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For decades, space experts have warned of orbits around the planet growing so crowded that two satellites might one day slam into one another, producing swarms of treacherous debris.

It happened Tuesday. And the whirling fragments could pose a threat to the International Space Station, orbiting 215 miles up with three astronauts on board, though officials said the risk was now small.

“This is a first, unfortunately,” Nicholas L. Johnson, chief scientist for orbital debris at the National Aeronautics and Space Administration, said of the collision.

It happened some 490 miles above northern Siberia, at around noon Eastern time. Two communications satellites — one Russian, one American — cracked up in silent destruction. In the aftermath, military radars on the ground tracked large amounts of debris going into higher and lower orbits.

“Nothing to this extent” has ever happened before, Mr. Johnson said. “We’ve had three other accidental collisions between what we call catalog objects, but they were all much smaller than this,” the objects always very small and moderate in size.

The communication satellites, he added, “are two relatively big objects.”

The American satellite was an Iridium, one of a constellation of 66 spacecraft. Liz DeCastro, corporate communications director of Iridium Satellite, based in Bethesda, Md., said that the satellite weighed about 1,200 pounds and that its body was more than 12 feet long, not including large solar arrays.

In a statement, the company said that it had “lost an operational satellite” on Tuesday, apparently after it collided with “a nonoperational” Russian satellite.

“Although this event has minimal impact on Iridium’s service,” the statement added, “the company is taking immediate action to address the loss.” The company’s hand-held phones can be used anywhere around the globe to give users voice and data communications.

Mr. Johnson said the Russian satellite was presumably nonfunctional. Officials at the Russian Embassy in Washington could not be reached for comment.

Mr. Johnson said the United States military’s tracking radars had yet to determine the number of detectable fragments. “It’s going to take a while,” he said. “It’s very, very difficult to discriminate all those objects when they’re really close together. And so over the next couple of days we’ll have a much better understanding.”

At a minimum, Mr. Johnson added, “I think we’re talking many, many dozens, if not hundreds.”

The debris could threaten the space station and its astronaut crew, he said.

“There are actually debris from this event which we believe are going through space station altitude already,” he said. The risk to the station, Mr. Johnson added, “is going to be very, very small.” In the worst case, he said, “We’ll just dodge them if we have to. It’s the small things you can’t see that are the ones that can do you harm.”

In Houston, International Space Station controllers have often adjusted its orbit to get out of the way of speeding space debris, which can move so incredibly fast that even small pieces pack a destructive wallop.

John Yembrick, a NASA spokesman in Washington, said the agency now judged the risk of collision with the speeding fragments to be “very small.” The threat, he added, is defined and acceptable.

Mr. Johnson, who works at the Johnson Space Center in Houston, said the new swarms of whirling debris might also eventually pose a threat to other satellites in an orbital chain reaction.

“What we’re doing now is trying to quantify that risk,” he said. “That’s a work in progress. It’s only been 24 hours. We put first things first,” meaning the station and preparing for the next shuttle mission.

William Harwood contributed reporting.