Nasa has confirmed that a defunct six-ton satellite has crashed back to earth, possibly over Canada, the Pacific Ocean, the Atlantic Ocean and the Indian Ocean.
The agency posted on its official Twitter site that the spacecraft crashed through the atmosphere early on Saturday morning some time between 3.45am and 4.45am GMT.
Most of it is believed to have burned up.
Unconfirmed reports on Twitter suggested some of the debris may have fallen near a town south of Calgary in western Canada.
The Upper Atmosphere Research Satellite was Nasa's biggest spacecraft to tumble out of orbit, uncontrolled, in 32 years.
UARS was launched aboard space shuttle Discovery in 1991. Nasa decommissioned the satellite in 2005, after moving it into a lower orbit that cut its life short by two decades.
Bits of space junk re-enter the atmosphere virtually every day. No injuries have ever been reported from it.
Some 26 pieces of the satellite -- representing 1,200 pounds of heavy metal -- were expected to rain down. The biggest surviving chunk should be no more than 300 pounds.
The Upper Atmosphere Research Satellite is the biggest Nasa spacecraft to crash back to Earth, uncontrolled, since the Skylab space station and the more than 10-ton Pegasus 2 satellite, both in 1979.