The North American X-15 rocket plane was part of the USAF/NASA/USN X-series of experimental aircraft, including also the Bell X-1. The X-15 set numerous speed and altitude records in the early 1960s, reaching the edge of space and bringing back valuable data that was used in the design of later aircraft and spacecraft. It could be considered the first manned suborbital spacecraft ever flown.During the X-15 program, 13 flights (by eight pilots) met the USAF's criteria for a spaceflight by passing an altitude of 50 miles (80 km) and the pilots were accordingly awarded astronaut status by the USAF. Two X-15s pilots also qualified to receive NASA astronaut wings.Some respected aerospace researchers have placed the threshold of space at lower altitudes than the USAF and NASA, so many X-15 pilots could also be considered as astronauts. The "aeropause" region, where space-equivalent conditions are first encountered, starts at an altitude of 19 miles (30 km) above the Earth. Many X-15 pilots traveled through, and far above, the aeropause.Out of all the X-15 missions, two flights (by the same pilot) also qualified for the international FAI definition of a spaceflight by passing the 62.1 mile (100 km) mark.