Joseph Albert Walker (February 20, 1921 – June 8, 1966) (Capt, USAF) was an American World War II pilot, experimental physicist, NASA test pilot, and astronaut who was the first person to fly an airplane to space. He was one of twelve pilots who flew the North American X-15, an experimental spaceplane jointly operated by the Air Force and NASA.
In 1961, Walker became the first human in the mesosphere when piloting Flight 35, and in 1963, Walker made three flights above 50 miles, thereby qualifying as an astronaut according to the United States definition of the boundary of space. The latter two, X-15 Flights 90 and 91, also surpassed the Kármán line, the internationally accepted boundary of 100 kilometers (62.14 miles). Making the latter flights immediately after the completion of the Mercury and Vostok programs, Walker became the first person to fly to space twice. He was the only X-15 pilot to fly above 100 km during the program.
Walker died in a group formation accident on June 8, 1966.