The payload canister is moved into position beneath the payload changeout room for transfer of its cargo into the PCR.

On Launch Pad 39B, the payload canister is moved into position beneath the payload changeout room (PCR) for transfer of its cargo into the PCR. The canister holds the payload for Atlantis and mission STS-115, the Port 3/4 truss segment with two large solar arrays. The payload changeout room provides an environmentally clean or "white room" condition in which to receive a payload transferred from a protective payload canister. After the shuttle arrives at the pad, the rotating service structure will close around it and the payload will then be transferred into Atlantis' payload bay. Atlantis' launch window begins Aug. 28. During its 11-day mission to the International Space Station, the STS-115 crew of six astronauts will install the truss, a 17-ton segment of the space station's truss backbone. Photo credit: NASA/George Shelton