Two satellites have crashed into each other in orbit in what is thought to be the first accident of its kind.
The privately owned United States communications satellite collided with a defunct Russian satellite more than 800km above Russia, NASA says.
When the satellites hit each other over northern Siberia the impact produced a massive cloud of debris.
Some reports suggest hundreds of pieces of wreckage are now being tracked and it is thought the magnitude of the accident will not be known for weeks.
Any risk to the International Space Station or NASA's next shuttle launch, due later in February, is low, but experts will be monitoring the situation.
Tens of thousands of objects are routinely tracked through space, but when two collide with such force, the ensuing debris could damage other satellites.
It is hoped that most of the wreckage will burn up in the Earth's atmosphere.
Some 6,000 satellites have been sent into orbit since 1957 and about 3,000 remain in operation, according to NASA.
