The Pentagon wants to shoot the craft down from a ship

President George W Bush approved the option to fire a missile from a US Navy ship to destroy the satellite before it enters the atmosphere, officials said.

They say the satellite contains a hazardous material which could be fatal if inhaled by humans.

A US general denied claims that the main aim was to destroy secret parts.

General James Cartwright said confidential components would be burned up in the atmosphere and, in any case, that would not be a reason for shooting down the satellite.

We are better off taking the attempt than not

General James Cartwright

He said the National Reconnaissance Office "bird", launched in December 2006, had lost power and communications shortly after it entered space and a controlled re-entry was not possible.

The broken satellite had been predicted to reach the top of the Earth's atmosphere towards the end of February or early March and officials could not predict where it would land.

The general said the window of opportunity for the strike would open in the next three or four days and last for about a week.

Burning sensation

Deputy National Security Adviser James Jeffries said thousands of space objects fell to Earth each year, without threat to humans.

BROKEN SATELLITE Owner: National Reconnaissance Office satellite Mission: Classified Launched: Dec 2006 Weight: 5,000lb (2,270 kg) 2,500lb could survive re-entry Carrying hydrazine rocket fuel

"What makes this case is a little bit different... was the likelihood that the satellite upon descent to the Earth's surface could release much of its 1,000lb plus (454 kg) of its hydrazine fuel as a toxic gas," he said.

He said it was unlikely to hit a populated area, but "there was enough of a risk for the president to be quite concerned about human life".

The hydrazine rocket fuel, which would have been used up had the satellite been operational, is in a tank that would survive re-entry.

Gen Cartwright said that exposure to the hydrazine would have similar effects to inhaling chlorine or ammonia - a burning sensation in lungs and, if too close and too much, then possibly death.

If the hydrazine tank did burst, the danger zone would be an area covering the size of two football pitches. He said that blowing the satellite up would disperse the hydrazine harmlessly in space, leaving only small-scale satellite debris to fall harmlessly to Earth.

One shot

The US Navy plans to modify a Standard Missile 3 to be launched from an Aegis destroyer - usually part of the US Missile Defense System designed to intercept ballistic missiles.

Gen Cartwright said they planned to have one missile shot, but there would be three missiles available on three ships. If the first attempt was unsuccessful, a decision would be made whether a second attempt was required.

It must be done before it re-enters the Earth's atmosphere, otherwise the craft would be "next to impossible" to hit because of atmospheric disturbances.

"We are better off taking the attempt than not," Gen Cartwright said.

The general added that the space shuttle, currently on a mission to the International Space Station, would be back on Earth before the attempt was made.

China test

The largest uncontrolled re-entry by a US space agency (Nasa) craft was the abandoned Skylab space station in 1979.

Last year, China carried out a test using a ground-based ballistic missile to destroy a satellite in space, prompting international alarm and fears of a space arms race.

Gen Cartwright said there was no parallel with Beijing's actions as the Chinese satellite had been much further out from Earth, meaning its debris could be floating round for decades, endangering spacecraft.