KENNEDY SPACE CENTER, FLA. - Workers on Launch Pad 39B prepare to close Space Shuttle Atlantis' payload bay after closeout of the orbiter's aft compartments and installation of the aft doors. Inside is seen the orbiter's cargo, the 17-and-a-half-ton P3/P4 truss segment for the International Space Station. Prior to the closing, technicians finished performing the aft confidence test, which involves powering up and testing all of the aft systems, such as the main propulsion system circuits. Atlantis will fly on mission STS-115 with a crew of six to deliver and install the P3/P4 segment to the port side of the integrated truss system on the orbital outpost. The truss includes a new set of photovoltaic solar arrays. When unfurled to their full length of 240 feet, the arrays will provide additional power for the station in preparation for the delivery of international science modules over the next two years. STS-115 is expected to last 11 days and includes three scheduled spacewalks.