unidentified flying object | History, Sightings, & Facts | Britannica
unidentified flying object (UFO), also called flying saucer, any aerial object or optical phenomenon not readily identifiable to the observer. UFOs became a major subject of interest following the development of rocketry after World War II and were thought by some researchers to be intelligent extraterrestrial life visiting Earth. The first well-known UFO sighting occurred in 1947, when businessman Kenneth Arnold claimed to see a group of nine high-speed objects near Mount Rainier in Washington while flying his small plane. Arnold estimated the speed of the crescent-shaped objects as several thousand miles per hour and said they moved “like saucers