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 X-15 test pilots, left to right; Air Force Captain Joseph H. Engle, Air Force Major Robert A. Rushworth, NASA pilot John B. "Jack" McKay, Air Force pilot William J. "Pete" Knight, NASA pilot Milton O. Thompson, and NASA pilot Bill Dana.

 Of their 125 X-15 flights, 8 were above the 50 miles that constituted the Air Force's definition of the beginning of space "Engle 3, Dana 2, Rushworth, Knight, and McKay one each." NASA used the international definition of space as beginning at 62 miles above the earth.