And except for an observing session next week and another in late August, the Hubble Space Telescope is moving on to view other heavenly objects. Scientists who last week barely had time to sleep, let alone think, are finally turning their attention away from spectacular pictures and starting the long, difficult process of seeing what they can learn from the great comet crash of 1994. Comets tend to be a mixture of ice, rock and dust, along with substances, like carbon monoxide, that evaporate easily to form a halo and a tail. Scientists studying the chemical composition of the spots on Jupiter where S-L 9 hit thought they might see evidence of water and oxygen, two of the expected products when an icy comet vaporizes. But except for one unconfirmed report,researchers have found only ammonia, hydrogen sulfide and sulfur gas.
Asteroids are rockier than comets.
Yet it is possible for an asteroid to have a halo or a tail, made mostly of dust. But maybe this was a fragile asteroid. Amateur astronomer David Levy, who with Eugene and Carolyn Shoemaker discovered S-L 9, points out that comets were originally distinguished by their appearance. They are objects that look like fuzzy stars with tails, and in any previous century astronomers would have called this discovery a comet. The apparent absence of water at the impact sites provides a clue about how far the S-L 9 fragments penetrated Jupiter's atmosphere before exploding.