US and Russian communications satellites have collided in space in the first such reported mishap.
A satellite owned by the US company Iridium hit a defunct Russian satellite at high speed nearly 780km (485 miles) over Siberia on Tuesday, Nasa said.
The risk to the International Space Station and a shuttle launch planned for later this month is said to be low.
The impact produced a massive cloud of debris, and the magnitude of the crash is not expected to be clear for weeks.
The reportedly non-operational Russian satellite, weighing 950kg (2,094lb), had been launched in 1993, while the Iridium satellite weighed 560 kg and was launched in 1997.
When two such objects collide with such force, the ensuing debris can destroy other satellites, says the BBC's Andy Gallacher in Florida.
But Nasa said the risk to the ISS and its three astronauts was low as the station orbits the earth some 435km below the course of the collision.
It is hoped that most of the wreckage from the collision will burn up in the earth's atmosphere, our correspondent says.
Hundreds of pieces of wreckage are now being tracked, reports say, adding to the tens of thousands of objects that are routinely tracked through space.
Some 6,000 satellites have been sent into orbit since 1957.