Workers on Launch Pad 39B watch as Space Shuttle Atlantis' payload bay doors begin to close.

Workers on Launch Pad 39B watch closely as Space Shuttle Atlantis' payload bay doors begin to close. Inside the payload bay is the orbiter's cargo, the 17-and-a-half-ton P3/P4 truss segment for the International Space Station. Mission STS-115 is scheduled to lift off about 4:30 p.m. Aug. 27. The crew will deliver and install the P3/P4 segment to the port side of the integrated truss system on the International Space Station. 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. The mission is expected to last 11 days and includes three scheduled spacewalks. Photo credit: NASA/Kim Shiflett