Joseph Albert "Joe" Walker (February 20, 1921 – June 8, 1966) flew the world's first two spaceplane flights in 1963, thereby becoming the United States' seventh astronaut. Walker was a Captain in the United States Air Force, an American World War II pilot, an experimental physicist, a NASA test pilot, and a member of the U.S. Air Force Man In Space Soonest spaceflight program. His two X-15 experimental rocket aircraft flights in 1963 that exceeded the Kármán line – the altitude of kilometres ( miles), generally considered to mark the threshold of outer space – qualified him as an astronaut under the rules of the U.S. Air Force and the Fédération Aéronautique Internationale (FAI).