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.

Aerial object or optical phenomenon not readily explainable to the observer. Interest in UFOs increased with developments in aeronautics and astronautics after World War II. A U.S. government panel investigating sightings in the 1950s reported that 90percnt coincided with astronomical or meteorological phenomena or sightings of aircraft, birds, or hot gases, sometimes under unusual weather conditions. Some remained unexplained, however, and in the mid 1960s a few scientists concluded that a small percentage indicated the presence of extraterrestrial visitors. This sensational hypothesis, promoted in the press, met with prompt resistance from other scientists. A U.S. Air Force UFO study begun in 1968 firmly rejected the extraterrestrial hypothesis, but a large fraction of the public, and a few scientists, continued to support it. UFO reports vary widely in reliability. The unaided eye is easily fooled; radar sightings of UFOs, more reliable in some ways, may fail to distinguish physical objects from meteor trails, rain, or thermal discontinuities and are subject to radio interference. Seealso SETI.

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