The crew members of space shuttle mission STS-120 prepare for their return to NASA's Johnson Space Center.

The crew members of space shuttle mission STS-120 prepare for their return to NASA's Johnson Space Center in Houston following three days of terminal countdown demonstration test, or TCDT, activities. At Kennedy Space Center's Shuttle Landing Facility, Mission Specialist Doug Wheelock prepares to board his T-38 jet aircraft. The TCDT provides astronauts and ground crews with equipment familiarization, emergency egress training and a simulated launch countdown. The STS-120 mission will deliver the U.S. Node 2 module, named Harmony, aboard space shuttle Discovery to the International Space Station on a 14-day mission. Discovery's launch is targeted for Oct. 23 at 11:38 a.m. EDT. Photo credit: NASA/Kim Shiflett