No. 95: Sep-Oct 1994
Like supernova 1987A, also mentioned in this issue, comet P/Shoemaker-Levi-9 is generating headlines in the scientific and popular media. Everyone expected that the impact of this comet on Jupiter would provide us with some very interesting phenomena, but most thought that the debris kicked up by the impacts would reveal something new about Jupiter -- not about comets, since everyone knows what they are! Instead, the long-held theory that comets are simply dirty iceballs is now questioned. Shoemaker-Levi-9 was originally classified as a comet because: (1) Its fragments all appeared to be surrounded by comas like all well-behaved comets; and (2) It was easily torn apart by Jupiter's strong gravitational field as it narrowly missed Jupiter in 1992. This is just what one would expect from a loosely compacted dirty iceball.
But doubts about Shoemaker-Levi-9's true nature have arisen because of two observations: (1) No water vapor has been detected in the plumes thrown up by the impacts of the fragments; and (2) The comet's Fragment M, which was supposed to have vaporized in 1993, as comets are wont to do, apparently did not and left a scar on Jupiter like the other fragments. So, Shoemaker-Levi-9 had no discernible ice (unlike comets; it was fragile and easily fragmented (unlike rocky asteroids); and Fragment M did not have a coma (unlike comets but like asteroids)! Was Shoemaker-Levi-9 really a comet?
(Roylance, Frank D.; "Is That Really a 'Comet' Hitting Jupiter?" Baltimore Sun, July 20, 1994. Roylance, Frank D.; "Fragment M's Reappearance Puzzles Astronomers," Baltimore Sun, July 22, 1994.)
Reference. See our Catalog The Sun and Solar System Debris for more on the comet-asteroid question. Ordering information here.