什麼是不明飛行物；幽浮

觀測者尚不能識別的各種空中物體或光學現象。第二次世界大戰後，隨着航空和太空事業的發展，不明飛行物受人關注。1950年代美國政府建立科學小組探測觀察。報告透露，不明飛行物中有90%是天文和氣象現象或者是飛機、鳥類及熱氣體。有時因異常氣象狀況。有許多尚無法解釋。1960年代中期，有少數科學家認爲少數不明飛行物顯示有天外來客的出現。這種聳人聽聞的假說在報刊上發表後，立即受到其他科學家的反對。美國空軍於1968年開始進行研究，堅決摒棄天外來客說。但仍然衆說紛紜，大部分美國公衆和少數科學家仍然支持天外來客說。不明飛行物報告的可靠性有很大的差別。裸眼會產生錯覺，雷達探測雖然在某些方面較爲可靠，但不能將有形物體從流星餘跡、雨和熱躍變的徑跡中分辨出來；況且電子干擾及如積雲中潮溼氣團的反射都可產生假回波。亦請參閱SETI。

unidentified flying object (UFO)

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 government panel investigating sightings in the 1950s reported that 90% coincided with astronomical or meteorological phenomena or sightings of aircraft, birds, or hot gases, sometimes under unusual weather conditions. Some remained unexplained, 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 U.S. public, and a few scientists, still supported 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. See also SETI.