US and Russian communications satellites have collided in space in the first such reported accident.
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 massive clouds of debris, and the magnitude of the crash is not expected to be clear for weeks.
There are thousands of man-made objects orbiting the earth, but this is thought to be the first time two intact spacecraft have hit each other, the BBC's Andy Gallacher in Miami says.
Nasa is now tracking the hundreds of pieces of wreckage from the collision.
It is hoped that most of it will burn up in the Earth's atmosphere, our correspondent adds.
The concern is whether the debris will spread and pose any risk to the ISS, which is orbiting the earth some 435km below the course of the collision.
According to the Washington Post, a NASA memo said officials determined the risk to be "elevated" but have estimated it as "very small and within acceptable limits".
Nasa spokesman John Yembrick said the ISS has the "capability of doing a debris-avoidance manoeuvre if necessary".
He said this had happened on just eight previous occasions during the course of its 60,000-plus orbits.
There are reported to be no plans to delay the launch of Nasa's space shuttle Discovery later this month.
Communications firm Iridium, based in Bethesda, Maryland, said it "lost an operational satellite" after it was struck on Tuesday by the Russian satellite.
It said its clients may experience some brief outages but was working to replace its 560kg satellite, launched in 1997, with one of its in-orbit spares within the next 30 days.
The firm described it as an "extremely unusual, very low-probability event", stressing that it was not caused by any fault on its part.
Some officials have said the Russian satellite, launched in 1993 and weighing 950kg (2,094lb), was out of control.
Some 6,000 satellites have been sent into orbit since 1957.
Around 3,000 remain in operation, according to NASA.