( AFP )- A 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.
Armed with two specially modified interceptor missiles, the USS Lake Erie has been tasked to intercept the satellite over the Pacific, the officials said, adding that the Aegis cruiser is already in waters off Hawaii.
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 window for the intercept has been open since Sunday, but the Pentagon has waited for the space shuttle Atlantis to land first at the end of its current mission to the International Space Station. That is scheduled to occur at around 9:05 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.
A notice to airmen posted Friday on a Pentagon website advised of the closure of a wide span of the northern Pacific beginning 0230 GMT Thursday to 0530 GMT.
Another notice posted on Tuesday advised of a closure 24 hours later of the same area for the same length of time.
Federal Aviation Administration spokeswoman Laura Brown said Friday's notice was a "draft" used in an exercise to prepare for the satellite shoot down and should not have been posted.
"It hasn't been issued by the FAA," she said.
The Pentagon is essentially employing the US missile defense system for the shoot-down attempt.
It is training a panoply of Aegis warships, radars and computerized command networks on the schoolbus-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, or "kinetic kill vehicle," that will be 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 12 of 14 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, which will make it more difficult to track than the ballistic missiles that have been intercepted with SM-3 missiles.
"We're looking at a cold body in space, a body that has been shut down for some time and so it doesn't have the traditional heating up that a ballistic missile has," he said.
The Pentagon hopes to use the warmth the satellite absorbs from sunlight to track it as it descends.
Whereas the closing speeds between SM-3 missiles and ballistic warheads in tests have been about 10 kilometers (six miles) per second, the satellite alone is moving at a speed of eight to 10 kilometers per second and the SM-3 will travel at about four kilometers per second, the official said.
That means that if the USS Lake Eerie 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 it down because it will be gathering speed as it falls 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.
Working in favor of the shoot-down is the fact that the satellite is a much bigger target than a one-meter (three-foot) long warhead.
"The system itself is very accurate so hopefully that will translate into being able to hit the tank like we need to hit the tank," said a defense official.
Defense Secretary Robert Gates, who departs 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.
General Kevin Chilton, the head of the US Strategic Command, will lead the operation.