KENNEDY SPACE CENTER, FLA. - 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. Waving on her way to the plane at the Shuttle Landing Facility is Mission Specialist Stephanie Wilson. 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

KENNEDY SPACE CENTER, FLA. - 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. Boarding the plane at the Shuttle Landing Facility are Mission Specialists Piers Sellers and Stephanie Wilson. 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

KENNEDY SPACE CENTER, FLA. - 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. Waving on his way to the plane at the Shuttle Landing Facility is Thomas Reiter of Germany, who represents the European Space Agency. 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

KENNEDY SPACE CENTER, FLA. - 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

KENNEDY SPACE CENTER, FLA. - The plane carrying the crew of mission STS-121 taxis onto the Shuttle Landing Facility at NASA's Kennedy Space Center. The crew was at the Center for a three-day series of Terminal Countdown Demonstration Test (TCDT) activities. 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

KENNEDY SPACE CENTER, FLA. - 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 to the plane at the Shuttle Landing Facility are Commander Steven Lindsey (left) and Mission Specialist Michael Fossum. 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