WASHINGTON – The U.S. Navy is preparing to shoot a faltering U.S. spy satellite out of the sky in the next two weeks using a tactical missile that was manufactured as a defensive weapon to head off enemy aircraft, the Pentagon announced on Thursday.

While it’s not uncommon for space junk to fall out of the sky, military officials said they are particularly concerned in this case because much of the 1,000 pounds of the frozen rocket fuel called hydrazine on the spacecraft could survive the descent and pose health risks, such as damage to skin and lung tissue, if it lands in a populated area.

Speaking to reporters, Marine Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff, and James Jeffrey, the deputy national security adviser, said the Navy’s window of opportunity to strike the satellite before it enters the Earth’s atmosphere begins in the next three or four days. Cartwright said the window would likely remain open for seven or eight days.

If the satellite is not intercepted, it is expected to enter the atmosphere in late February or early March.

“This has no aerodynamic properties,” Cartwright said of the satellite. “Once it hits the atmosphere, it tumbles, it breaks apart. It is very unpredictable and next to impossible to engage. So what we’re trying to do here is catch it just prior to the last minute, so it’s absolutely low as possible, outside the atmosphere, so that the debris comes down as quickly as possible.”

Three Navy ships will deploy somewhere in the Pacific Ocean to launch the missile. Cartwright declined to specify an exact location of the launch beyond stating it would be from somewhere in the Northern Hemisphere. At the time of the launch, officials said the satellite would be about 150 miles above the Earth.

Software for the Strategic Missile 3, designed as an anti-aircraft system, is being modified to adjust for the bus-sized satellite. Military officials have “high confidence” that the missile will strike the satellite and hopefully puncture its 40-inch wide-tank that holds the hydrazine.

If the first attempt fails, officials would have to decide whether to take another shot, but it could be “next to impossible” to strike on a subsequent attempt, Cartwright said.

President Bush’s decision to order the missile strike on the satellite comes in the wake of China’s decision to intercept one of its own weather satellites last year in an attempt to test an anti-satellite missile system. The U.S. and other nations criticized that move.

Michael Griffin, the NASA administrator, said the United States’ approach is decidedly different than that of China, which conducted its missile strike without notifying other nations.

“We’ve started that notification well over a month ago and we’re continuing to keep people informed,” Griffin said. “And we have a consequence management plan that is in place.”

Members of Congress were offered a classified briefing by the Defense Department early Thursday before the plan was announced.

“As the administration moves forward, I am advising that they do so in an open manner and in consultation with our allies and the international community,” Rep. Ellen Tauscher, D-Calif., the chairwoman of the House subcommittee on strategic forces, said in a statement. “Just like our partners in space, we need to be responsible for the risks we create. This is a matter of public safety.”

Cartwright told reporters that the use of the missile was “a one-time deal,” when asked whether the Standard Missile 3 was being tested as anti-satellite weapon technology.

Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics, said the Bush administration seemed to be overreacting. While hydrazine is toxic, McDowell noted that toxic debris often falls out of the sky.

“This seems to be … an excuse to show off our new toy and in particular to show the Chinese that you can blow up a satellite, but so can we,” McDowell said.

The faulty spacecraft is a test satellite that was launched by the National Reconnaissance Office, a spokeswoman for the intelligence agency confirmed. The cost of the satellite and the contractor are classified. Reuters reported last year that the satellite, which was built by Lockheed Martin at a cost of hundreds of millions of dollars, was launched in December 2006, but has been out of touch since soon after reaching its low orbit.

A spokesman for Lockheed Martin declined to comment.

Thousands of pieces of space debris have fallen from space into Earth’s atmosphere, but there are no previous reports of death or injuries.

In this case, administration officials said as much as half of the satellite debris, potentially including an intact fuel tank, could make it into the atmosphere and could spread toxic rocket fuel over an area the size of two football fields.

“This is all about trying to reduce the danger to human beings,” Jeffrey said.

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