Space Shuttle OV-103, Discovery, pulls away from the International Space Station on Monday, March 7, 2011. This is the final voyage of Discovery, after 39 missions in 27 years. NASA is retiring the Shuttle fleet this year, due to concerns regarding the stresses on the vehicles from repeated launching and landings. While designed, built, and maintained for multiple missions, the Shuttles are, like any heavily used machine, subject to wear and tear, and at some point become a safety risk above and beyond the dangers inherent in space flight. Endeavor, OV-105, is being prepped for it’s final mission in the VAB at Kennedy Space Center.