Mission STS-117 Mission Specialists James Reilly left and Danny Olivas sit to participate in a simulated launch countdown.

Mission STS-117 Mission Specialists James Reilly (left) and Danny Olivas sit in the mid-deck of Space Shuttle Atlantis at Launch Pad 39A to participate in a simulated launch countdown that is part of the prelaunch preparations known as terminal countdown demonstration test (TCDT). The TCDT also includes M-113 armored personnel carrier training and payload familiarization. The STS-117 mission is No. 21 to the International Space Station. Mission payloads aboard Atlantis include the S3/S4 integrated truss structure, a third set of solar arrays and batteries. The crew of six astronauts will install the truss to continue assembly of the station. Launch is scheduled for no earlier than March 15. Photo credit: NASA/Kim Shiflett.