After a three-day series of Terminal Countdown Demonstration Test (TCDT) activities, the crew of mission STS-121 are leaving NASA's Kennedy Space Center to return to Houston. Walking toward the plane at the Shuttle Landing Facility are Mission Specialists Lisa Nowak and Piers Sellers. The TCDT includes equipment familiarization, emergency egress training and a simulated launch countdown. Mission STS-121 is scheduled to launch July 1. Photo credit: NASA/Kim Shiflett