单选题
Unidentified Flying Object
Unidentified Flying Object (UFO) is any object or light, reportedly sighted in the sky, that cannot be immediately explained by the observer. Sightings of unusual flying objects date back to ancient times, but UFOs (sometimes called flying saucers) became widely discussed only after the first widely publicized US sighting in 1947. Many thousands of such observations have since been reported worldwide.
At least 90 percent of UFO sightings can be identified as conventional objects, although time-consuming investigations are often necessary for such identification. The objects most often mistaken for UFOs are bright planets and stars, aircraft, birds, balloons, kites, aerial flares, peculiar clouds, meteors, and satellites. The remaining sightings most likely can be attributed to other mistaken sightings or to inaccurate reporting, tricks, or delusions, although to disprove all claims made about UFOs is impossible.
From 1947 to 1969 the US Force investigated UFOs as a possible threat to national security. A total of 12,618 reports were received, of which 701 reports, or 5.6 percent, were listed as unexplained. The air force concluded that "no UFO reported, investigated, and evaluated by the Air Force has ever given any indication of threat to our national security." Since 1969 no agency of the US government has had any active program of UFO investigation.
Some persons, however, believe that UFOs are extraterrestrial spacecraft, even though no scientifically valid evidence supports that belief. The possibility of extraterrestrial civilizations is not the stumbling block; most scientists grant that intelligent life may well exist elsewhere in the universe. A fully convincing UFO photograph has yet to be taken, however, and the scientific method requires that highly speculative explanations should not be adopted unless all of the more ordinary explanations can be ruled out.
UFO enthusiasts persist, however, and some persons even claim to have been captured and taken aboard UFOs. No one has produced scientifically acceptable proof of these claims. Behavioral scientist Carl Sagan once proposed that "certain psychological needs are met by belief in superior beings from other worlds."