American astronaut who left his spacecraft for more than two hours of extravehicular activity during the Gemini 9 mission in 1966. Cernan joined the U.S. Navy in 1956, became a test pilot, and earned a master's degree in aeronautical engineering. In 1963 he was selected by NASA to serve as an astronaut. Cernan also participated in the Apollo 10 and Apollo 17 moon missions. He resigned from the navy and the space program in 1976 to enter private business.