Fifteen years ago today, the Space Shuttle Discovery lifted off from Kennedy Space Center on its final mission. The launch came at 4:53pm on February 24, 2011, marking the 39th and last flight for the most-flown shuttle in NASA history. Discovery delivered supplies and a humanoid robot to the International Space Station. Today, the shuttle is preserved at the Smithsonian, ending a 27-year run that helped shape modern spaceflight.