Nasa's UARS satellite plunges over Pacific

The spacecraft was expected to fall to Earth by about 0500 GMT - officials say it is not possible yet to give a precise time, BBC News reports.

The Upper Atmosphere Research Satellite (UARS) is the largest American space agency satellite to return uncontrolled into the atmosphere in about 30 years.

Officials said the risk to public safety was remote.

A statement on the Nasa UARS website read: "The Joint Space Operations Center at Vandenberg Air Force Base in California said the satellite penetrated the atmosphere over the Pacific Ocean. The precise re-entry time and location are not yet known with certainty."

There have been some unconfirmed reports on Twitter that suggested debris might have fallen in western Canada.

Most of the decommissioned spacecraft should simply have burnt up, but modelling work indicated perhaps 500kg could have survived to the surface.

Any pieces of debris should have been scattered over a 800km path; but with more than 70% of the Earth's surface covered by water, many experts said the pieces were most likely to end up in the ocean.

Stephen Cole, a Nasa spokesman in Washington DC, told BBC News: "You have to remember that they're very, very small pieces, even though the original satellite was large - as large as a bus. Most of that burns up in the atmosphere and just a few dozen pieces survive. They're highly damaged, and if they're in the ocean - they're gone."

UARS was deployed in 1991 from the space shuttle Discovery on a mission to study the Earth's upper atmosphere.

It contributed important new understanding on subjects such as the chemistry of the protective ozone layer and the cooling effect volcanoes can exert on the global climate.

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