To related article, Timing Is EverythingIn late October, preliminary data analyses from three of Galileo's instruments indicated that one of the SL9 fragments exploded into a 7-km (4-mile)-diameter fireball. This Fragment G fireball on July 18, when first detected by the Ultraviolet Spectrometer (UVS) and Photopolarimeter-Radiometer (PPR), was about 8,000 kelvins (14,000 degrees Fahrenheit), which is hotter than the Sun's surface. Five seconds later, the Near-Infrared Mapping Spectrometer (NIMS) detected it, recording the fireball's expansion, rise, and cooling for a minute and a half, until it was hundreds of kilometers across and only about 400 K (260 deg F). Galileo has thus provided a unique data set on SL9, that is, the only profile of the size and temperature of the fireball during the first few minutes following the impact itself.
Five of the Galileo Orbiter's 11 instruments observed the comet impacts. The remote-sensing instruments mounted on the scan platform were four of the five--the Solid-State Imaging (SSI) camera, NIMS, PPR, and UVS. The fifth was the Plasma Wave Subsystem (PWS), included because it might be able to detect radio-frequency emissions caused by the impacts. A sixth instrument, the Dust Detector Subsystem (DDS), was configured to watch for any dust streams coming from Jupiter, which could take 1 to 2 months to reach Galileo.
The normal Galileo observing strategy is for a single instrument to be prime for a given observation, and other instruments to ride along where practical. However, there were unique constraints on the SL9 sequence development, particularly in staffing and budget. Observational opportunities were simply divided up between the four instruments (NIMS, SSI, PPR, and UVS) with most of the opportunities being assigned to the SSI and NIMS because of the anticipated likelihood of success. Most of the PPR observations were designed to provide near-real-time data return to help determine impact times. For all but one PPR observation, data were stored directly in the spacecraft's central computer and played back to the ground on a nearly daily basis. A similar approach was used for the PWS, which observed nearly continuously from before the first impact through the entire sequence with data being returned in near real time.
Of the PPR-observed events, nothing was seen on B; a clear signal was seen for G, H, and L; a faint signal was detected for Q1; and S was missed as a result of an out-of-tolerance shift in the time of impact. After preliminary processing of the PWS data, no impact-induced signals have been detected. SSI captured the K, N, and W fragment impacts. The NIMS, UVS, and PPR have returned data on the G event, where the huge fireball was measured. The PPR and SSI data so far indicate that the impacts produced near-infrared signals lasting a surprising 20 to 40 seconds with intensities ranging from 1 to 10 percent of the total brightness of Jupiter (for Q1 and K, respectively). As of late November, only data from impacts W and R remain scheduled for playback.
One of the fascinating riddles that SL9 posed is why the Hubble Space Telescope and Earth-based observers saw some of the impacts at the same time Galileo did. When the observations were made, Galileo was at a viewing angle that should have allowed it to observe early events hidden from the Earth by Jupiter's horizon. One possible explanation for the simultaneous observations is that "something was happening high enough to be seen beyond the curve of the planet," speculates Dr. Torrence V. Johnson, Galileo Project Scientist. However, he points out that how the material got there is another question. "There may have been earlier, smaller impacts going on that were too faint for Galileo to detect that sent plumes high into the upper atmosphere, from which the main G impact fragment flash was reflected. Or it is possible that the main flash was reflected off of a train of dust that was following the main G fragment. We will learn more as our modeling continues."
One question that Galileo may help answer concerns the size of the comet fragments. Were they large, several kilometers in diameter, as some predicted? Or were they much smaller--only half a kilometer across--or even just loosely held-together piles of rubble or wisps of dust?
A related question is how deeply the comet pieces penetrated into Jupiter's atmosphere before exploding. Single, large, solid fragments would have been expected to penetrate further and bring up water from Jupiter's presumed water-rich atmospheric layers, while rubble piles or rubble swarms might only have caused meteor storms in the upper atmosphere with no deep penetration. Preliminary spectroscopic data imply that the fragments did not penetrate very deeply, since little or no water was splashed up into the stratosphere. However, the spectacular show argues in favor of the large-fragment, deep-plunging model.
Another interesting question is why Jupiter's icy satellite Europa did not reflect the bright flashes from the dark side of Jupiter, as expected. Europa's shadowed, reflective, icy surface should have served as an excellent mirror for the brilliant flashes and subsequent glowing fireballs. But it didn't happen that way. As the recent information about the Fragment G fireball proves, Galileo may be best able to answer questions about optical flashes.
Perhaps the most perplexing question is what caused those immense black patches to remain in Jupiter's high atmosphere. The largest patches are much bigger than the whole planet Earth and much darker and more prominent than the Great Red Spot. Initially, they were expected to fade and disappear in a few days, but they seem to be persisting. Conceivably, as Galileo nears its target, it will also be able to help explain these aspects of the SL9 impacts at Jupiter.
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To Fragment W Impact photo 
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