"Numerous satellites over the years have come out of orbit and fallen harmlessly. We are looking at potential options to mitigate any possible damage this satellite may cause."

Photo reconnaisance

Jeffrey Richelson, a senior fellow with the National Security Archive, said that the spacecraft was likely to be a photo reconnaisance satellite used to gather visual information from space about governments and groups, such as construction at suspected nuclear sites or training camps.

The satellites also can be used to survey damage from hurricanes, fires and other natural disasters.

The largest uncontrolled re-entry by a Nasa spacecraft occured when Skylab, the 78-ton abandoned space station, fell from orbit in 1979.

Its debris dropped harmlessly into the Indian Ocean and across a remote section of western Australia.

In 2000, Nasa engineers successfully directed a safe de-orbit of the 17-ton Compton Gamma Ray Observatory, using rockets aboard the satellite to bring it down in a remote part of the Pacific Ocean.

Protecting secrets

John Pike, director of the defence research group GlobalSecurity.org, told the Associated Press news agency that spy satellites are typically disposed of through a controlled re-entry into the ocean so that no one else can access the spacecraft, he said.

Pike estimated that the spacecraft weighed about 9072kg and is the size of a small bus.

He said the satellite would create significantly less debris than the Columbia space shuttle crash in 2003. 

Pike suggested that it was unlikely that authorities would decide to shoot the satellite down with a missile as it would create debris that could hit the ground.