All U.S. missile defense plans will rely on a constellation of low-orbiting satellites known as SBIRS Low that are supposed to resolve what is probably the toughest challenge of missile defense: finding and tracking with pinpoint accuracy complex warheads traveling at 15,000 miles an hour and surrounded by dozens of decoys. However, there is substantial debate over whether defense contractors can come up with the crucial satellite technology -- especially on the timetable that Congress and the Pentagon are demanding.