SL9 Impact Details Explained

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One of the most well know images is the HST image of the G impact just after its appearance on the limb of Jupiter. This image is the one in the top left in the series below. There are a few features that stand out on this image: A dark thin ring, a dark streak inside this ring, a broad horseshoe-shaped feature to the south, and a small black spot toward the following limb. The small black spot is actually the impact site of fragment D that stuck Jupiter the day before fragment G hit. The thin ring is possibly a shock wave in the atmosphere moving outward from where the fragment exploded below the cloudtops. At the time the image was taken the ring measured about one half of an Earth-diameter. If this ring was a shock wave it must have been traveling radially outward at about Mach 1.7 (580 meters per second) based on its time of impact. The dark streak inside this ring is probably the path of the fragment with the entry point being on the south end. Note that the streak ends near the center of the thin ring. The broad horseshoe-shaped feature appears to represent the resettling debris from the fireball. In some wavelengths the bands of Jupiter can be seen through this broad feature. When compared to impact simulations, this pattern fits quite well with a 45 degree angle of entry.

The fragment G fireball, when first detected by the Galileo spacecraft, 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 Kelvin (260 F). 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. 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. "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, "There may have been earlier, smaller impacts going on that were too faint for Galileo to detect."