America is poised to shoot down a rogue spy satellite which is on collision course with Earth.
President George Bush has ordered his military chiefs to destroy the disabled 5,000lb spacecraft while it is still hurtling through space on a wayward path that takes it over Britain.
His decision came amid growing fears over the danger posed by the satellite's substantial load of explosive rocket fuel.
Without intervention, it would be expected to crash land early next month.
The Pentagon said the satellite would be shot down before it enters the Earth's atmosphere by a missile fired from a U.S. Navy cruiser.
This should mean fragments of the disabled satellite will fall harmlessly into the ocean or burn up in the atmosphere.
Confirming the emergency strike last night, Defence Department spokesman Bryan Whitman said: "We have been looking at ways to mitigate the possible risk to human lives and to demonstrate our continuing commitment to safe and responsible space operations."
General James Cartwright, vice-chairman of the Joint Chiefs of Staff, said that the "window of opportunity" for such a shoot-down will open in the next three or four days and last for seven or eight days.
He did not say whether the Pentagon has decided on an exact launch date.
He said a Navy missile known as Standard Missile 3 would be fired in an attempt to intercept the satellite just prior to its re-entering-Earth's atmosphere.
It would be "next to impossible" to hit the satellite after that because of atmospheric disturbances. One goal, said General Cartwright, is to hit the fuel tank in order to minimise the amount of fuel that returns to Earth.
He also said that if an initial shoot-down attempt fails, a decision will be made whether to take a second shot.
Another main goal of the satellite's destruction is to prevent its sensitive technology from falling into the wrong hands.
Defence and intelligence expert John Pike said: "What they have to be worried about is that a souvenir collector is going to find some piece, put it on eBay and the Chinese buy it."
Known by its military designation US193, the satellite was launched in December 2006. It lost power and its central computer failed almost immediately after, leaving it uncontrollable.
It carried a highly innovative secret imaging sensor.
It would be due to hit the earth during the first week of March, though it is not known where.
Officials say about half of the spacecraft would be expected to survive its blazing descent through the atmosphere and would scatter debris, some of it potentially hazardous, over several hundred miles.
The satellite is fitted with thrusters - small engines used to position it in space. They contain the toxic rocket fuel hydrazine, which can harm anyone who comes into contact with it. Officials have said it carries about 1,000lbs of the propellant.
The U.S. State Department was contacting foreign governments yesterday to assure them the strike was purely precautionary.
"You want to make sure that everybody understands exactly what actions are being taken so there are no misunderstandings and also to reassure people vis-a-vis treaty obligations," said spokesman Sean McCormack.
Shooting down a satellite is particularly sensitive because of the controversy surrounding China's anti-satellite test last year, when Beijing shot down one of its defunct weather satellites, drawing immediate criticism from the U.S. and other countries.
A key concern at that time was the debris created by Chinese satellite's destruction - and that will also be a focus now, as the U.S. determines exactly when and under what circumstances to shoot down its own errant satellite.
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