GREENBELT -- The last major comet fragments hit Jupiter yesterday, one with a fizzle and the other with a medium-sized bang, ending the weeklong celestial fusillade that has pockmarked the solar system's largest planet.
Astronomers said P/Shoemaker-Levy 9's final chunk vaporized in a fireball in the Jovian atmosphere around 4 a.m. yesterday, although tiny pieces will continue to rain down for at least another month. About 3 3/4 hours earlier, another fragment probably struck but left no visible mark.
Eugene Shoemaker, a co-discoverer of the comet, admitted he felt some "postpartum depression" at the end of the bombardment, which sculpted a necklace of rust-colored splotches circling the planet's southern hemisphere. A similar-sized body strikes Jupiter only once every 10,000 years, he estimated.
But the comet "did such an extraordinary job," he said, that he did not regret its passing. At least 21 mountain-sized pieces landed in the six days that shook Jupiter, although not all left visible marks.
Most of the Earth's major telescopes probably watched at one time or another, in what has been called the largest coordinated astronomical observation campaign ever mounted.
Dr. Shoemaker said that University of Arizona scientists had reported sighting a huge, faint ring around one impact site covering almost one-quarter of Jupiter's surface. The giant planet, which consists mostly of hydrogen, has 1,300 times the volume of Earth.
If the ring's existence is confirmed, he said, it could be a seismic wave from an exploding fragment that shot deep into the planet's interior, then refracted and disturbed its upper atmosphere.
One of the most intriguing questions raised by the impacts, several astronomers said, is how far the pieces of Shoemaker-Levy 9 penetrated Jupiter's atmosphere.
Andrew P. Ingersoll, of the California Institute of Technology, said he had analyzed Hubble Space Telescope images of the site of the largest impact, which occurred Monday, and identified a 4,800-mile wide surface ring as a kind of Jovian sonic boom triggered by the explosive end of Fragment G. (All comet fragments were identified by letters of the alphabet.)
Monday's big bang spread out from the impact site through the planet's upper atmosphere at a speed of 2,000 mph, he said, though at far too low a frequency for the human ear to hear. Hubble images also show a smaller sonic wave moving more slowly at a lower altitude, he said, where water clouds are thought to swirl.
The upper atmosphere's ring appears far more powerful than the deeper one, he said, suggesting that most of the fragment's energy was spent in the higher region. In addition, he pointed out, little or no water vapor has been detected in plumes thrown up by the explosions.
This evidence suggests, he said, that all the fragments vaporized in the top 60 miles or so of Jupiter's atmosphere -- a glancing blow considering the planet's enormous size.
Not everyone is ready to concede that the comet inflicted only flesh wounds.
But if they failed to penetrate far, Dr. Shoemaker said, it was probably because the large shards were broken up into small pieces just before impact by Jupiter's gravitation grip, more than 2 1/2 times that of Earth.
Now that the impacts have halted, astronomers will have a chance to pore over their images and other data and reconstruct the collisions in more detail. Dr. Ingersoll expects the sulfur-rich spots that appeared at the impact sites to linger for more than a year.
A measure introduced yesterday in Congress, meanwhile, asks the National Aeronautic and Space Administration to draft a plan to deploy a network of telescopes to watch for hefty asteroids and comets that cross the Earth's orbit.
Dr. Shoemaker said that astronomers could probably spot and track 80 percent of these so-called Earth-crossing objects, but that the other 20 percent pass by so infrequently that sighting them would be extremely difficult. "At best we would only get about a two-year warning," he said.