The orbiter Atlantis, on top of the orbiter transporter, begins its turn after rolling out of the Orbiter Processing Facility, heading for the Vehicle Assembly Building (VAB). In the VAB it will be mated with the twin Solid Rocket Boosters and External Tank already stacked on the Mobile Launcher Platform. Atlantis is being readied for mission STS-121, the second Return to Flight mission to the International Space Station. This is Atlantis 27th space flight and the mission is the 18th flight to the Space Station. It is a Utilization and Logistics Flight. Launch is scheduled during a planning window between Sept. 9 and Sept. 24.