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]
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Denise Chow was the assistant managing editor at Live Science before moving to NBC News as a science reporter, where she focuses on general science and climate change. Before joining the Live Science team in 2013, she spent two years as a staff writer for Space.com, writing about rocket launches and covering NASA's final three space shuttle missions. A Canadian transplant, Denise has a bachelor's degree from the University of Toronto, and a master's degree in journalism from New York University.