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KENNEDY SPACE CENTER, FLA. - Technicians from NASA's Michoud Assembly Facility in Louisiana begin removing foam from the external tank designated for mission STS-121. The work is the first step in removing and replacing the external tank's four liquid hydrogen main engine cutoff sensors, which indicate whether the tank still has fuel during its climb to orbit. After the foam removal, the technicians will remove a cover, called the "manhole," to provide access to the area to begin the removal and replacement of the sensors. Photo credit: NASA/Jim Grossmann