WAS P/SHOEMAKER-LEVY 9 A COMET OR AN ASTEROID?
About four days before impact, at a distance of 2.3 million miles from Jupiter, nucleus “G” of comet P/Shoemaker-Levy 9 apparently penetrated Jupiter’s powerful magnetic field, the magnetosphere. (Jupiter’s magnetosphere is so vast, if visible from Earth, it would be about the size of the full Moon.) Hubble’s Faint Object Spectrograph (FOS) recorded dramatic changes at the magnetosphere crossing that provided a rare opportunity to gather more clues on the comet’s true composition. During a two minute period on July 14, HST detected strong emissions from ionized magnesium (Mg II), an important component of both comet dust and asteroids. However, if the nuclei were ice-laden — as expected of a comet nucleus — astronomers expected to detect the hydroxyl radical (OH). Hubble did not see OH, casting some doubt on the cometary nature of comet P/Shoemaker-Levy 9. Eighteen minutes after comet P/Shoemaker- Levy 9 displayed the flare-up in Mg II emissions, there was also a dramatic change in t
