Fifty years ago, the U.S. Air Force closed its UFO investigation program, Project Blue Book. Its initial predecessor was Project Sign, created in 1947 (after Roswell). The problem with Sign was that it allowed for the notion that UFOs were of extraterrestrial origin. So the Air Force replaced it with Project Grudge in 1949. It was shut down in 1951 as it labeled all UFOs as hoaxes, although they couldn’t explain 25% of them.