Astronomers reacted with whoops and slurps of champagne last night as photographs of the impact of the first fragment of comet P/Shoemaker-Levy 9 with Jupiter showed that it left a bright splotch on the planet's surface.
Based on the photo, scientists estimated the size of the first fragment -- among the broken comet's smallest -- at about one kilometer, or 0.6-mile.
That's bigger than many predictions, and suggested that the much bigger chunks of rock and ice due to hit the planet this
week will produce a dramatic display for scientists around the world.
Hubble Space Telescope astronomer Heidi Hammel, who is leading the imaging team, said the space telescope first spotted the impact as a bright plume of hot gas that rose more than 600 miles above the edge of the planet.
Ninety minutes later, during the telescope's next orbit, the plume had grown into a bright circle estimated at one-third to one-half the diameter of Earth.
Hubble astronomer Harold Weaver estimated the energy released by the comet's impact and explosion at 200,000 megatons of TNT.
Mr. Weaver said the largest fragment, expected to strike Jupiter next week, will pack 25 times as much energy as the first.
"It's a champagne experience," said an elated Carolyn Shoemaker, a co-discoverer of the comet, as Hubble's first raw image of the impact site was shown to her at the Space Telescope Science Institute in Baltimore.
Minutes earlier, she admitted to shedding "a momentary tear" as the predicted time for the first collision passed at 4 p.m. EDT yesterday.
"I discovered I had a real emotional attachment to that comet. It has been a thing a beauty to me," she said.
In all, at least 21 visible fragments of the comet are expected to plunge into Jupiter's atmosphere during the seven-day barrage.
Virtually every major observatory in the world, in orbit and touring the solar system is watching. But Hubble is expected to have the best view.
Scientists at two overseas observatories -- Calar Alto near Grenada, Spain, and the Nordic Optical Telescope in La Silla, Chile -- were the first yesterday to report seeing signs of the first impact.
At 4:14 p.m. EDT, they said, the plume appeared at the edge of the planet and glowed brighter than Io, one of Jupiter's moons.
The cometary collision is the first ever predicted and the first to be witnessed and recorded by the world's astronomers.
"We are very privileged," said Eugene Shoemaker, of the Lowell Observatory, who with his wife, Carolyn, and comet-hunter David Levy discovered the comet in March 1993.
Based on the size of the plume seen yesterday, Mr. Shoemaker estimated that the original comet, before it broke apart during a swing past Jupiter in July 1992, was more than 10 kilometers (about 6 miles) in diameter. That would make it comparable in size to Halley's Comet.
Collisions of objects that big with planets are very rare, scientists said.
"My guess is you don't get a hit of a 10-kilometer size object more than once every 1,000 years," Mr. Shoemaker said.
Because comets are moving at high speeds -- this one has been clocked at 130,000 mph -- they unleash tremendous energy measured in millions of megatons of TNT when they collide with a planet.
An impact of an icy comet similar to Shoemaker-Levy 9 about 65 million years ago is believed to have caused environmental devastation on Earth that caused the extinction of 70 percent of all living species, including the dinosaurs.
The impacts on Jupiter are not expected to leave any lasting damage, however.
Jupiter is composed mostly of hydrogen and helium gas. Mr. Shoemaker said there is no oxygen there to sustain combustion, and the heat produced at the impact site -- while formidable -- is too little to ignite nuclear fusion and create a second "sun."