Photos: Looking back at NASA's X-15 aircraft

The X-15 was launched in midair from a B-52, so that it could conserve the fuel it would need to reach super high speeds and altitudes. NASA says that the hypersonic aircraft--there were three in total, built by North American Aviation--would eventually set unofficial world records for speed (4,520 mph, or Mach 6.7) and altitude (354,200 feet). The X-15s made a total of 199 flights over a decade, from June 8, 1959, to October 24, 1968.