On a cloudy Florida day Space Shuttle Atlantis awaits launch atop a mobile launch platform at Launch Pad 39A.

On a cloudy Florida day, Space Shuttle Atlantis awaits launch atop a mobile launch platform at Launch Pad 39A. At the right of the pad is the 290-foot tall, 300,000-gallon water tank that is part of the sound suppression system during launches. In the foreground is the crawlerway. a 130-foot-wide roadway with a 5-percent grade leading to the top of the launch pad. Rollback of the pad's rotating service structure, or RSS, is one of the milestones in preparation for the launch of mission STS-117 on June 8. Rollback started at 10:56 p.m. EDT June 7 and was complete at 11:34 p.m. The RSS, the massive structure to the left of the shuttle, provides protected access to the orbiter for changeout and servicing of payloads at the pad. The structure is supported by a rotating bridge that pivots about a vertical axis on the side of the pad's flame trench. The hinge column rests on the pad surface and is braced to the fixed service structure. Support for the outer end of the bridge is provided by two eight-wheel, motor-driven trucks that move along circular twin rails installed flush with the pad surface. The track crosses the flame trench on a permanent bridge. The RSS is 102 feet long, 50 feet wide and 130 feet high. The structure has orbiter access platforms at five levels to provide access to the payload bay while the orbiter is being serviced in the RSS. Each platform has independent extendable planks that can be arranged to conform to a payload's configuration. This mission is the 118th shuttle flight and the 21st U.S. flight to the International Space Station and will deliver and install the S3/S4 truss segment, deploy a set of solar arrays and prepare them for operation. Photo credit: NASA/Ken Thornsley