WASHINGTON — The Pentagon's missile defense program has long been as contentious as it is pricey. But a small part of that system, ship-mounted missiles designed to track and destroy enemy warheads, has proved more affordable and successful.
The Navy will activate that system as soon as today when the window opens for an unusual Pentagon attempt to shoot down a failing spy satellite that is hurtling toward Earth with 1,000 pounds of toxic rocket fuel aboard.
In eight years of testing, warships equipped with Aegis radar systems have hit 12 targets in 14 Pacific Ocean attempts, compiling a better record than the costlier land-based system of interceptor missiles in Alaska and California.
But the task of bringing down the satellite will be much harder, Navy officials warned. The satellite is traveling faster, higher and, perhaps most important, colder than the enemy missiles the system was built to hit.
"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 that a ballistic missile has," said a Navy official, noting that heat is one of the primary ways an interceptor finds its target.
"The typical clues that both the interceptors and the combat system look for have to be changed. That's the difficulty," the official said, speaking on condition of anonymity because he was not authorized to discuss the effort publicly.
Pentagon officials said today's scheduled landing of the space shuttle Atlantis in Florida marks the beginning of a nearly weeklong window to shoot down the satellite. There were signals that the attempt could come just hours after the shuttle's touchdown.
Ships and aircraft were issued a notice Tuesday by federal officials to avoid the north Pacific test area today, said Pentagon Press Secretary Geoff Morrell. But Morrell emphasized that no final decision had been made about when to take the shot.
"If a shot is not taken within the 24 hours after that notice went out, there will likely be another [notice] that goes out," he said.
According to a Web posting by Ted Molczan, a well-known amateur satellite tracker in Canada, the federal notice warns ships and planes to avoid a restricted zone just west of Hawaii for 2 1/2 hours beginning at 7:30 Pacific time tonight.
The 5,000-pound spy satellite went bad soon after its 2006 launch and has been orbiting out of control. The Bush administration's decision last week to shoot it down marks the first such attempt since Cold War-era tests in the 1980s.
The Chinese government has expressed concern, and Russian officials have charged that it is a veiled missile test. Some U.S. nonproliferation groups have questioned whether the threat of a fuel-tank rupture justifies the effort and expense involved in blasting the satellite.
Pentagon officials defended the plan and expressed confidence about chances for success. But Navy officials said the mission would not be easy.
The interceptor missile's "kill vehicle" -- the very top of the rocket that gets pushed toward the satellite -- is equipped with an infrared seeker, which looks for signs of heat and uses retrorockets to guide itself into the target's path.
Since the cold satellite lacks the heat that an enemy missile would have, the operation's planners are relying on the sun to raise the satellite's temperature enough to allow the kill vehicle to find the spacecraft, the Navy official said -- an indication that the shoot-down would be attempted in the daytime.
Navy officials plan to use a Raytheon-made Standard Missile 3. Three of the missiles, which cost $10 million apiece, have been readied for the shoot-down attempt. Navy experts have reconfigured the missile's software to help it find the cold satellite in the even colder upper atmosphere.
"Once the weapon goes into track, then I think it's a done deal," the Navy official said, using the military term for locking onto a target.
The speed of the intercept will cause additional complications. The satellite and the missile will be flying toward each other at about 22,000 mph, nearly twice the speed of a missile defense test. That gives the interceptor, which is not armed with explosives, only a small margin of error.
Even the satellite's size -- military officials have compared it to a bus -- may not be much help. The Navy official said the target is the satellite's fuel tank, which is holding the half-ton of hydrazine fuel that U.S. officials warn could turn into a toxic gas if the tank cracked open when it hit Earth.
With the satellite spinning out of control, hitting a specific part of it adds to the difficulty.
"This is a very big object. If the area of concern is the gas tank, you need to be in that location," the Navy official said. He added that software technologies allow the missile to home in on a small part of a larger target, but the specifics remain classified.
Navy officials said the cruiser Lake Erie, which has fired missiles in 10 of the Navy's 14 missile defense tests, has been designated as the ship to fire at the satellite. The destroyer Decatur will be nearby as a backup.
Although Pentagon officials initially said that three ships would be involved in the test, a second Navy official said the third ship, the destroyer Russell, might not participate.