More fragments of comet Shoemaker-Levy 9 rained down on Jupiter yesterday in a stunning display that amazed astronomers but promises to be just a warm-up for much larger impacts over the next three days.
An infrared image snapped by the W. M. Keck Observatory in Hawaii early yesterday shows two circular plumes of hot gas glowing white, like headlights in the fog, southwest of Jupiter's Great Red Spot.
They mark the spots where fragments "A" and "C" -- the first and third of the broken comet's 21 visible pieces -- slammed down Saturday afternoon and early yesterday morning. Fragment "B" apparently had little visible effect.
Four more pieces were to have struck the planet by 4 a.m. today, including fragment "G" -- one of the largest in the "string-of-pearls" formation, packing 25 times the mass and energy of fragment "A."
"This is just the orchestra warm- ing up," comet co-discoverer David Levy said of the earlier impacts.
Even at a mile or less in diameter, however, "A" was no popgun.
"If 'A' had hit North America, it would have made a crater about 20 kilometers [12.4 miles] in diameter," said comet co-discoverer Eugene Shoemaker of the Lowell Observatory.
Had it struck near the Baltimore-Washington area, he said, "it would have taken us all out." The hole itself would have been bigger than Baltimore City.
The "ejecta blanket" of rock, dirt and debris would have extended out ward 100 miles.
"And a tremendous amount of pulverized material would have been spread [in the upper atmosphere] all over the Northern Hemisphere, causing severe climatic changes -- probably the worst natural disaster ever witnessed by man," he said.
"We can be very glad this comet was heading for Jupiter and not the Earth," said Dr. Heidi Hammel, an astronomer from the Massachusetts Institute of Technology and imaging team leader for the Hubble Space Telescope.
On Jupiter, which is composed mostly of hydrogen gas, the comet-missile of ice, rock and organic chemicals is believed to have plunged 100 miles or more into the planet's atmosphere at 130,000 miles per hour.
As fragment "A" was slowed by the increasingly dense atmosphere, scientists believe, its immense kinetic energy was converted quickly to heat -- up to 30,000 degrees. That created an expanding bubble of hot gas that rose through the atmosphere, erupting through the cloud tops as a towering "fireball" or plume that swept far above the clouds.
On Saturday, the Hubble Space Telescope made an extraordinary 12-minute series of five photographs showing that plume rising above Jupiter's southwest horizon, then cooling, collapsing and flattening out more than 100 miles above the surrounding clouds of ammonia.
The central plume spread to a diameter of 2,600 to 3,900 miles -- one-third to one-half that of the Earth.
Dr. Hammel estimated the fireball's maximum height above the cloud tops at the impact point at more than 600 miles.
Hubble's photos confirmed scientists' theories of how the never-before-witnessed event would play out a half-billion miles away. Nevertheless, it "absolutely knocked our socks off," Dr. Hammel said. "We were all incredibly astonished at what we were seeing."
When the impact photograph first appeared Saturday night on a computer screen at the Space Telescope Science Institute in Baltimore, astonished team members -- who had been prepared to see nothing from this first, relatively small fragment -- erupted in whoops, cheers and applause, and passed around a bottle of champagne.
"We all just literally hit the ceiling," said astronomer and comet co-discoverer Dr. Carolyn Shoemaker. "It was too exciting to believe."
Surrounding the plume was a ripplelike ring that puzzled scientists.
Mr. Shoemaker, who was trained as a geologist, argued that it was a seismic wave, like that generated by earthquakes. But Dr. Hammel doubted that, proposing that it represented "fallout" from the fireball.
She also hinted at "some reports of some very interesting chemistry going on" in the plume, but she would not elaborate.