Unidentified Flying Object
       
       
       A UFO is the reported sighting of an object or light seen in the sky or on land, whose appearance, trajectory(轨迹), actions, motions, lights, and colors do not have a logical, conventional, or natural explanation, and which cannot be explained, not only by the original witness, but by scientists or technical experts who try to make a common sense identification after examining the evidence.UFOs (sometimes called flying saucers) became widely discussed only after the first widely publicized U.S. 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, hoaxes(恶作剧), or delusions, although to disprove all claims made about UFOs is impossible.
       From 1947 to 1969 the U.S. Air Force investigated UFOs as a possible threat to national security. A total of 12,618 reports was 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 U.S. government has had any active program of UFO investigation.
       In 1997 the U.S. Central Intelligence Agency (CIA) admitted that the U.S. military had deceived the American public in an effort to hide information about high-altitude spy planes. These planes, the Lockheed U-2A(美国头号军火商洛克西德-马丁公司(Lockheed Martin)生产的侦察机) and the Lockheed SR-71, accounted for over half of the UFO reports during the late 1950s and 1960s.
       Some persons nevertheless 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 of a craftlike object has yet to be taken, however, and the scientific method(科学方法论) 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 abducted and taken aboard UFOs. No one has produced scientifically acceptable proof of these claims.