Space shuttle Discovery sits atop NASA's 747 Shuttle Carrier Aircraft (SCA) ready to transport it from Kennedy Space Center to the Washington D.C., on April 17, 2012 in Cape Canaveral, Florida. Following the retirement of the shuttle fleet, Discovery will fly to Washington for display at the Smithsonian's National Air and Space Museum. (Photo by Lofty Ambitions)

The space shuttle Discovery attached to its 747 transport takes off from Kennedy Space Center on April 17, 2012. (U.S. Navy/Chief Petty Officer James B. Clark)

Spectators atop the U.S. Capitol Building watch as Discovery and a a NASA T-38 aircraft fly past the Washington Monument, on April 17, 2012. (Reuters/Kevin Lamarque)

Discovery and the shuttle carrier aircraft land at Dulles International Airport in Chantilly, Virginia, on Tuesday, April 17, 2012. (AP Photo/Manuel Balce Ceneta)

Its final flight complete, the space shuttle Discovery, sits on the Dulles International Airport tarmac under blue skies, on April 17, 2012. Discovery will be lifted from the carrier aircraft shortly, and will towed to the National Air and Space Museum's Udvar-Hazy Center for display. (Reuters/Gary Cameron)