Unit 91 In popular culture, the term UFO–or unidentified flying object–refers to a suspected alien spacecraft, although its definition encompasses any unexplained aerial phenomena. UFO sightings have been reported throughout recorded history and in various parts of the world, raising questions about life on other planets and whether extraterrestrials have visited Earth. They became a major subject of interest–and the inspiration behind numerous films and books–following the development of rocketry after World War II.通俗地说，所谓 UFO--或者不明飞行物--指疑似外星人的飞船，不过它的定义也包括无法解释的空中现象。
在全世界各地不同时期都有 UFO 目击的报告，引起人们关于外星生命以及地外生命是否访问过地球的争论。
在二战后火箭技术的发展下，它们成了人们感兴趣的一大主题--也是数不清的电影和书籍背后的灵感来源。
Flying Saucers2 The first well-known UFO sighting occurred in 1947, when businessman Kenneth Arnold claimed to see a group of nine high-speed objects near Mount Rainier in Washington while flying his small plane. Arnold estimated the speed of the crescent-shaped objects as several thousand miles per hour and said they moved “like saucers skipping on water.” In the newspaper report that followed, it was mistakenly stated that the objects were saucer-shaped, hence the term flying saucer.飞碟首个著名的飞碟目击事件发生在 1947 年，一个叫 Kenneth Arnold 的商人声称他在华盛顿的雷尼尔山附近玩小型飞机模型时看到了一组9个高速移动的物体。
Arnold 估计这些月牙形的物体速度达每小时数千英里，还说他们的移动“就像是打水漂”。
在后来的新闻中错误地报道成了这些物体是碟形的，于是就有了飞碟这一称呼。
3 Sightings of unidentified aerial phenomena increased, and in 1948 the U.S. Air Force began an investigation of these reports called Project Sign. The initial opinion of those involved with the project was that the UFOs were most likely sophisticated Soviet aircraft, although some researchers suggested that they might be spacecraft from other worlds, the so-called extraterrestrial hypothesis (ETH). Within a year, Project Sign was succeeded by Project Grudge, which in 1952 was itself replaced by the longest-lived of the official inquiries into UFOs, Project Blue Book, headquartered at Wright-Patterson Air Force Base in Dayton, Ohio. From 1952 to 1969 Project Blue Book compiled reports of more than 12,000 sightings or events, each of which was ultimately classified as (1) “identified” with a known astronomical, atmospheric, or artificial (human-caused) phenomenon or (2) “unidentified.” The latter category,approximately 6 percent of the total, included cases for which there was insufficient information to make an identification with a known phenomenon.之后不明的空中现象频发，在1948 年美国空军开始对这些报告进行调查，称为Sign计划。
这一计划的最初动机是怀疑这些 UFO 很可能是苏联尖端飞行器，虽然有些研究员认为它们可能是来自其他世界的飞船，也就是所谓的外星人假说（ETH）。
在一年内，Grudge 计划继承了Sign计划，到了1952 年它又被 UFO 官方研究计划中历时最长的 Blue Book计划取代，其总部位于俄亥俄州代顿市的Wright-Patterson 空军基地。
在 1952 年至1969 年间，Blue Book 编辑了一万两千多宗目击事件的报告，每一个最后都标上“已确认”或者“未确认”，“已确认”的包括已知的天文现象、大气现象和人造（人为引起的）现象。
后者约占总数的 6%，包括了一些信息不足，无法用已知的现象定性的事件。
The Robertson Panel and the Condon Report4 An American obsession with the UFO phenomenon was under way. In the hot summer of 1952 a provocative series of radar and visual sightings occurred near National Airport in Washington, D.C. Although these events were attributed to temperature inversions in the air over the city, not everyone was convinced by this explanation. Meanwhile, the number of UFO reports had climbed to a record high. This led the Central Intelligence Agency to prompt the U.S. government to establish an expert panel of scientists to investigate the phenomena. The panel was headed by H.P. Robertson, a physicist at the California Institute of Technology in Pasadena, Calif., and included other physicists, an astronomer, and a rocket engineer. The Robertson Panel met for three days in 1953 and interviewed military officers and the head of Project Blue Book. They also reviewed films and photographs of UFOs. Their conclusions were that (1) 90 percent of the sightings could be easily attributed to astronomical and meteorologic phenomena (e.g., bright planets and stars, meteors, auroras, ion clouds) or to such earthly objects as aircraft, balloons, birds, and searchlights, (2) there was no obvious security threat, and (3) there was no evidence to support the ETH. Parts of the panel's report were kept classified until 1979, and this long period of secrecy helped fuel suspicions of a governmentcover-up.小组和 Condon 报告而美国人已经开始对UFO现象着迷。
在 1952 年炎热的夏天，在华盛顿特区的国家机场附近发生了一系列令人激动的雷达和肉眼观测报告。
虽然这些报告被归结于城市上空空气的温度反转现象，并不是所有人都能接受这个解释。
同时，UFO 报告的数量攀升到历史最高。
这导致了中情局提请美国政府组建一支由科学家组成的专家小组对这一现象进行调查。
这个小组由加州帕萨迪纳市的加州理工学院物理学家 H.P. Robertson 领头，有数名物理学家、一名宇航员和一名火箭工程师。