A US warship is moving into position to try to shoot down an out-of-control US spy satellite as early as Wednesday before it tumbles into the Earth's atmosphere, Pentagon officials said Tuesday.

Armed with two specially modified interceptor missiles, the USS Lake Erie has been tasked to intercept the satellite over the Pacific and shoot it down into the ocean, the officials said, adding that a cruiser, the Aegis, is already in waters off Hawaii.

spy satellite
©AFP/Us Navy-HO/Michael Hight
This picture released by the US Navy shows Lt. Cmdr. Andrew Bates operating the radar system control during a ballistic missile defense drill on February 16 aboard the guided-missile cruiser USS Lake Erie. The US warship is moving into position to try to shoot down a defunct US spy satellite as early as Wednesday before it tumbles into the Earth's atmosphere, Pentagon officials said Tuesday.

The USS Decatur, a guided missile destroyer, is carrying a third interceptor missile in case the first two attempts fail, defense officials said. Another destroyer, the USS Russell, was still in port on Tuesday.

"I'm confident they'll be able to do something," said a senior Navy official. "Once the weapon goes into track, then I think it's a done deal."

The Pentagon has waited for the space shuttle Atlantis to land first at the end of its mission to the International Space Station. That is scheduled for 9:07 am Wednesday.

"Touchdown of the Atlantis opens the window of opportunity for the US military to shoot down that rapidly decaying US intelligence satellite," said Geoff Morrell, the Pentagon press secretary.

"There is a very low risk because our orbits are quite different," Atlantis Commander Steve Frick said. "The satellite is well below us (where) we are now, but of course we are going to land before they break up that satellite."

The Pentagon is essentially employing the US missile defense system for the shoot-down attempt at an estimated cost of 40 to 60 million dollars.

It is training a panoply of Aegis warships, radars and computerized command networks on the school bus-sized satellite.

Software changes have been made to the SM-3 interceptor missiles so that they will recognize a satellite in space instead of a ballistic missile -- their normal programmed target -- officials said.

The three-stage missile will carry a maneuverable non-explosive warhead guided from the ground until it can use its infrared sensor to steer itself into a shattering collision with the satellite at an altitude of 150 nautical miles.

US Navy ships have intercepted ballistic missile warheads in this way in tests, but the Navy official, who briefed reporters on condition of anonymity, said the satellite poses a somewhat different problem.

It is colder and moving through space at a much higher speed, making it more difficult to track than the ballistic missiles.

If the USS Lake Erie misses with its first shot, it will probably have to wait a day to try again. The longer the wait, the harder the satellite will be to shoot down as it gathers speed, falling towards the Earth's atmosphere.

The plan is to hit a tank on the satellite carrying the toxic propellant hydrazine, which officials say could pose a threat to humans if it survives re-entry.

"The system itself is very accurate so hopefully that will translate into being able to hit the tank," said a defense official.

Defense Secretary Robert Gates, who leaves Wednesday on a trip to Australia and Southeast Asia, has been empowered President George W. Bush to authorize the shoot-down, Morrell said.

"Based on the advice he gets he's prepared to do so from the road if necessary," he said.

France urged the United States to take all necessary safety measures.

"We hope all measures will be taken to reduce as much as possible the consequences of destroying this satellite for the safety and integrity of other space objects," foreign ministry spokeswoman Pascale Andreani told reporters.

The plan drew criticism on Sunday from Russia, whose defense ministry said it looked like a veiled weapons test and an "attempt to move the arms race into space." China has also voiced concern.

Washington has denied seeking to cover up the satellite's technological secrets or to make a show of strength after China used a missile to shoot down an old weather satellite in January 2007.