PCR a worker prepares the mechanism to open the doors of the canister containing the S3 S4 integrated truss.

In the payload changeout room (PCR) on Launch Pad 39A, a worker prepares the mechanism to open the doors of the canister containing the S3/S4 integrated truss. The PCR is the enclosed, environmentally controlled portion of the rotating service structure that supports cargo delivery to the pad and subsequent vertical installation into the orbiter payload bay. The truss is the payload for Space Shuttle Atlantis on mission STS-117 to the International Space Station. The Atlantis crew will install the new truss segment, retract a set of solar arrays and unfold a new set on the starboard side of the station. Launch is targeted for March 15. Photo credit: NASA/Jack Pfaller