The payload canister moves past NASA's Vehicle Assembly Building on its way to Launch Pad 39B.

The payload canister moves past NASA's Vehicle Assembly Building on its way to Launch Pad 39B. Inside are the payloads for mission STS-121: the multi-purpose logistics module Leonardo, with supplies and equipment for the International Space Station; the lightweight multi-purpose experiment support structure carrier; and the integrated cargo carrier, with the mobile transporter reel assembly and a spare pump module. The payload will be transferred from the canister to Space Shuttle Discovery's payload bay at the pad. Discovery is scheduled to launch on mission STS-121 from Launch Pad 39B in a window that opens July 1 and extends to July 19. Photo credit: NASA/Kim Shiflett