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CAPE CANAVERAL, Fla. -- STS-124 Commander Mark Kelly sits in the cockpit of NASA's Shuttle Training Aircraft, or STA, preparing to practice space shuttle landings. The STA is a Grumman American Aviation-built Gulf Stream II jet that was modified to simulate an orbiter's cockpit, motion and visual cues, and handling qualities. In flight, the STA duplicates the orbiter's atmospheric descent trajectory from approximately 35,000 feet altitude to landing on a runway. Because the orbiter is unpowered during re-entry and landing, its high-speed glide must be perfectly executed the first time. The crew for space shuttle Discovery's STS-124 mission is at Kennedy for a full launch dress rehearsal, known as the terminal countdown demonstration test, or TCDT. Providing astronauts and ground crews with an opportunity to participate in various simulated countdown activities, TCDT includes equipment familiarization and emergency training. Discovery's launch is targeted for May 31. Photo credit: NASA/Kim Shiflett