By: Robert Z. Pearlman Published: 04/17/2012 07:11 AM EDT on SPACE.com WASHINGTON — Space shuttle Discovery, NASA's fleet leader and the world's most flown spacecraft, lifted off for the last time on Tuesday (April 17) to be delivered to the Smithsonian for its permanent display. Discovery took off atop NASA's Shuttle Carrier Aircraft, a modified Boeing 747, leaving the Kennedy Space Center in Florida at 6:58 a.m.