of Jupiter and were not directly observable from the Earth. However, their sites became visible within minutes, thanks to the rapid rotation of the planet. The Hubble provided a series of beautiful images of the sequence. The Galileo probe was even better positioned, and its instruments recorded the entire phenomenon.
The fall of the fragments at 60 kilometers (38 miles) per second first produced in Jupiter’s atmosphere a fireball with temperatures exceeding 20,000 degrees, then decreasing quickly. Some material was ejected and then fell 10 minutes later, causing a further increase in temperature. A big black spot, about 6,000 km in diameter, appeared in the location of each impact. One could observe the spectral signature of molecules not yet detected in Jupiter’s atmosphere: water, observed by Galileo in the near infrared; CO, HCN, CS and OCS, whose emission was detected by millimeter radio telescopes; finally S2, CS2 and H2S, observed by the Hubble space telescope in the ultraviolet. Some were produced by chemical reactions caused by the temperature increase. Others, such as water, were brought by the comet.
Such an event is certainly uncommon, but not unique; models of collisions with comets and asteroids predict that the fall on Jupiter of an object a kilometer in diameter, as was the comet SL9, occurs on average every 500 to 1,000 years.
Moreover, a phenomenon of this kind was most likely observed by Cassini in December 1690. In the case of Earth, these falls are fortunately much more rare and occur on average every million years. However, it is likely that the oceanic water was brought to Earth by the fall of comets and asteroids, which was much more frequent during the billion years that followed the formation of the Solar system.