Smack Jupiter with a rocky snowball the size of Baltimore-Washington International Airport moving at 37 miles a second, and you produce what looks for all the world like a black eye.
Pictures snapped by the Hubble Space Telescope show the region of Jupiter struck early yesterday by a 2-mile-wide chunk of comet P/Shoemaker-Levy 9 as having a sharp, black central ring nearly the diameter of Earth, partly surrounded by a much larger fan of dark material that resembles a bruise.
"I feel sorry for Jupiter; it's really getting pummeled. But I think it's going to hang in there," said Dr. Heidi Hammel, a Massachusetts Institute of Technology astronomer who is leading the Hubble imaging team at the Space Telescope Science Institute in Baltimore.
The gigantic bruise was caused by the impact of fragment "G," the seventh of 21 due to strike the planet in all, and the first of the broken comet's four largest visible fragments.
Had fragment "G" struck Earth, scientists said, the crater would have been the size of Rhode Island and the debris tossed from inside it would have swept around the globe.
Eugene Shoemaker, a Lowell Observatory astronomer and co-discoverer of the comet, yesterday estimated the energy released by the impact at the equivalent of 6 million megatons of TNT.
That was a sharp revision from his earlier estimate of 250 million megatons, a calculation error he blamed on haste.
Even at 6 million megatons, it is an impressive display, 25 times as violent as the impact of fragment "A," which fell on Saturday. All the nuclear weapons on Earth total about 13,000 megatons. A megaton is 1 million tons of TNT.
Scientists have just begun to debate the physics of what they're seeing. But Dr. Hammel theorized yesterday that the central ring at the impact site may be an "atmospheric wave" -- a ripple moving out from the impact zone through the planet's ammonia cloud tops.
The dark crescent, she said, may be bits of the comet itself, mixed with hot gas ripped from the atmosphere by the impact's resulting fireball, and then deposited toward the southeast in a thin apron of fallout.
A separate sequence of Hubble photographs taken 90 minutes earlier captured the fireball caused by fragment "G" rising more than 1,300 miles above the clouds at the impact site -- twice as high as the fireball photographed Saturday after the impact of much-smaller fragment "A."
Within less than 20 minutes, the plume had cooled, collapsed and spread out over the clouds. The spectacle "just overwhelms anything we've ever seen before," said Dr. Imke de Pater, a University of California astronomer who was observing in infrared wavelengths at the W. M. Keck Observatory in Hawaii.
The Keck Observatory's infrared sensors -- which detect heat -- were overloaded by the fireball rising from the impact of fragment "G," unable to record all the energy they were receiving. For a few seconds, Dr. de Pater, said, the explosion was brighter in the infrared than the planet itself.
A Keck Observatory infrared photograph of the fireball from fragment "G" showed an enormous glowing ball protruding from the planet's southwest horizon.
Based on the size of the explosion, Mr. Shoemaker estimated that fragment "G" was about 3 kilometers (1.8 miles) in diameter before it plunged into Jupiter's atmosphere.
A second fragment of apparently similar size -- tagged "H" by scientists -- slammed into the planet at 3:30 p.m. yesterday.
No Hubble Space Telescope observations were planned for the "H" impact, however, because the orbiting observatory was passing through the "South Atlantic Anomaly" at the time. That's a region above the Atlantic Ocean where a quirk in Earth's magnetic field allows radiation to interfere with the telescope's sensitive light detectors.
Scientists are expecting 12 more of the comet's visible fragments to strike Jupiter before the show ends Friday. The largest still to fall, fragments "K" and "Q1," were due to hit at 6:18 a.m. today and at 3:59 p.m. tomorrow, respectively.
Astronomers are expecting the Q fragments to be followed in quick succession by fragments "R" and "S" -- all of them hitting very close to each other.
"This is going to make one heck of a mess," said Dr. Hammel. "We're looking forward to some very interesting chemistry because you're stirring up the atmosphere a lot."
As big as they are, however, all of the impacts are too small to cause Jupiter any permanent damage, and far too feeble to ignite nuclear fusion on the planet, which like the fusion-powered sun is composed mostly (89 percent) of hydrogen gas.
Peak temperatures created by comet fragments boring into Jupiter's atmosphere at 60 kilometers (37 miles) a second are estimated at 30,000 degrees. That is less than 10 percent of the temperature needed to ignite fusion, scientists say.
"It's not going to turn Jupiter on," said Mr. Shoemaker.
Because all the impacts are falling in a narrow band of latitude in the planet's southern hemisphere, however, scientists say the plumes could eventually merge to form a noticeable new band around the already colorfully banded planet.
Shoemaker-Levy 9 is believed to have originated as a single, 10-kilometer (6-mile) wide comet in orbit around the sun. Sometime in the recent past -- perhaps in the 1960s, it passed close enough to Jupiter to be captured by the planet's enormous gravitational field. It then moved into an elongated orbit around Jupiter that extended 30 million miles from the planet at its farthest.
In July 1992, astronomers calculate, it passed within 75,000 miles of the planet's surface, a close pass that caused the loosely bound comet to come apart in the tug of Jupiter's gravity. By the time it was discovered in March 1993, the comet had become a chain of as many as 21 visible fragments.
Astronomers soon realized the "string-of-pearls" formation was headed for a July 1994 collision with the giant planet.
Because the impacts have left such a dramatic mark on the planet, professional astronomers are slightly more encouraging
about the prospects for amateurs trying to see some trace of the collisions in their backyard equipment.
University of Maryland astronomer Dr. Lucy McFadden said a respected amateur observer in England has reported seeing spots from the first impacts along Jupiter's southern latitudes.
Based on that report, Mr. Shoemaker said "it should be possible for amateurs with very good observing conditions to see the spots."