This time their target is not an incoming warhead or a dummy test target, but a doomed experimental satellite the size of a school bus and weighing 5,000 pounds. It went dead shortly after being launched in December 2006. It contains a half-ton of hydrazene, a rocket fuel that officials said can burn the lungs and even is deadly in extended doses.

The fuel tank is believed sturdy enough to survive re-entry, based on studies of the fuel tank that fell to earth after the shuttle Columbia disaster in 2003. Officials said that the slushy frozen fuel would have then been released wherever it came down.

The military and NASA have calculated that the best opportunity to shoot down the satellite with an interceptor missile is just before it re-enters the atmosphere and starts to tumble and break apart on a random path toward the surface, an opportunity that begins in three to four days, and extends for eight days after that. At that point, its debris would quickly be dragged out of orbit.

In many ways, the task resembles shooting down an intercontinental nuclear missile, although in this case the target is larger, its path is better known, and if a first shot misses, it will continue to circle the earth for long enough to allow a second or even a third try.

The weapon of choice, after modifications that are still under way, is the Standard Missile 3 carried by the Navy’s Aegis cruisers, originally fielded for use against airplanes, cruise missiles and the like. The defensive missiles and supporting radar were already being modified and tested to shoot down enemy warheads, so the software is now being reprogrammed to home in on the radar and other signatures of a large satellite instead of a ballistic missile, officials said. Although White House, military and NASA officials described the president’s decision as motivated solely by wanting to avoid a spread of toxic fuel in an inhabited area, it has implications for missile defense and antisatellite weapons.