An unidentified flying object, or UFO, in its most general definition, is any apparent anomaly in the sky that is not identifiable as a known object or phenomenon. Culturally, UFOs are associated with claims of visitation by extraterrestrial life or government-related conspiracy theories, and have become popular subjects in fiction. While UFOs are often later identified, sometimes identification may not be possible owing to the usually low quality of evidence related to UFO sightings (generally anecdotal evidence and eyewitness accounts). Stories of fantastical celestial apparitions have been told since antiquity, but the term "UFO" (or "UFOB") was officially created in 1953 by the United States Air Force (USAF) to serve as a catch-all for all such reports. In its initial definition, the USAF stated that a "UFOB" was "any airborne object which by performance, aerodynamic characteristics, or unusual features, does not conform to any presently known aircraft or missile type, or which cannot be positively identified as a familiar object." Accordingly, the term was initially restricted to those fraction of cases which remained unidentified after investigation, as the USAF was interested in potential national security reasons and/or "technical aspects" (see Air Force Regulation 200-2). During the late 1940s and through the 1950s, UFOs were often referred to popularly as "flying saucers" or "flying discs". The term UFO became more widespread during the 1950s, at first in technical literature, but later in popular use. UFOs garnered considerable interest during the Cold War, an era associated with a heightened concern for national security. Various studies have concluded that the phenomenon does not represent a threat to national security nor does it contain anything worthy of scientific pursuit (e.g., 1951 Flying Saucer Working Party, 1953 CIA Robertson Panel, USAF Project Blue Book, Condon Committee).

Studies have established that the majority of UFO observations are misidentified conventional objects or natural phenomena—most commonly aircraft, balloons, noctilucent clouds, nacreous clouds, or astronomical objects such as meteors or bright planets with a small percentage even being hoaxes.[note 1] Between 5% and 20% of reported sightings are not explained, and therefore can be classified as unidentified in the strictest sense. While proponents of the extraterrestrial hypothesis (ETH) suggest that these unexplained reports are of alien spacecraft, the null hypothesis cannot be excluded that these reports are simply other more prosaic phenomena that cannot be identified due to lack of complete information or due to the necessary subjectivity of the reports. While UFOs have been the subject of extensive investigation by various governments and although a few scientists have supported the extraterrestrial hypothesis, almost no scientific papers about UFOs have been published in peer-reviewed journals.[7] There was, in the past, some debate in the scientific community about whether any scientific investigation into UFO sightings is warranted with the general conclusion being that the phenomena was not worthy of serious investigation beyond a cultural artifact.[8][9][10][11][12][13][14] The void left by the lack of institutional scientific study has given rise to independent researchers and groups, including the National Investigations Committee on Aerial Phenomena (NICAP) in the mid-20th century and, more recently, the Mutual UFO Network (MUFON)[15] and the Center for UFO Studies (CUFOS).[16] The term "Ufology" is used to describe the collective efforts of those who study reports and associated evidence of unidentified flying objects.[17] UFOs have become a prevalent theme in modern culture,[18] and the social phenomena have been the subject of academic research in sociology and psychology.Unexplained aerial observations have been reported throughout history. Some were undoubtedly astronomical in nature: comets, bright meteors, one or more of the five planets that can be seen with the naked eye, planetary conjunctions, or atmospheric optical phenomena such as parhelia and lenticular clouds. An example is Halley's Comet, which was recorded first by Chinese astronomers in 240 BC and possibly as early as 467 BC. Such sightings throughout history often were treated as supernatural portents, angels, or other religious omens. Some current-day UFO researchers have noticed similarities between some religious symbols in medieval paintings and UFO reports[19] though the canonical and symbolic character of such images is documented by art historians placing more conventional religious interpretations on such images.[20] On January 25, 1878, the Denison Daily News printed an article in which John Martin, a local farmer, had reported seeing a large, dark, circular object resembling a balloon flying "at wonderful speed." Martin, according to the newspaper account, said it appeared to be about the size of a saucer, the first known use of the word "saucer" in association with a UFO.[21] In April 1897 thousands of people reported seeing "airships" in various parts of the United States. Many signed affidavits. Scores of people even reported talking to the pilots. Thomas Edison was asked his opinion, and said, "You can take it from me that it is a pure fake."[22][23] On February 28, 1904, there was a sighting by three crew members on the USS Supply 300 miles (483 km) west of San Francisco, reported by Lieutenant Frank Schofield, later to become Commander-in-Chief of the Pacific Battle Fleet. Schofield wrote of three bright red egg-shaped and circular objects flying in echelon formation that approached beneath the cloud layer, then changed course and "soared" above the clouds, departing directly away from the earth after two to three minutes. The largest had an apparent size of about six Suns, he said.[24][25] The three earliest known pilot UFO sightings, of 1,305 similar sitings cataloged by NARCAP, took place in 1916 and 1926. On January 31, 1916, a UK pilot near Rochford reported a row of lights, resembling lighted windows on a railway carriage, that rose and disappeared. In January 1926 a pilot reported six "flying manhole covers" between Wichita, Kansas, and Colorado Springs, Colorado. In late September 1926 an airmail pilot over Nevada said he had been forced to land by a huge, wingless, cylindrical object.[26] On August 5, 1926, while traveling in the Humboldt Mountains of Tibet's Kokonor region, Russian explorer Nicholas Roerich reported, members of his expedition saw "something big and shiny reflecting the sun, like a huge oval moving at great speed. Crossing our camp the thing changed in its direction from south to southwest. And we saw how it disappeared in the intense blue sky. We even had time to take our field glasses and saw quite distinctly an oval form with shiny surface, one side of which was brilliant from the sun."[27] Another description by Roerich was of a "shiny body flying from north to south. Field glasses are at hand. It is a huge body. One side glows in the sun. It is oval in shape. Then it somehow turns in another direction and disappears in the southwest."[28] In the Pacific and European theatres during World War II, "foo fighters" (metallic spheres, balls of light and other shapes that followed aircraft) were reported and on occasion photographed by Allied and Axis pilots. Some proposed Allied explanations at the time included St. Elmo's fire, the planet Venus, hallucinations from oxygen deprivation, or German secret weapons.[29][30] In 1946 more than 2,000 reports were collected, primarily by the Swedish military, of unidentified aerial objects over the Scandinavian nations, along with isolated reports from France, Portugal, Italy and Greece. The objects were referred to as "Russian hail" and later as "ghost rockets" because it was thought that the mysterious objects were possibly Russian tests of captured German V1 or V2 rockets. Although most were thought to be such natural phenomena as meteors, more than 200 were tracked on radar by the Swedish military and deemed to be "real physical objects." In a 1948 top secret document, Swedish authorities advised the USAF Europe that some of their investigators believed these craft to be extraterrestrial in origin.UFOs have been subject to investigations over the years that varied widely in scope and scientific rigor. Governments or independent academics in the United States, Canada, the United Kingdom, Japan, Peru, France, Belgium, Sweden, Brazil, Chile, Uruguay, Mexico, Spain, and the Soviet Union are known to have investigated UFO reports at various times. Among the best known government studies are the ghost rockets investigation by the Swedish military (1946–1947), Project Blue Book, previously Project Sign and Project Grudge, conducted by the USAF from 1947 until 1969, the secret U.S. Army/Air Force Project Twinkle investigation into green fireballs (1948–1951), the secret USAF Project Blue Book Special Report No. 14[32] by the Battelle Memorial Institute, and the Brazilian Air Force's 1977 Operação Prato (Operation Saucer). France has had an ongoing investigation (GEPAN/SEPRA/GEIPAN) within its space agency Centre national d'études spatiales (CNES) since 1977; the government of Uruguay has had a similar investigation since 1989.Project Sign in 1948 produced a highly classified finding (see Estimate of the Situation) that the best UFO reports probably had an extraterrestrial explanation. A top secret Swedish military opinion given to the USAF in 1948 stated that some of their analysts believed that the 1946 ghost rockets and later flying saucers had extraterrestrial origins. (For document, see Ghost rockets.) In 1954 German rocket scientist Hermann Oberth revealed that an internal West German government investigation, which he headed, had arrived at an extraterrestrial conclusion, but this study was never made public.Project Sign was dismantled and became Project Grudge at the end of 1948. Angered by the low quality of investigations by Grudge, the Air Force Director of Intelligence reorganized it as Project Blue Book in late 1951, placing Ruppelt in charge. Blue Book closed down in 1970, using the Condon Committee's negative conclusion as a rationale, thus ending official Air Force UFO investigations. However, a 1969 USAF document, known as the Bolender memo, along with later government documents, revealed that non-public U.S. government UFO investigations continued after 1970. The Bolender memo first stated that "reports of unidentified flying objects that could affect national security ... are not part of the Blue Book system," indicating that more serious UFO incidents already were handled outside the public Blue Book investigation. The memo then added, "reports of UFOs which could affect national security would continue to be handled through the standard Air Force procedures designed for this purpose."[note 2] In addition, in the late 1960s a chapter on UFOs in the Space Sciences course at the U.S. Air Force Academy gave serious consideration to possible extraterrestrial origins. When word of the curriculum became public, the Air Force in 1970 issued a statement to the effect that the book was outdated and that cadets instead were being informed of the Condon Report's negative conclusion.Air Force Regulation 200-2,[34] issued in 1953 and 1954, defined an Unidentified Flying Object ("UFOB") as "any airborne object which by performance, aerodynamic characteristics, or unusual features, does not conform to any presently known aircraft or missile type, or which cannot be positively identified as a familiar object." The regulation also said UFOBs were to be investigated as a "possible threat to the security of the United States" and "to determine technical aspects involved." The regulation went on to say that "it is permissible to inform news media representatives on UFOB's when the object is positively identified as a familiar object," but added: "For those objects which are not explainable, only the fact that ATIC [Air Technical Intelligence Center] will analyze the data is worthy of release, due to many unknowns involved.J. Allen Hynek, a trained astronomer who served as a scientific advisor for Project Blue Book, was initially skeptical of UFO reports, but eventually came to the conclusion that many of them could not be satisfactorily explained and was highly critical of what he described as "the cavalier disregard by Project Blue Book of the principles of scientific investigation."[35] Leaving government work, he founded the privately funded CUFOS, to whose work he devoted the rest of his life. Other private groups studying the phenomenon include the MUFON, a grass roots organization whose investigator's handbooks go into great detail on the documentation of alleged UFO sightings. Like Hynek, Jacques Vallée, a scientist and prominent UFO researcher, has pointed to what he believes is the scientific deficiency of most UFO research, including government studies. He complains of the mythology and cultism often associated with the phenomenon, but alleges that several hundred professional scientists—a group both he and Hynek have termed "the invisible college"—continue to study UFOs in private.The study of UFOs has received little support in mainstream scientific literature. Official studies ended in the U.S. in December 1969, following the statement by the government scientist Edward Condon that further study of UFOs could not be justified on grounds of scientific advancement.[10][36] The Condon Report and its conclusions were endorsed by the National Academy of Scientists, of which Condon was a member. On the other hand, a scientific review by the UFO subcommittee of the American Institute of Aeronautics and Astronautics (AIAA) disagreed with Condon's conclusion, noting that at least 30 percent of the cases studied remained unexplained and that scientific benefit might be gained by continued study. Critics argue that all UFO evidence is anecdotal[37] and can be explained as prosaic natural phenomena. Defenders of UFO research counter that knowledge of observational data, other than what is reported in the popular media, is limited in the scientific community and that further study is needed.[18][38] No official government investigation has ever publicly concluded that UFOs are indisputably real, physical objects, extraterrestrial in origin, or of concern to national defense. These same negative conclusions also have been found in studies that were highly classified for many years, such as the UK's Flying Saucer Working Party, Project Condign, the U.S. CIA-sponsored Robertson Panel, the U.S. military investigation into the green fireballs from 1948 to 1951, and the Battelle Memorial Institute study for the USAF from 1952 to 1955 (Project Blue Book Special Report No. 14). Some public government reports have acknowledged the possibility of physical reality of UFOs, but have stopped short of proposing extraterrestrial origins, though not dismissing the possibility entirely. Examples are the Belgian military investigation into large triangles over their airspace in 1989–1991 and the 2009 Uruguayan Air Force study conclusion (see below). Some private studies have been neutral in their conclusions, but argued that the inexplicable core cases call for continued scientific study. Examples are the Sturrock panel study of 1998 and the 1970 AIAA review of the Condon Report.U.S. investigations into UFOs include: The Interplanetary Phenomenon Unit (IPU), established by the U.S. Army sometime in the 1940s, and about which little is known. In 1987, British UFO researcher Timothy Good received from the Army's director of counter-intelligence a letter confirming the existence of the IPU. The letter stated that "the aforementioned Army unit was disestablished during the late 1950s and never reactivated. All records pertaining to this unit were surrendered to the U.S. Air Force Office of Special Investigations in conjunction with operation BLUEBOOK." The IPU records have never been released.[39] Project Blue Book, previously Project Sign and Project Grudge, conducted by the USAF from 1947 until 1969 The secret U.S. Army/Air Force Project Twinkle investigation into green fireballs (1948–1951) Ghost rockets investigations by the Swedish, UK, U.S., and Greek militaries (1946–1947) The secret CIA Office of Scientific Investigation (OS/I) study (1952–53) The secret CIA Robertson Panel (1953) The secret USAF Project Blue Book Special Report No. 14 by the Battelle Memorial Institute (1951–1954) The Brookings Report (1960), commissioned by NASA The public Condon Committee (1966–1968) The private, internal RAND Corporation study (1968)[40] The private Sturrock panel (1998) Thousands of documents released under FOIA also indicate that many U.S. intelligence agencies collected (and still collect) information on UFOs. These agencies include the Defense Intelligence Agency (DIA), FBI,[5] CIA, National Security Agency (NSA), as well as military intelligence agencies of the Army and U.S. Navy, in addition to the Air Force.[note 3] The investigation of UFOs has also attracted many civilians, who in the U.S formed research groups such as NICAP (active 1956–1980), Aerial Phenomena Research Organization (APRO) (active 1952–1988), MUFON (active 1969–), and CUFOS (active 1973–). In November 2011, the White House released an official response to two petitions asking the U.S. government to acknowledge formally that aliens have visited this planet and to disclose any intentional withholding of government interactions with extraterrestrial beings. According to the response, "The U.S. government has no evidence that any life exists outside our planet, or that an extraterrestrial presence has contacted or engaged any member of the human race."[41][42] Also, according to the response, there is "no credible information to suggest that any evidence is being hidden from the public's eye."[41][42] The response further noted that efforts, like SETI and NASA's Kepler space telescope and Mars Science Laboratory, continue looking for signs of life. The response noted "odds are pretty high" that there may be life on other planets but "the odds of us making contact with any of them—especially any intelligent ones—are extremely small, given the distances involved.Following the large U.S. surge in sightings in June and early July 1947, on July 9, 1947, United States Army Air Forces (USAAF) intelligence, in cooperation with the FBI,[5] began a formal investigation into selected sightings with characteristics that could not be immediately rationalized, which included Kenneth Arnold's and that of the United Airlines crew. The USAAF used "all of its top scientists" to determine whether "such a phenomenon could, in fact, occur." The research was "being conducted with the thought that the flying objects might be a celestial phenomenon," or that "they might be a foreign body mechanically devised and controlled."[43] Three weeks later in a preliminary defense estimate, the air force investigation decided that, "This 'flying saucer' situation is not all imaginary or seeing too much in some natural phenomenon. Something is really flying around."[44] A further review by the intelligence and technical divisions of the Air Materiel Command at Wright Field reached the same conclusion. It reported that "the phenomenon is something real and not visionary or fictitious," that there were objects in the shape of a disc, metallic in appearance, and as big as man-made aircraft. They were characterized by "extreme rates of climb [and] maneuverability," general lack of noise, absence of trail, occasional formation flying, and "evasive" behavior "when sighted or contacted by friendly aircraft and radar," suggesting a controlled craft. It was therefore recommended in late September 1947 that an official Air Force investigation be set up to investigate the phenomenon. It was also recommended that other government agencies should assist in the investigation.This led to the creation of the Air Force's Project Sign at the end of 1947, one of the earliest government studies to come to a secret extraterrestrial conclusion. In August 1948, Sign investigators wrote a top-secret intelligence estimate to that effect, but the Air Force Chief of Staff Hoyt Vandenberg ordered it destroyed. The existence of this suppressed report was revealed by several insiders who had read it, such as astronomer and USAF consultant J. Allen Hynek and Capt. Edward J. Ruppelt, the first head of the USAF's Project Blue Book.[45] Another highly classified U.S. study was conducted by the CIA's Office of Scientific Investigation (OS/I) in the latter half of 1952 in response to orders from the National Security Council (NSC). This study concluded UFOs were real physical objects of potential threat to national security. One OS/I memo to the CIA Director (DCI) in December read: the reports of incidents convince us that there is something going on that must have immediate attention ... Sightings of unexplained objects at great altitudes and traveling at high speeds in the vicinity of major U.S. defense installations are of such a nature that they are not attributable to natural phenomena or any known types of aerial vehicles. The matter was considered so urgent that OS/I drafted a memorandum from the DCI to the NSC proposing that the NSC establish an investigation of UFOs as a priority project throughout the intelligence and the defense research and development community. It also urged the DCI to establish an external research project of top-level scientists, now known as the Robertson Panel to analyze the problem of UFOs. The OS/I investigation was called off after the Robertson Panel's negative conclusions in January 1953.Studies show that after careful investigation, the majority of UFOs can be identified as ordinary objects or phenomena. The most commonly found identified sources of UFO reports are: Astronomical objects (bright stars, planets, meteors, re-entering man-made spacecraft, artificial satellites, and the Moon) Aircraft (aerial advertising and other aircraft, missile launches) Balloons (toy balloons, weather balloons, large research balloons) Other atmospheric objects and phenomena (birds, unusual clouds, kites, flares) Light phenomena mirages, Fata Morgana, ball lightning, moon dogs, searchlights and other ground lights, etc.) Hoaxes A 1952–1955 study by the Battelle Memorial Institute for the USAF included these categories as well as a "psychological" one. An individual 1979 study by CUFOS researcher Allan Hendry found, as did other investigations, that only a small percentage of cases he investigated were hoaxes.
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