NASA's Space Shuttle "Endeavour" prepared for launch at the Kennedy Space Center.

As the Rotating Service Structure rolls back, NASA's space shuttle "Endeavour" stands on Launch Complex 39A at the Kennedy Space Center on April 28, 2011. As the countdown ticks toward her twenty fifth and final mission (STS 134), Endeavour will launch at apprximately 3:47 pm on April 29 and fly to the International Space Station with a crew of six. The primary payload is the two billion dollar Alpha Magnetic Spectrometer which is supported by 60 institutes in 16 countries. The payload will be attached to the exterior of the station to conduct multiple projects lasting throughout the life of the orbiting station. (UPI Photo / Joe Marino, Bill Cantrell