The open doors of the payload canister reveal its cargo the Port 3/4 truss segment for mission STS-115.

Inside the payload changeout room, the open doors of the payload canister reveal its cargo, the Port 3/4 truss segment for mission STS-115. The payload will be removed from the canister and later installed in Space Shuttle 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/Jack Pfaller