The rocket-powered X-15 was part of a fleet of X-plane experimental aircraft operated jointly by NASA and the U.S. Air Force. In the early 1960s, the X-15 set a number of speed and altitude records, reaching the edge of space (an altitude of more than 62 miles or 100 kilometers) on two separate occasions in 1963.
Currently, the X-15 still holds the official world record for the fastest speed ever reached by a manned aircraft: Mach 6.72, which is 6.72 times the speed of sound, or 4,520 mph (7,274 km/h).
The X-15 was retired in 1970, but the program featured many notable NASA and Air Force test pilots, including Neil Armstrong, the man who would go on to become the first person to step foot on the moon.
Interestingly, during the illustrious X-15 program, 13 flights by eight different pilots exceeded an altitude of 50 miles (80 km), meeting the U.S. Air Force's standard for spaceflight. As a result, the Air Force test pilots were awarded Air Force astronaut wings, and the civilian pilots were granted NASA astronaut wings. [Related Image Gallery: Breaking the Sound Barrier]