Lockheed Martin technicians clean the area around the manhole on the external tank designated for mission STS-121.

Lockheed Martin technicians from NASA's Michoud Assembly Facility in Louisiana clean the area around the "manhole" on the external tank designated for mission STS-121. The work is part of the process 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 cleaning, the manhole will be removed to provide access to the area of the sensors for their removal. Photo credit: NASA/Jim Grossmann