one of the ‘Original Seven’ chosen by NASA in 1959 for the Mercury series of flights. Grounded for health reasons, he became director of NASA's Flight Crew Operations Branch for ten years. In 1972 he was returned to flight status, and in 1975 he finally flew in space, then the oldest person to have done so at the age of 51.

Although chosen for the second Mercury orbital mission, he was grounded because of a minor heart irregularity. As NASA's flight-crew director he was responsible for astronaut training and the selection of crews for various flights, including the Apollo Moon missions. He finally flew on the Apollo–Soyuz Test Project, in which a US Apollo spacecraft docked in orbit with a Soviet Soyuz spacecraft.

He subsequently left NASA to join a private company, Space Services Inc, which unsuccessfully attempted to develop rockets for a commercial launch service.