Donald K. 'Deke' Slayton (March 1, 1924 – June 13, 1993) was the only one of the Mercury 7 — the astronauts chosen for America's first manned-space effort, Project Mercury — who did not fly, due to an erratic heart rate (idiopathic atrial fibrillation). Slayton had been scheduled to fly in 1962 on the second orbital flight that was flown instead by Scott Carpenter.
A US Air Force pilot, Slayton resigned his commission in 1963 and worked for NASA in a civilian capacity. A long medical program led to him being restored to full flight status in 1973, when he was chosen as pilot for the Apollo-Soyuz Test Project, designed to allow a docking between the American Apollo space program and the Soyuz space program of the Soviet Union. On July 17, 1975, the two craft joined up in orbit, and astronauts Slayton, Thomas Stafford and Vance D. Brand conducted crew transfers with cosmonauts Aleksey A. Leonov and Valeriy Kubasov.