The Greyhound bus-sized intelligence satellite failed shortly after launch in 2006. Intended to conduct both electronic eavesdropping and photographic intelligence-gathering, the satellite contains a large tank of unused toxic fuel called hydrazine. The fuel would pose a health risk if the tank survived re-entry and landed in a populated area. The satellite has been gradually moving closer to the atmosphere and could come down some time in the next several weeks.
Since the satellite cannot be maneuvered to fall into the ocean, the plan calls for firing a modified Navy SM-3 anti-missile interceptor from an Aegis battle management system equipped warship in the northern Pacific, as the satellite nears the atmosphere.
It will be the first time a missile defense interceptor will be used against a satellite, something that has not been attempted since the 1980s, when the Pentagon tested an anti-satellite missile from a jet fighter.
From WaPo here.