Just when you think you know a comet, it throws you a curve ball called fragment M.
Yesterday, astronomers at W. M. Keck Observatory in Hawaii reported that they had seen signs of an impact on Jupiter that seemed to be that of the comet P/Shoemaker-Levy 9's missing fragment M, which was thought to have evaporated last year.
The report, if confirmed, suggests that -- far from evaporating -- M was still a hunk of matter perhaps as long as five football fields when it shot into the planet Wednesday morning.
"We're very pleased at that report," said Eugene Shoemaker, a co-discoverer of the comet and one of a group of astronomers who believed all along that M was still out there. "It's delightful to see there was . . . still a body there, and that it came on in."
The reappearance of fragment M was one more complication for astronomers who are trying to figure out whether Shoemaker-Levy 9 fits the current definition of a comet.
The once-in-a-lifetime comet extravaganza on Jupiter, which has been full of scientific surprises, drew to a close at about 4 a.m. today with the collision of fragment W. It was the last of more than 20 visible pieces of the original 6-mile-wide comet that broke up during a close brush with Jupiter in July 1992.
For scientists who were prepared to see nothing at all, it has been a week of surprises. Among the latest:
* Alongside the enormous impact plume of the comet's fragment Q2, astronomers said they saw the barely discernible impact spot of its twin, Q1. Scientists believe Q1, like several others that have struck with little effect, was a loosely packed object that broke apart in a final fizzle as it approached Jupiter.
* Yesterday, the first true-color photo of the collisions' aftermath taken from Hubble shows the giant impact plume from fragment G, which fell Monday morning, as a thin veil of brown debris. More than two times larger than Earth, the plume appears to float unperturbed above the windy ammonia cloud tops, which can be seen through the plume.
The worldwide efforts have been the largest coordinated astronomical observation campaign ever conducted. They are expected to generate hundreds of scientific papers.
Even though the bombardment is officially over, professional and amateur astronomers plan to keep looking at Jupiter for at least another month. They hope to learn how Jupiter's stratospheric winds will change and eventually dissipate the lingering impact plumes, and how the comet has affected the planet's radiation belts.
They also want to watch for any additional impacts as the hazy trailing "wing" of the train of cometary material continues to rain down on the planet.
"Don't turn off the set," urged David Levy, an author, comet-hunter and co-discoverer of Shoemaker-Levy 9. In the coming weeks, the impact points will move from the "back" of the planet as seen from Earth, to the front.
When it does, he said, even relatively small pieces of the comet may produce visible meteor flashes as they streak into Jupiter's atmosphere and obvious fireballs as they explode and erupt above the cloud tops.
Fragment M earned its letter designation soon after the "string-of-pearls" comet was discovered in March 1993. But by last summer it had disappeared from view.
Astronomers assumed it had a loosely bound nucleus of water ice that simply vaporized, leaving nothing but a dissipating cloud of dust, invisible to Earth's telescopes. They never expected to see it again.
Its reappearance on Jupiter Wednesday morning presented a new puzzle for astronomers.
If M could be stripped of its coma, or halo, of visible dust and gas, leaving only a bare rock that disappears from view, perhaps Shoemaker-Levy 9 was not an icy comet from the outermost reaches of the solar system, but a rocky asteroid deflected from the asteroid belts between the orbits of Jupiter and Mars.
On the other hand, astronomers have watched for more than a year as fragments of Shoemaker-Levy 9 appeared to break up or disappear. Fragment Q became Q1 and Q2.
Fragment P also split, and J and M vanished.
"We haven't seen asteroids break up into smaller fragments; we have seen comets break up," said Harold Weaver, a comet specialist and principal investigator for the Hubble Space Telescope's observations.
For Dr. Shoemaker, the comet hunter, there is no mystery. "It's a classical problem of observing comets," he said. "Many comets have been observed to turn off and disappear, and all you have is a bare nucleus and it's too faint to see."
But Dr. Weaver is not convinced.
"I'm still not sure it's a comet," he said. "The only evidence we have is the fragility. There is no evidence of any icy material in this object. It's very strange."
At a similar distance, Halley's Comet was observed giving off water vapor, he said. But astronomers have not seen any signs of water in Shoemaker-Levy 9, or in its impact plumes.
Still more curious, Dr. Weaver said, when Hubble's spectrographs focused on fragment G during a final look just as it entered Jupiter's powerful magnetic field last week, they got their very first reading on just what was coming off the comet.
It wasn't water, however, but ions of magnesium, a metal that is much less volatile.
So, is Shoemaker-Levy 9 a new kind of waterless comet astronomers don't understand, or a new, fragile and dusty kind of asteroid they've never seen before?
"It's been a real puzzle," Dr. Weaver said.