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President Bush has ordered the Pentagon to undertake an unprecedented effort to use a Navy missile to try to destroy a broken U.S. spy satellite - and thereby minimize the risk to humans from its toxic fuel - by intercepting it just before it re-enters the atmosphere. "This is the first time we've used a tactical missile to engage a spacecraft," said Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff, who would not say exactly what the odds of success are.

The attack plan: A single missile, of a type known as the Standard Missile 3, would be launched from a Navy ship at sea. The missile was originally designed to intercept a ballistic missile in flight, not a spacecraft, so the missile and its software have been modified. Ideally, the missile will strike the satellite directly just before it re-enters Earth's atmosphere over the Pacific, breaking it into small pieces that will mostly fall through the atmosphere rather than become space debris. It would be "next to impossible" to hit the satellite after that because of atmospheric disturbances, Cartwright said. If the first shot misses, a second try may be made. "This is all about trying to reduce the danger to human beings," said deputy national security adviser James Jeffrey.

Timing: Jeffrey did not say when the attempted intercept would be conducted, but the satellite, known by its military designation US 193, is expected to hit Earth during the first week of March. A "window of opportunity" for hitting the satellite has been calculated to begin in three or four days, Cartwright said. It could last as long as eight days. The Pentagon will wait until the shuttle Atlantis returns to Earth, a flight scheduled for Monday.

Assessing the danger: Many experts in the field said the danger of anyone being harmed by the falling satellite is extremely small. Much larger spacecraft, including Skylab and the shuttle Challenger, have fallen to Earth without consequences.

About the satellite: The 5,000-pound spacecraft, about the size of a school bus, was launched in December 2006 and failed almost immediately, leaving it uncontrollable. While information about the spacecraft is classified, experts believe it is the first of a new generation of smaller and more precise spy satellites. It circles the globe 16 times a day, carries a secret imaging sensor and is outfitted with thrusters - small engines used to position it in space. They contain the toxic rocket fuel hydrazine, which can be deadly in extended doses. About half of the spacecraft is expected to survive its blazing descent through the atmosphere and will scatter debris over several hundred miles.