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KENNEDY SPACE CENTER, FLA. -- In the white room on Launch Pad 39A, STS-120 Mission Specialist Doug Wheelock is ready to enter space shuttle Discovery for a simulated launch countdown. The countdown is the culmination of the prelaunch terminal countdown demonstration test, or TCDT. The TCDT at NASA's Kennedy Space Center provides astronauts and ground crews an opportunity to participate in various launch preparation activities, including equipment familiarization and emergency training. The STS-120 mission will deliver the U.S. Node 2 module, named Harmony, aboard space shuttle Discovery to the International Space Station. Discovery is targeted to launch on its 14-day mission at 11:38 a.m. EDT on Oct. 23. Photo credit: NASA/George Shelton