One fragment of Comet Shoemaker-Levy 9 lit a 7-kilometer (5- mile) fireball hotter than the Sun's surface last July 18 as it plunged into Jupiter's night sky in full view of NASA's Galileo spacecraft, scientists have reported.
After a preliminary analysis of data on the comet's fragment G from three instruments reaching from the ultraviolet into the near infrared, they "have characterized a comet impact directly for the first time in history," said Galileo Project Scientist Dr. Torrence V. Johnson of the Jet Propulsion Laboratory.
The fragment G fireball, when first detected by the ultraviolet spectrometer and photopolarometer-radiometer, was apparently about 7 kilometers (5 miles) in diameter, with a temperature of at least 8,000 degrees Kelvin (14,000 degrees Fahrenheit), hotter than the sun's surface. Five seconds later the infrared spectrometer detected it, and recorded the fireball's expansion, rise and cooling for a minute and a half, until it was hundreds of miles across and only at about 400 degrees K (260 F).
"Very simply, it looks to us like an expanding, cooling bubble of hot gas," said infrared Principal Investigator Dr. Robert Carlson of JPL. "But obviously it will take more complicated models to explain all the data."
Observations made by Galileo's ultraviolet spectrometer, the near infrared mapping spectrometer, the photopolarimeter- radiometer and the imaging system for six of the impacts (fragments G, H, K, L, Q1, and W) were reported by the scientists at the annual meeting of the Division of Planetary Sciences of the American Astronomical Society in Bethesda, Md.
When these observations were made in late July, Galileo was 240 million kilometers (150 million miles) from Jupiter and at such an angle that it could observe early events hidden from the Earth by the limb or horizon of Jupiter. Each of the impacts began with a rapid rise in brightness, or flash, typically reaching about 10 percent of Jupiter's total brightness, followed by a plateau of brightness and then a slow decline. In ultraviolet light, the total event lasts only about 10 seconds, but at infrared wavelengths it could go on for 90 seconds or more.
But there remains a mystery: the Hubble Space Telescope and Earth-based observatories saw some of the impacts start just as soon as Galileo did -- as if looking through Jupiter. "In effect, we are apparently seeing something we didn't think we had any right to see, " said Dr. Andrew P. Ingersoll of the California Institute of Technology, a member of both the Hubble Space Telescope and Galileo science teams.
There are possible explanations for the timing of the reported observations. "If everyone's times are right -- and we think they are -- it seems clear that something was happening high enough to be seen beyond the curve of the planet, before and during the events Galileo saw," said Galileo Project Scientist Dr. Torrence V. Johnson of JPL.
"The Hubble observations of fragments G and W could conceivably be due to scattering of light from the Galileo events off comet dust or other material at very high altitude," Johnson added, "but 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."