Prior to Project Mercury, there was no protocol for selecting astronauts, so NASA would set a far-reaching precedent with both their selection process and initial choices for astronauts. At the end of 1958, various ideas for the selection pool were discussed privately within the national government and the civilian space program, and also among the public at large. Initially, there was the idea to issue a widespread public call to volunteers. Thrill-seekers such as rock climbers and acrobats would have been allowed to apply, but this idea was quickly shot down by NASA officials who understood that an undertaking such as space flight required individuals with professional training and education in flight engineering. By late 1958, NASA officials decided to move forward with test pilots being the heart of their selection pool. On President Eisenhower's insistence, the group was further narrowed down to active duty military test pilots, which set the number of candidates at 508. These candidates were USN or USMC naval aviation pilots (NAPs), or USAF pilots of Senior or Command rating. These aviators had long military records, which would give NASA officials more background information on which to base their decisions. Furthermore, these aviators were skilled in flying the most advanced aircraft to date, giving them the best qualifications for the new position of astronaut. During this time, women were banned from flying in the military and so could not successfully qualify as test pilots. This meant that no female candidates could earn consideration for the title of astronaut. Civilian NASA X-15 pilot Neil Armstrong was also disqualified, though he had been selected by the US Air Force in 1958 for its Man in Space Soonest program, which was replaced by Mercury. Although Armstrong had been a combat-experienced NAP during the Korean War, he left active duty in 1952. Armstrong became NASA's first civilian astronaut in 1962 when he was selected for NASA's second group, and became the first man on the Moon in 1969.