Nasa scientists are closely monitoring the skies after two satellites crashed into each other over Siberia, in what experts have said is the first collision of its kind.
The accident, which took place more than 400 miles above the earth's surface on Tuesday, has left a large cloud of debris drifting in space. Nasa officials are keeping watch to see if the wreckage could endanger other spacecraft, although they said it was unlikely that the International Space Station could be damaged.
"It will be weeks at least before the true magnitude of these clouds are known," Nasa said in an alert message. "The risk to the space station is considered to be very small and within acceptable limits."
The agency said that it was more concerned about the threat to an array of monitoring satellites, which it said were of "highest interest for immediate consideration".
Air Force Lieutenant Colonel Les Kodlick of the US strategic command, said: "We believe it's the first time that two satellites have collided in orbit." The command's joint space operations centre was tracking 500 to 600 new bits of debris, some as small as 10cm (3.9 inches) across, in addition to the 18,000 or so other man-made objects it has catalogued, he said.
The incident is thought to have involved a 12-year-old satellite belonging to the US company Iridium and a defunct Russian Cosmos satellite that was put into orbit in 1993. The craft, which weighed 560kg and 950kg respectively, apparently smashed into each other at high speed. Both satellites were used for telecommunications, with the US satellite an active part of Iridium's network of 66 craft which provide satellite telephone access to more than 250,000 people worldwide.
It is unclear what caused the crash, but the Russian satellite is thought to have shut down some time ago and would have had no steering mechanism.
Although there have been collisions in the past, they only involved spent rocket parts or small satellites. The scale of this crash, said officials, was unprecedented.
"In the past almost 20 years there have been three other accidental collisions between objects in orbit, but they've all been very minor," said Nicholas Johnson, chief scientist at Nasa's orbital debris programme. "The most debris ever produced in an event was four … this is two intact spacecraft colliding and we have hundreds of debris out there."
In the past abandoned or dysfunctional satellites have caused problems, with some pushed into extremely wide "graveyard" orbits that move them out of the way of other spacecraft. In several cases, rogue satellites have been shot out of the sky to prevent them crashing to Earth.
