GREENBELT -- Astronomers said yesterday that they have detected, for the first time, the rocky debris of one of the comet fragments that smacked into Jupiter last week.
Melissa McGrath, an astronomer with NASA's Goddard Space Flight Center in Greenbelt, said the Hubble Space Telescope found magnesium, silicon and perhaps iron in the rust-colored plume kicked up after fragment "G" slammed into Jupiter's atmosphere early Monday morning.
She said there are still no signs of water, deepening suspicion that Shoemaker-Levy 9 was not a chain of icy comet shards at all, but pieces of an unfamiliar species of asteroid.
"I think it's a distinct possibility it's not a comet," she said. "An asteroid? Maybe, or maybe something unusual, something between the two we don't usually see."
The 37-mile-per-second collision left the giant planet, which is composed mostly of hydrogen and helium gas, with a bruise bigger than Earth.
Dr. McGrath said she was not able to detect any water directly, or any oxygen atoms torn loose from the hydrogen in water by the estimated 6 million megaton force of the explosion.
Andrew P. Ingersoll of the California Institute of Technology, who is using one of Hubble's cameras to study the impact sites, said Shoemaker-Levy 9 is looking more and more like an asteroid. "I think we're all inclined toward that assumption, but we're a little reluctant to say so," he said.
Comets are, in effect, dirty snowballs, made of loosely-compacted ice with rocky cores, born in the solar system's frigid exurbs beyond Pluto. Asteroids are thought to be boulders, most of which whiz around the sun in a region between Mars and Jupiter.
Astronomers first thought Shoemaker-Levy 9 was a comet because its pieces looked as if they had a comet's characteristic "coma," a glowing cloak of water vapor, dust and gas.
And it seemed to act more like a snow cone than a stone: it split into at least 21 major pieces in its first, relatively gentle nudge from the forces of Jupiter's gravity.
In part, scientists are puzzled about the identity of the object -- known formally as P/Shoemaker-Levy 9 -- because it was so hard to see. Astronomers calculate that the largest fragments were between a half-mile and 3 miles in diameter. When they hit Jupiter, the pieces were 477 million miles away from Earth.
Dr. Ingersoll, part of the team using the Hubble Space Telescope to watch the collisions, said physicists at the Sandia National Laboratories in New Mexico came up with their own estimate of the size of the Shoemaker-Levy fragments based on the height and speed of the plumes and the distribution of matter ejected.
Employing powerful computers used to design nuclear weapons, the physicists estimated that the Shoemaker-Levy 9 fragments were, on average, between 1.5 miles and 1.9 miles in diameter.
Two of the fragments could fit in the straits spanned by the 4.3-mile-long Chesapeake Bay Bridge, with room to spare.
"But who knows?" Dr. Ingersoll said.
Dr. McGrath said she is sure that the silicon and magnesium Hubble detected are from the comet fragment, and not Jupiter itself, because the elements are too heavy to be swirling in the planet's mostly hydrogen atmosphere.
Both astronomers spoke at a news conference held by the National Aeronautics and Space Administration at Goddard.