unidentified flying object or UFO, an object or light reportedly seen in the sky whose appearance, trajectory, and general dynamic and luminescent behavior do not readily suggest a logical, conventional explanation. Some of these phenomena have been photographed and others correlated with radar echoes. Throughout history there have been reports of strange objects in the sky. In the 20th cent. a number of observers claim to have seen vehicles, or "flying saucers," which some believe are space ships visiting the earth from other planets. Because the objects are often shining and in that part of the sky opposite the sun, most investigators, official and unofficial alike, tend to interpret them as reflections of the sun's rays from airplanes. Some UFOs, when pursued by planes, have proved to be weather balloons or other objects of unquestionably terrestrial origin. Fireballs, meteors, and other meteorological phenomena account for most of the relatively few UFOs that observers report seeing at night. However, there are some sightings that investigators are unable to explain in terms of known phenomena.
See D. Ritchie, UFO: The Definitive Guide to Unidentified Flying Objects and Related Phenomena (1994); B. Steiger, ed., Project Blue Book: The Top Secret UFO Findings Revealed (repr. 1995); J. Clark, The UFO Book: Encyclopedia of the Extraterrestrial (1997); B. Saler et al., UFO Crash at Roswell: The Genesis of a Modern Myth ; J. A. Hynek et al., Night Siege: The Hudson Valley UFO Sightings (2d ed. 1998); J. A. Hynek, The UFO Experience: A Scientific Inquiry (1999); N. Pope, Open Skies, Closed Minds: For the First Time a Government UFO Expert Speaks Out (1999).