For as long as astronauts have been flying off into space, there have strange sightings of some kind- sometimes a bright light nearby, sometimes a far away light making acute right turns. However, many of these sightings have gone unheard of by the public, mostly because NASA censors such reports. Whether such censorship is meant to prevent public panic (which is highly unlikely considering decades of Hollywood conditioning) one can only speculate since NASA itself does not give any reason for it.
One of the most prominent UFO sighting made by a NASA astronaut was made by Major Gordon Cooper. Cooper was the last American astronaut to fly into space alone when he shot into space on May 5, 1963. During his final orbit around the earth, Cooper reported to a tracking station in Australia that he had seen a glowing, greenish object approaching his capsule. This sighting was confirmed by the tracking station in Australia which picked up the object on radar. In addition, the sighting of the UFO was reported by the National Broadcast Company which had been on a live coverage of the flight. However, when Cooper landed, reporters were instructed by NASA that they would be able to question him on anything other than the UFO sighting.
Major Cooper believed in the presence of UFOs. Twelve years earlier, he had observed saucer-shaped discs while flying over West Germany. While he filed a report over it, most astronauts were reluctant to discuss his findings, and so was NASA. A chain of other NASA astronauts have also reported UFO sightings. These include Donald Slayton (1951), Joseph Walker (1962), Frank Borman (1965) and even Neil Armstrong and Edwin Aldrin in their historic flight in 1969. But even with the level-headed accounts of these astronauts, we may never be able to ascertain whether aliens are real as long as NASA keeps censoring these accounts.