The US might have to delay shooting down a defunct spy satellite on Thursday due to bad weather.

Waves in the Pacific are too big for US warships to get into a correct position to fire a missile at the spacecraft.

The US Navy has been on standby to destroy the "spysat" before it enters Earth's atmosphere.

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The satellite, known as USA 193, stopped communicating and lost control a few hours after it was launched on 14 December 2006.

Officials say the shootdown was approved amid concerns that toxic hydrazine fuel on board could harm or kill humans if inhaled.

The US has denied the shootdown is a response to a successful anti-satellite test carried out by China last year, which prompted fears of a space arms race. But Russia has challenged America's rationale for carrying out the action.

Once the space shuttle Atlantis had returned safely to Earth, the Pentagon announced the opening of a window of opportunity for it to try to shoot down USA 193.

The military plans to destroy the satellite with an SM-3 missile fired from a Navy cruiser parked on the western side of Hawaii.

The US government has issued notices to aviators and mariners to remain clear of a section of the Pacific Ocean from 0230 to 0500 GMT on Thursday.

Precision strike

The US intends to intercept the 2,300kg (5,000lbs) spacecraft - believed by some commentators to be a radar imaging reconnaissance satellite - when it is at an altitude of 240km above the ground.

Hitting USA 193 just at the edge of the Earth's atmosphere minimizes the amount of debris that would remain in space.

But the missile, launched from a Navy cruiser, will have to do better than just striking the bus-sized spacecraft.

It needs to pierce the satellite's spherical fuel tank, containing more than 450kg (1,000lbs) of toxic hydrazine, which would otherwise be expected to survive re-entry.

With the satellite's thermal control system gone, the fuel would now be frozen solid, allowing the tank to resist the heat of re-entry. If the tank were to land intact, it could leak toxic gas over an area the size of two football pitches.

US General James Cartwright, vice-chairman of the joint chiefs of staff, said exposure to the fuel would have similar effects to inhaling chlorine or ammonia - a burning sensation in the lungs and, if too close and too much, possibly death.

Window of opportunity

By rupturing the one metre-wide tank, the military hopes to disperse as much hydrazine as possible in space before the National Reconaissance Office (NRO) "bird" falls to Earth.

Officials expect that over 50% of the debris will fall to Earth within the first 15 hours after the strike - or within its first two revolutions of Earth.

Gen Cartwright, and Gen Kevin Chilton, head of Strategic Command, will advise US defence secretary Robert Gates on exactly when to launch the missile.

The mission could go forward on any day until 29 February, when the satellite is projected to have re-entered the atmosphere, making it virtually impossible to attempt to hit with a missile.

Left to its own devices, about half of the spacecraft would be expected to survive the blazing descent through the atmosphere, scattering debris in a defined "corridor" which runs across the Earth's surface.

Professor Richard Crowther, a space debris expert with the Science and Technology Facilities Council (STFC), said that if struck with the missile, about 25% of USA 193 is likely to survive the fall to Earth.

"The smaller the debris is the more likely you are to get burn-through. So if you fragment something before re-entry, less mass will survive to hit the Earth," he told BBC News.

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