Astronaut Gordon Cooper, who died in 2004, spent much of his life convinced that extraterrestrial intelligences are visiting Earth and that the American government is covering up this fact. Here’s what Wikipedia says about Cooper’s UFO beliefs and experiences:
Cooper claimed to have seen his first UFO while flying over West Germany in 1951, although he denied reports he had seen a UFO during his Mercury flight. However, these claims are controversial because of a conflict Cooper had with the brass at NASA. In 1957, when Cooper was 30 and a captain, he was assigned to Fighter Section of the Experimental Flight Test Engineering Division at Edwards Air Force Base in California. He acted as a test pilot and project manager. On May 3 of that year, he had a crew setting up an Askania-camera precision landing system on a dry lake bed. This camera system would take pictures at one frame per second as an aircraft landed. The crew consisted of James Bittick and Jack Gettys who began work at the site just before 0800, using both still and motion picture cameras. According to his accounts, later that morning they returned to report to Cooper that they saw a “strange-looking saucer” like aircraft that did not make a sound either on landing or take off. According to his accounts, Cooper realized that these men, who on a regular basis have seen experimental aircraft flying and landing around them as part of their job of filming those aircraft, were clearly worked up and unnerved. They explained how the saucer hovered over them, landed 50 yards away from them using three extended landing gears and then took off as they approached for a closer look. Being photographers with cameras in hand, they of course shot images with 35mm and 4-by-5 still cameras as well as motion film. There was a special Pentagon number to call to report incidents like this. He called and it immediately went up the chain of command until he was instructed by a general to have the film developed (but to make no prints of it) and send it right away in a locked courier pouch. As he hadn’t been instructed to not look at the negatives before sending them, he did. He said the quality of the photography was excellent as would be expected from the experienced photographers who took them. What he saw was exactly what they had described to him. He did not see the movie film before everything was sent away. He expected that there would be a follow up investigation since an aircraft of unknown origin had landed in a highly classified military installation, but nothing was ever said of the incident again. He was never able to track down what happened to those photos. He assumed that they ended up going to the Air Force‘s official UFO investigation, Project Blue Book, which was based at Wright-Patterson Air Force Base. He held claim until his death that the government is indeed covering up information about UFOs. He gives the example of President Harry Truman who said on April 4, 1950, “I can assure you that flying saucers, given that they exist, are not constructed by any power on Earth.” He also pointed out that there were hundreds of reports made by his fellow pilots, many coming from military jet pilots sent to respond to radar or visual sightings from the ground. He was quite convinced till the day he died that he had seen UFOs and was a strong advocate to make the government come clean with what it knew.  In his memoirs Cooper wrote he had seen other unexplained aircraft several times during his career and also said hundreds of similar reports had been made, often by military jet pilots responding to radar or visual sightings from the ground. He further claimed these sightings had been “swept under the rug” by the US government. Throughout his later life Cooper expressed repeatedly in interviews he had seen extra-terrestrial crafts and described his recollections for the documentary Out of the Blue.
Out of the Blue, by the way, is a “must see” film for anyone wanting a broad overview of UFO history and phenomena.
And here’s Cooper doing a sit down television interview: