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KENNEDY SPACE CENTER, FLA. - After a several-hour trip from the Canister Rotation Facility, the payload canister arrives on Launch Pad 39B. Inside the canister is the payload for Atlantis and mission STS-115, the Port 3/4 truss segment with two large solar arrays. The canister will be positioned alongside the rotating service structure and beneath the payload changeout room (PCR) for transfer of the truss into the PCR. 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