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KENNEDY SPACE CENTER, FLA. - In the pre-dawn hours, STS-121 Pilot Mark Kelly (left) and Commander Steven Lindsey look at the Shuttle Training Aircraft (STA) they will be flying to practice landings in preparation for the July 1 launch of Space Shuttle Discovery. 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. Photo credit: NASA/Kim Shiflett