Space Shuttle Discovery crew member Nicole Stott smiles at the camera as she prepares to board the orbiter for the final space flight in this still image taken from video, February 24, 2011. [Photo/Agencies] 

Crew members of the STS-133 space shuttle Discovery gather together on the launchpad at the Kennedy Space Center in Florida before boarding the orbiter for the final space flight in this still image taken from video February 24, 2011.[Photo/Agencies]