A US and a Russian satellite have collided in space hundreds of miles above Earth in what is believed to be the first major crash of two spacecraft in orbit.

The collision – which occurred nearly 500 miles over Siberia on Tuesday – caused massive debris clouds to shoot out into the atmosphere and posed a risk to astronauts aboard the International Space Station.

Nasa said that it would take weeks to determine the full magnitude of the crash, during which an Iridium commercial satellite from a Maryland-based company was destroyed after it was struck by a spent Russian satellite.

“We knew this was going to happen eventually,” said Mark Matney, an orbital debris scientist at Johnson Space Centre in Houston.

Nasa believes that any risk to the space station and its three astronauts is low as it orbits about 270 miles below the collision course.

There also should be no danger to the space shuttle scheduled to launch with seven astronauts later this month, officials said, but that would be re-evaluated in the coming days.

The Russian satellite, which was launched in 1993 and weighed nearly a tonne, was out of control. The Iridium commercial satellite was launched in 1997 and weighed 1,235 pounds.

No one has any idea yet how many pieces were generated or how big they may be.

“Right now, they’re definitely counting dozens,” Mr Matney said. “I would suspect that they’ll be counting hundreds when the counting is done.”

The Bethesda-based company that owns the Iridium commercial satellite said that it had "lost an operational satellite”.

"While this is an extremely unusual, very low-probability event, the Iridium constellation is uniquely designed to withstand such an event, and the company is taking the necessary steps to replace the lost satellite with one of its in-orbit spare satellites," the company said in a statement.

The company, which operates 66 communication satellites, said that the accident was not the result of a technological failure.

"This satellite loss may result in very limited service disruption in the form of brief, occasional outages," it said, adding that it expected to have solved the problem by Friday.

There have been four other cases of space objects colliding in orbit, Nasa said. But those were considered minor and involved parts of spent rockets or small satellites.

Nicholas Johnson, an orbital debris expert at the Houston space centre, said that the risk of damage from Tuesday’s collision was greater for the Hubble Space Telescope and Earth-observing satellites, which are in higher orbit and nearer the debris field.

There are about 17,000 pieces of man-made debris orbiting Earth. The items are tracked by the US Space Surveillance Network, which is operated by the military and which detected the two debris clouds created by the crash on Tuesday.

Litter in orbit has increased in recent years, in part because of the deliberate break-up of old satellites.

The problem is so bad that orbital debris is now the biggest threat to a space shuttles, surpassing the dangers of lift-off and return to Earth. Nasa is in regular touch with the Space Surveillance Network to keep the space station a safe distance from any encroaching objects.

“The collisions are going to become more and more important in the coming decades,” Mr Matney said.

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