WASHINGTON: - 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 finally happened on Tuesday. And the whirling fragments could pose a threat to the International Space Station (ISS) and its three astronauts on board.

The National Aeronautics and Space Administration (Nasa) believes any risk to the ISS and its crew is low. The space station orbits about 430km below the collision, which occurred nearly 800km over Siberia. Officials also said there should be no danger to the space shuttle set to launch with seven astronauts on Feb 22, but added this would be re-evaluated in the coming days.

'This is a first, unfortunately,' said Mr Nicholas Johnson, chief scientist for orbital debris at Nasa, who works at the Johnson Space Centre in Houston.

After the two communications satellites - one Russian, the other American - cracked up in silent destruction, military radars on the ground tracked large amounts of debris going into higher and lower orbits.

The American craft weighed 560kg and the Russian craft nearly a tonne.

The US satellite belongs to Iridium Satellite, based in Bethesda, Maryland, which operates a network of 66 satellites to provide wireless telephone and data services worldwide.

A company statement said that the collision would have minimal impact on service, but that customers might experience limited disruptions. Iridium will replace the lost satellite within 30 days with a spare already in orbit.

The Russian satellite, launched in June 1993, is believed to have stopped working five to 10 years ago. Russia's civilian space agency, Roscosmos, confirmed the collision but a spokesman was quoted as saying that the satellite likely 'belonged in the past to the military' and was therefore not the agency's responsibility.

Mr Johnson said that at the beginning of this year, there were roughly 17,000 pieces of man-made debris orbiting Earth. The items, at least 10cm in size, are being tracked by the US Space Surveillance Network, which is operated by the military. The network detected two debris clouds created on Tuesday.

Litter in orbit has increased in recent years, in part because of the deliberate break-ups of old satellites. It has become so bad that orbital debris is now the biggest threat to a space shuttle in flight, surpassing the dangers of liftoff and return to Earth.

Nasa is in regular touch with the Space Surveillance Network to keep the ISS a safe distance from any encroaching objects and also shuttles in flight.

Mr Johnson said the US military's tracking radars had yet to determine the number of detectable fragments. He estimated that there would be 'many, many dozens, if not hundreds' but it would take time to come up with accurate data.

'It's very, very difficult to discriminate between all those objects when they're really close together. And so over the next couple of days, we'll have a much better understanding,' he said.

'What we're doing now is trying to quantify that risk. That's a work in progress.'

NEW YORK TIMES, BLOOMBERG, ASSOCIATED PRESS, REUTERS