O'Neill saw great potential in the United States space program, especially the Apollo missions. He applied to the Astronaut Corps after NASA opened it up to civilian scientists in 1966. Later, when asked why he wanted to go on the Moon missions, he said, "to be alive now and not take part in it seemed terribly myopic". He was put through NASA's rigorous mental and physical examinations. During this time he met Brian O'Leary, also a scientist-astronaut candidate, who became his good friend. O'Leary was selected for Astronaut Group 6 but O'Neill was not.