Investigators can explain all but a small percentage of UFO reports. The remainder may be due to an unknown phenomenon or merely to limitations in human perception, memory, and research. Most scientists believe that there is not enough reliable evidence to connect these sightings with life from other planets. Some UFO's are called flying saucers. This term was coined by the press in 1947 to describe a sighting by Kenneth Arnold, a civilian pilot, who reported unknown objects speeding through the air.
Beginning in 1952, the United States Air Force, in a program called Project Blue Book, investigated about 12,000 UFO reports to determine whether UFO's were a potential threat to national security. In addition, from 1966 to 1968, the Air Force sponsored an independent study of UFO's by scientists at the University of Colorado. The Colorado scientists advised the Air Force that further study of UFO's was not likely to produce useful information concerning a security threat. As a result, the Air Force ended Project Blue Book in 1969.
Nevertheless, many people throughout the world continue to believe that UFO's are spacecraft from other planets. Accounts of encounters with alien visitors have appeared in many books, newspaper articles, motion pictures, and TV programs. Some people have even reported that they have been abducted by aliens.