UFOs are aerial objects, sighted by visual or electronic means, which fail to identify themselves, and which remain unexplained after review of the evidence by competent personnel with scientific training. (In popular usage, UFO includes any aerial object or optical phenomenon which the observer cannot explain). Inexplicable aerial objects with erratic movements at high velocities are believed to have caused fatal air accidents; and have resulted in ground and air sighter panic, and a considerable expenditure of money and time in governmental investigations. They disrupt air traffic control procedures and may be interpreted as hostile attacks on radar screens, thus leading to the danger of accidental military over-reaction. They arouse concern in the public mind, especially on the basis of reports of abductions and other phenomena dangerous to human life. The phenomena and reports of them have a dubious effect on human culture at the mythic and spiritual levels.
Despite world wide reports of UFOs, the central problem remains whether in fact they exist and what in fact they are. Consideration of the problem is complicated by the large number of reports which prove to result from misidentification of aircraft, celestial objects, balloons, birds, and meteorological phenomena, as well as deliberate falsification of evidence by hoaxers. There is therefore a marked reluctance on the part of witnesses to report such objects, for fear of the official ridicule with which honest reports of genuinely puzzling phenomena have been received in the past. It is further complicated by psychological studies attributing such sightings to individual or collective hallucinations and people who, consciously or unconsciously, seek recognition.
UFOs have been seen throughout history and have been described according to the culture within which they were observed, whether as gods, magicians, or as interplanetary travellers. UFO reports have been made in almost every country of the world. More than 20,000 sightings have been catalogued, of which more than 1,500 concern objects described in detail from a short distance. Such reports bear a certain similarity to one another. In the USA, it has been estimated that about half the population believes in the existence of flying saucers and about one person in 20 claims to have seen one. SEPRA, the space study unit in France, has documented 2,300 cases in the period 1979-1993, some of which were hoaxes, some capable of rational explanation, but more than 40% could not be easily explained. A Belgian unit has logged 1,500 sightings based on reports to the gendarmerie.