NASA Astronauts and STS-133 mission crew members, L-R: Mission Specialists Nicole Stott, Michael Barratt, Pilot Eric Boe, Commander Steve Lindsey, Mission Specialists Alvin Drew, and Steve Bowen pose for a photograph in front of the space shuttle Discovery after they landed at Kennedy Space Center in Cape Canaveral, Florida, completing Discovery's 39th and final flight on March 9, 2011. [Photo/Agencies]

The space shuttle Discovery is seen from the International Space Station with the southwestern coast of Morocco in the background after the undocking of the two spacecraft in this photo provided by NASA and taken March 7, 2011. [Photo/Agencies]