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It could reach a top speed of 4,520 miles per hour (7,274 km/h), or Mach 6.72. During the X-15 program, 13 flights by eight pilots met the Air Force spaceflight criterion by exceeding the altitude of 50 miles (80 km), thus qualifying these pilots as being astronauts.
Guinness World Records has recognized the world speed record set by NASA's hypersonic X-43A aircraft earlier this year in an experimental flight over the Pacific Ocean. Using a scramjet engine, the unpiloted, 12 foot-long aircraft achieved Mach 6.83 -- almost seven times the speed of sound -- or nearly 5,000 mph, in a March 27 flight.