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CAPE CANAVERAL, Fla. -- An engineer uses a backscatter device to examine space shuttle Discovery's external fuel tank on Launch Pad 39A at NASA's Kennedy Space Center in Florida. The device bounces radiation off the tank, allowing technicians to see under the tank's foam insulation. The foam cracked during initial loading operations for Discovery’s STS-133 launch attempt on Nov. 5, and technicians later identified two cracked stringers, which are the composite aluminum ribs located vertically on the tank’s intertank area. Those two stringers have been replaced and reinforced with doublers, which are shaped metal pieces twice as thick as the original stringers. Launch is no earlier than Dec. 17 at 8:51 p.m. EST. For more information on STS-133, visit www.nasa.gov/mission_pages/shuttle/shuttlemissions/sts133/. Photo credit: NASA/Frank Michaux