User:Tony Sidaway/Unidentified flying object

Unidentified flying object (commonly abbreviated UFO) is a popular term for any phenomenon seen in the sky that is not readily identified. Research by military and civilian groups shows that after investigation UFOs are generally identified either directly or by applying Occam's Razor. Therefore the USAF, who originally coined the term in 1952, define UFOs as objects remaining unidentified after scrutiny by expert investigators. Colloquially the term UFO is used to describe an as-yet unidentified sighting. In popular culture the term UFO is often used as a synonym for alien spacecraft. Because of these different meanings that have become associated with UFO, some investigators now prefer to use the broader term Unidentified Aerial Phenomena (or UAP).

Some studies have established that a small percentage of reported UFOs are hoaxes. The majority of reports indicate something real, perhaps appearing anomalous, but most of these represent honest misidentifications of conventional objects such as aircraft, balloons, or astronomical objects such as meteors or bright planets. This leaves a small percentage of reported sightings, usually between around 5-15%, classified as unidentified flying objects.

Modern reports and the first official investigations of UFOs began during World War II with sightings of so-called foo fighters by Allied airplane crews, and in 1946 with widespread sightings of European "ghost rockets". UFO reports became even more common after the first widely publicized United States UFO sighting, by private pilot Kenneth Arnold in mid 1947 (which gave rise to the popular terms "flying saucer" and "flying disc"). Since then, millions of people believe they have seen UFOs and tens of thousands of such reports have been catalogued.

History


Unexplained aerial observations have been reported throughout history. Some were undoubtedly astronomical in nature: comets, bright meteors, one or more of the five planets which 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 B.C. and possibly as early as 467 B.C.

Other historical reports seem to defy prosaic explanation, but assessing such accounts is difficult. Whatever their actual cause, such sightings throughout history were often treated as supernatural portents, angels, or other religious omens. Some objects in medieval paintings seem strikingly similar to UFO reports. Art historians explain those objects as religious symbols, often represented in many other paintings of Middle-Age and Renaissance.

Shen Kuo (1031–1095), a Song Chinese government scholar-official and prolific polymath inventor and scholar, wrote a vivid passage in his Dream Pool Essays (1088) about an unidentified flying object. He recorded the testimony of eyewitnesses in 11th century Anhui and Jiangsu (especially in the city of Yangzhou), who stated that a flying object with opening doors would shine a blinding light from its interior (from an object shaped like a pearl) that would cast shadows from trees for ten miles in radius, and was able to take off at tremendous speeds.

Early modern reports
Before the terms "flying saucer" were coined in 1947 and "UFO" in 1952, there were a number of reports of unidentified aerial phenomena. These reports date from the mid-nineteenth to early twentieth century. They include:


 * On January 25, 1878, The Denison Daily News wrote that local farmer John Martin had reported seeing a large, dark, circular flying object resembling a balloon flying "at wonderful speed." Martin also said it appeared to be about the size of a saucer, the first known use of the word "saucer" in association with a UFO.
 * On November 17, 1882, a UFO was observed by astronomer Edward Walter Maunder of the Greenwich Royal Observatory and some other European astronomers. Maunder in The Observatory reported "a strange celestial visitor" that was "disc-shaped", "torpedo-shaped", "spindle-shaped", or "just like a Zeppelin" dirigible (as he described it in 1916).  It moved rapidly from horizon to horizon.  The sighting was during high auroral activity; therefore Maunder assumed it was some extraordinary auroral phenomenon never before seen and called it an "auroral beam".
 * On February 28, 1904, there was a sighting by three crew members on the USS Supply 300 miles west of San Francisco, reported by Lt. 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.
 * 1916 and 1926: the three oldest known pilot UFO sightings, of 1305 cataloged by NARCAP. On January 31, 1916, a UK pilot near Rochford reported a row of lights, like 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 was forced to land by a huge, wingless cylindrical object.
 * On August 5, 1926, while traveling in the Humboldt Mountains of Tibet's Kokonor region, Nicholas Roerich reported that members of his expedition saw "something big and shiny reflecting sun, like a huge oval moving at great speed".
 * 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 but were dismissed by scientists as St. Elmo's Fire or illusions.
 * On February 25, 1942, the U.S. Army detected a unidentified aircraft both visually and on radar over the Los Angeles, California region. No readily apparent explanation was offered. The incident later became known as the Battle of Los Angeles, or the West coast air raid.
 * In 1946, there were over 2000 reports of unidentified aircraft in the Scandinavian nations, along with isolated reports from France, Portugal, Italy and Greece, then referred to as "Russian hail", and later as "ghost rockets", because it was thought that these mysterious objects were Russian tests of captured German V1 or V2 rockets. Over 200 were tracked on radar and deemed to be "real physical objects" by the Swedish military.

The Kenneth Arnold sightings
The post World War II UFO phase in the United States began with a famous sighting by American businessman Kenneth Arnold on June 24, 1947 while flying his private plane near Mount Rainier, Washington. He reported seeing nine brilliantly bright objects flying across the face of Rainier towards nearby Mount Adams at "an incredible speed", which he "calculated" as at least 1200 miles per hour by timing their travel between Rainier and Adams.



Although there were other 1947 U.S. sightings of similar objects that preceded this, it was Arnold's sighting that first received significant media attention and captured the public's imagination. Arnold described what he saw as being "flat like a pie pan", "shaped like saucers and were so thin I could barely see them...", "half-moon shaped, oval in front and convex in the rear. ...they looked like a big flat disk" (see Arnold's drawing at right), and flew "like a saucer would if you skipped it across the water". (One of the objects, however, he would describe later as being almost crescent-shaped, as shown in illustration at left.) Arnold’s descriptions were widely reported and within a few days gave rise to the terms flying saucer and flying disk. Arnold’s sighting was followed in the next few weeks by hundreds of other reported sightings, mostly in the U.S., but in other countries as well.

After reports of the Arnold sighting hit the media, other cases began to be reported in increasing numbers. In one instance a United Airlines crew sighting of nine more disc-like objects over Idaho on the evening of July 4. At the time, this sighting was even more widely reported than Arnold’s and lent considerable credence to Arnold’s report.

American UFO researcher Ted Bloecher, in his comprehensive review of newspaper reports (including cases that preceded Arnold's), found a sudden surge upwards in sightings on July 4, peaking on July 6–8. Bloecher noted that for the next few days most American newspapers were filled with front-page stories of the new "flying saucers" or "flying discs". Reports began to rapidly tail off after July 8, when officials began issuing press statements on the Roswell UFO incident, in which they explained the debris as being that of a weather balloon.

Over several years in the 1960s, Bloecher (aided by physicist James E. McDonald) discovered 853 flying disc sightings that year from 140 newspapers from Canada, Washington D.C, and every U.S. state except Montana.

Identification of UFOs


Studies show that after careful investigation, the majority of UFOs can be identified as ordinary phenomena (see Identification studies of UFOs). For example, a 1979 study by CUFOS researcher Allan Hendry found that only a small percentage of cases were hoaxes and that most sightings were actually honest misidentifications of prosaic phenomena. Hendry attributed most of these to inexperience or misperception.

The most commonly identified sources of UFO reports include:
 * Astronomical phenomena (bright stars, planets, meteors, re-entering man-made spacecraft, artificial satellites, and the moon)
 * Aircraft (advertising planes and other aircraft, missile launches)
 * Other atmospheric objects and phenomena (balloons, birds, clouds, kites, flares)
 * Light phenomena (mirages, Fata Morgana, moon dogs, searchlights and other ground lights, etc.)
 * Hoaxes

A study by the Battelle Memorial Institute of US Air Force reports included these categories as well as psychological phenomena. Hendry found that 88.6% of the cases he studied had a clear prosaic explanation, and he discarded a further 2.8% due to unreliable or contradictory witnesses or insufficient information. The remaining 8.6% of reports could not definitively be explained by prosaic phenomena, although he felt that a further 7.1% could probably be explained, leaving 1.5% without plausible explanation. Hendry's figure for unidentified cases is considerably lower than many other UFO studies such as Project Blue Book or the Condon Report which have found rates of unidentified cases ranging from 6 to 30%.

UFO hypotheses
To account for unsolved UFO cases, several hypotheses have been proposed.


 * The Extraterrestrial hypothesis (ETH), "The idea that some UFOs may be spacecraft sent to Earth from another civilization, or on a planet associated with a more distant star", attributed to Donald B. Keyhoe, 1950 (Edward Condon, 1968 )
 * The Interdimensional hypothesis, that UFOs are objects crossing over from other dimensions or parallel universe, proposed by Jacques Vallée.
 * The Paranormal/Occult Hypothesis; A variant of the Interdimensional Hypothesis, invoked to explain so-called paranormal aspects sometimes associated with UFO reports
 * That UFOs are time machines or vehicles from a future Earth, perhaps made by our descendants.
 * The Psychosocial Hypothesis, that what people report as UFO experiences is the result of psychological misperception mechanisms and is strongly influenced by popular culture.
 * That UFOs represent poorly understood or still unknown natural phenomena, such as ball lightning or sprites.
 * The Earthquake lights/Tectonic Strain hypothesis: UFOs are caused by strains in Earth's crust near earthquake faults, which can also supposedly induce hallucinations.
 * That UFOs are military flying saucers; top secret or experimental aircraft unfamiliar to most people.

Investigation and responses
UFOs have been subject to various investigations over the years, varying widely in scope and scientific rigor. Governments or independent academics in the United States, Canada, the United Kingdom, France, Belgium, Sweden, Brazil, Mexico, Spain, and the Soviet Union are known to have investigated UFO reports at various times. Among the best known government studies are Project Blue Book, previously Project Sign and Project Grudge, conducted by the United States Air Force from 1947 until 1969, the secret U.S. Army/Air Force Project Twinkle investigation into green fireballs (1948–1951), and Brazilian Air Force Operation Saucer (1977). A public research effort conducted by the Condon Committee, which reported in 1968, marked the end of the US government's official investigation of UFOs.

The investigation of UFOs also attracted many civilians who in the U.S formed groups such as National Investigations Committee On Aerial Phenomena (NICAP, active 1956-1980), Aerial Phenomena Research Organization (APRO, 1952-1988), Mutual UFO Network (MUFON, 1969-), and Center for UFO Studies (CUFOS, 1973-).

American investigations
Starting July 9, 1947 Army Air Force (AAF) intelligence, in cooperation with the FBI, began a formal investigation into selected sightings with characteristics that could not be immediately rationalized, which included Arnold’s and the United crew’s. The AAF used "all of its scientists" to determine whether or not "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." 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."

A further review by the intelligence and technical divisions of the Air Materiel Command at Wright Field reached the same conclusion, 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 thus recommended in late September 1947 that an official Air Force investigation be set up to investigate the phenomenon.

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 ETH conclusion. In August 1948, Sign investigators wrote a top-secret intelligence estimate to that effect. 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 Dr. J. Allen Hynek and Edward J. Ruppelt, the first head of the USAF's Project Blue Book. (Ruppelt, Chapt. 3)

Project Sign was dismantled and became Project Grudge at the end of 1948, and then Project Blue Book In 1952. Blue Book closed down in 1970, ending the official Air Force UFO investigations. However, a 1969 USAF document, known as the Bolender memo, plus later government documents revealed that nonpublic U.S. government UFO investigations continued after 1970. The Bollender memo first stated that "reports of unidentified flying objects which could affect national security... are not part of the Blue Book system," indicating that more serious UFO incidents were already handled outside of 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."

An early U.S. Army study, of which little is known, was called the Interplanetary Phenomenon Unit (IPU). In 1987, British UFO researcher Timothy Good received a letter confirming the existence of the IPU from the Army Director of Counter-intelligence, in which it was stated, "...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.

Use of UFO instead of flying saucer was first suggested in 1952 by Capt. Edward J. Ruppelt, the first director of Project Blue Book, who felt that flying saucer did not reflect the diversity of the sightings. Ruppelt suggested that UFO should be pronounced as a word &mdash; you-foe. However it is generally pronounced by forming each letter: U.F.O. His term was quickly adopted by the Air Force, which also briefly used "UFOB" circa 1954, for Unidentified Flying Object. Ruppelt recounted his experiences with Project Blue Book in his memoir, The Report on Unidentified Flying Objects (1956), also the first book to use the term.

Air Force Regulation 200-2, issued in 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." As with any then-ongoing investigation, Air Force personnel did not discuss the investigation with the press.

Well known American investigations include:


 * Project Blue Book, previously Project Sign and Project Grudge, conducted by the United States Air Force 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, U.K., U.S., and Greek militaries (1946–1947)
 * 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 Sturrock Panel (1998)

Canadian investigation
In Canada, the Department of National Defence has dealt with reports, sightings and investigations of UFOs across Canada. In addition to conducting investigations into crop circles in Duhamel, Alberta, it still identifies the Falcon Lake incident in Manitoba and the Shag Harbour incident in Nova Scotia as "unsolved".

The Canadian studies include Project Magnet (1950–1954) and Project Second Story (1952–1954)

French investigation
In March 2007, the French Centre National d'Études Spatiales (CNES) published an archive of UFO sightings and other phenomena online.

French studies include GEPAN/SEPRA/GEIPAN (1977–), within the French space agency CNES, the longest ongoing government-sponsored investigation, and the private French COMETA panel (1996–1999)

British investigation
The UK conducted various investigations into UFO sightings and related stories. The contents of some of these investigations have since been released to the public.

Eight file collections on UFO sightings, dating from 1978 to 1987, were first released on May 14, 2008, to the UK National Archives by the Ministry of Defence. Although kept secret from the public for many years, most of the files have low levels of classification and none is classified Top Secret. 200 files are set to be made public by 2012. The files are correspondence from the public sent to government officials, such as the MoD and Margaret Thatcher. The MoD released the files under the Freedom of Information Act due to requests from researchers. These files include, but are not limited to, UFOs over Liverpool and the Waterloo Bridge in London.

On October 20, 2008 more UFO files were released. One case released detailed that in 1991 an Alitalia passenger aircraft was approaching Heathrow Airport when the pilots saw what they described as a "cruise missile" flew extremely close to the cockpit. The pilots believed that a collision was imminent. UFO expert Dr David Clarke says that this is one of the most convincing cases for a UFO he has come across.

British investigations include The UK's Flying Saucer Working Party. Its final report, published in 1951, remained secret for over 50 years. The Working Party concluded that all UFO sightings could be explained as misidentifications of ordinary objects or phenomena, optical illusions, psychological delusions or hoaxes. The report stated: ‘We accordingly recommend very strongly that no further investigation of reported mysterious aerial phenomena be undertaken, unless and until some material evidence becomes available’. Another notable investigation was Project Condign (1997–2000)

A secret study of UFOs undertaken for the UK’s Ministry of Defence (MoD) between 1996 and 2000 and was publicly released in 2006. The report is titled "Unidentified Aerial Phenomena in the UK Defence Region" and was code-named Project Condign. The report confirmed earlier findings that the main causes of UFO sightings are misidentification of man-made and natural objects. The report noted: "No artefacts of unknown or unexplained origin have been reported or handed to the UK authorities, despite thousands of UAP reports. There are no SIGINT, ELINT or radiation measurements and little useful video or still IMINT." It concluded: "There is no evidence that any UAP, seen in the UKADR [UK Air Defence Region], are incursions by air-objects of any intelligent (extraterrestrial or foreign) origin, or that they represent any hostile intent." In contrast to the official government position, Nick Pope, the head of the UK government UFO desk for a number of years, is an advocate of the ETH, based on the inexplicable (to him) cases he reviewed, such as the Rendlesham UFO incident and the so-called Cosford Incident.

Other
The Air Force's Project Blue Book files indicate that approximately 1% of all unknown reports came from amateur and professional astronomers or other users of telescopes (such as missile trackers or surveyors). In the 1970s, astrophysicist Peter A. Sturrock conducted two surveys of the American Institute of Aeronautics and Astronautics and American Astronomical Society. About 5% of the members polled indicated that they had had UFO sightings. In 1980, a survey of 1800 members of various amateur astronomer associations by Gert Helb and astronomer J. Allen Hynek of the Center for UFO Studies (CUFOS) found that 24% responded "yes" to the question "Have you ever observed an object which resisted your most exhaustive efforts at identification?"

Astronomer Clyde Tombaugh, who admitted to six UFO sightings, including three green fireballs supported the Extraterrestrial hypothesis (ETH) for UFOs and stated he thought scientists who dismissed it without study were being "unscientific." Another astronomer was Dr. Lincoln LaPaz, who had headed the Air Force's investigation into the green fireballs and other UFO phenomena in New Mexico. LaPaz reported two personal sightings, one of a green fireball, the other of an anomalous disc-like object. Even later UFO debunker Dr. Donald Menzel filed a UFO report in 1949.

Some studies were neutral in their conclusions, but argued the inexplicable core cases called for continued scientific study. Examples are the Sturrock Panel study of 1998 and the 1970 AIAA review of the Condon Report.

National Press Club press conference on November 12, 2007
On November 12, 2007, Former Arizona Governor Fife Symington moderated a panel of former high-ranking government, aviation and military officials from seven countries at the National Press Club; discussing the UFO topic and governmental investigations. The press conference was open for credentialed media and congressional staff only.

Physical evidence
Besides visual sightings, reports sometimes include indirect physical evidence, including many cases studied by the military and various government agencies of different countries. Indirect physical evidence would be data obtained from afar, such as radar contact and photographs. More direct physical evidence involves physical interactions with the environment at close range, such as repors of "close encounters", "landing traces," electromagnetic interference, and physiological/biological effects.
 * Radar contact and tracking, sometimes from multiple sites. These are often considered among the best cases since they usually involve trained military personnel and control tower operators, simultaneous visual sightings, and aircraft intercepts. One such recent example were the mass sightings of large, silent, low-flying black triangles in 1989 and 1990 over Belgium, tracked by multiple NATO radar and jet interceptors, and investigated by Belgium's military (included photographic evidence). Another famous case from 1986 was the JAL 1628 case over Alaska investigated by the FAA.
 * Photographic evidence, including still photos, movie film, and video, including some in the infrared spectrum (rare).
 * Recorded visual spectrograms
 * Recorded gravimetric (example) and magnetic disturbances (extremely rare)
 * Landing physical trace evidence, including ground impressions, burned and/or desiccated soil, burned and broken foliage, magnetic anomalies, increased radiation levels, and metallic traces. See, e.g. Height 611 UFO Incident or the 1964 Lonnie Zamora's Socorro, New Mexico encounter, considered one of the most inexplicable of the USAF Project Blue Book cases). A well-known example from December 1980 was the USAF Rendlesham Forest Incident in England. Another less than two weeks later, in January 1981, occurred in Trans-en-Provence and was investigated by GEPAN, then France's official government UFO-investigation agency. Project Blue Book head Edward J. Ruppelt described a classic 1952 CE2 case involving a patch of charred grass roots. Catalogs of several thousand such cases have been compiled, particularly by researcher Ted Phillips.
 * Physiological effects on people and animals including temporary paralysis, skin burns and rashes, corneal burns, and symptoms superficially resembling radiation poisoning, such as the Cash-Landrum incident in 1980. One such case dates back to 1886, a Venezuelan incident reported in Scientific American magazine.
 * Animal/cattle mutilation cases, that some feel are also part of the UFO phenomenon. Such cases can and have been analyzed using forensic science techniques.
 * Biological effects on plants such as increased or decreased growth, germination effects on seeds, and blown-out stem nodes (usually associated with physical trace cases or crop circles)
 * Electromagnetic interference (EM) effects, including stalled cars, power black-outs, radio/TV interference, magnetic compass deflections, and aircraft navigation, communication, and engine disruption. A list of over 30 such aircraft EM incidents was compiled by NASA scientist Dr. Richard F. Haines. A famous 1976 military case over Tehran, recorded in CIA and DIA classified documents, resulted in communication losses in multiple aircraft and weapons system failure in an F-4 Phantom II jet interceptor as it was about to fire a missile on one of the UFOs. This was also a radar/visual case.
 * Remote radiation detection, some noted in FBI and CIA documents occurring over government nuclear installations at Los Alamos National Laboratory and Oak Ridge National Laboratory in 1950, also reported by Project Blue Book director Ed Ruppelt in his book.
 * Actual hard physical evidence cases, such as 1957, Ubatuba, Brazil, magnesium fragments analyzed by the Brazilian government and in the Condon Report and by others. The 1964 Socorro/Lonnie Zamora incident also left metal traces, analyzed by NASA.
 * Misc: Recorded electromagnetic emissions, such as microwaves detected in the well-known 1957 RB-47 surveillance aircraft case, which was also a visual and radar case; polarization rings observed around a UFO by a scientist, explained by Dr. James Harder as intense magnetic fields from the UFO causing the Faraday effect.

These various reported physical evidence cases have been studied by various scientist and engineers, both privately and in official governmental studies (such as Project Blue Book, the Condon Committee, and the French GEPAN/SEPRA). A comprehensive scientific review of physical evidence cases was carried out by the 1998 Sturrock UFO panel.

Ufology
Ufology is a neologism describing the collective efforts of those who study UFO reports and associated evidence.

Reverse engineering
Attempts have been made to reverse engineer the possible physics behind UFOs through analysis of both eyewitness reports and the physical evidence, on the assumption that they are powered vehicles. Examples are former NASA and nuclear engineer James McCampbell in his book Ufology, NACA/NASA engineer Paul R. Hill in his book Unconventional Flying Objects, and German rocketry pioneer Hermann Oberth. Among subjects tackled by McCampbell, Hill, and Oberth was the question of how UFOs can fly at supersonic speeds without creating a sonic boom. McCampbell's proposed solution of a microwave plasma parting the air in front of the craft is currently being researched by Dr. Leik Myrabo, Professor of Engineering Physics at the Rensselaer Polytechnic Institute as a possible advance in hypersonic flight. In contrast, Hill and Oberth believed UFOs utilize an as yet unknown anti-gravity field to accomplish the same thing as well as provide propulsion and protection of occupants from the effects of high acceleration.

UFO categorization
Some researchers recommend that observations be classified according to the features of the phenomenon or object that are reported or recorded. Typical categories include:
 * Saucer, toy-top, or disk-shaped "craft" without visible or audible propulsion. (day and night)
 * Large triangular "craft" or triangular light pattern, usually reported at night.
 * Cigar-shaped "craft" with lighted windows (Meteor fireballs are sometimes reported this way, but are very different phenomena).
 * Other: chevrons, (equilateral) triangles, crescent, boomerangs, spheres (usually reported to be shining, glowing at night), domes, diamonds, shapeless black masses, eggs, pyramids and cylinders, classic "lights".

Popular UFO classification systems include the Hynek system, created by J. Allen Hynek, and the Vallée system, created by Jacques Vallée.

Hynek's system involves dividing the sighted object by appearance, subdivided further into the type of "close encounter" (a term from which the film director Steven Spielberg derived the title of his UFO movie, "Close Encounters of the Third Kind").

Jacques Vallée's system classifies UFOs into five broad types, each with from three to five subtypes that vary according to type.

Conspiracy theories
UFOs are sometimes an element of elaborate conspiracy theories in which governments are said to be intentionally covering up the existence of aliens, or sometimes collaborating with them. There are many versions of this story; some are exclusive, while others overlap with various other conspiracy theories.

In the U.S., opinion polls again indicate that a strong majority of people believe the U.S. government is withholding such information. Various notables have also expressed such views. Some examples are astronauts Gordon Cooper and Edgar Mitchell, Senator Barry Goldwater, Vice Admiral Roscoe H. Hillenkoetter (the first CIA director), Lord Hill-Norton (former British Chief of Defense Staff and NATO head), the 1999 high-level French COMETA report by various French generals and aerospace experts, and Yves Sillard (former director of the French space agency CNES, new director of French UFO research organization GEIPAN).

There is also speculation that UFO phenomena are tests of experimental aircraft or advanced weapons. In this case UFOs are viewed as failures to retain secrecy, or deliberate attempts at misinformation: to deride the phenomenon so that it can be pursued unhindered. This explanation may or may not feed back into the previous one, where current advanced military technology is considered to be adapted alien technology (see also: skunk works and Area 51).

It has also been suggested by a few paranormal authors that all or most human technology and culture is based on extraterrestrial contact. See also ancient astronauts.

Allegations of evidence suppression
Some also contend regarding physical evidence that it exists abundantly but is swiftly and sometimes clumsily suppressed by governments, aiming to insulate a population they regard as unprepared for the social, theological, and security implications of such evidence. See the Brookings Report.

There have been allegations of suppression of UFO related evidence for many decades. There are also conspiracy theories which claim that physical evidence might have been removed and/or destroyed/suppressed by some governments. (See also Men in Black) Some examples are:
 * On July 7, 1947, William Rhodes took photos of an unusual object over Phoenix, Arizona. The photos appeared in a Phoenix newspaper and a few other papers. According to documents from Project Bluebook, an Army counter-intelligence (CIC) agent and an FBI agent interviewed Rhodes on August 29 and convinced him to surrender the negatives. The CIC agent deliberately concealed his true identity, leaving Rhodes to believe both men were from the FBI. Rhodes said he wanted the negatives back, but when he turned them into the FBI the next day, he was informed he wouldn't be getting them back, though Rhodes later tried unsuccessfully. The photos were extensively analyzed and would eventually show up in some classified Air Force UFO intelligence reports. (Randle, 34–45, full account)
 * A June 27, 1950, movie of a "flying disk" over Louisville, Kentucky, taken by a Louisville Courier-Journal photographer, had the USAF Directors of counterintelligence (AFOSI) and intelligence discussing in memos how to best obtain the movie and interview the photographer without revealing Air Force interest. One memo suggested the FBI be used, then precluded the FBI getting involved. Another memo said "it would be nice if OSI could arrange to secure a copy of the film in some covert manner," but if that wasn't feasible, one of the Air Force scientists might have to negotiate directly with the newspaper. In a recent interview, the photographer confirmed meeting with military intelligence and still having the film in his possession until then, but refused to say what happened to the film after that.
 * In another 1950 movie incident from Montana, Nicholas Mariana filmed some unusual aerial objects and eventually turned the film over to the U.S. Air Force, but insisted that the first part of the film, clearly showing the objects as spinning discs, had been removed when it was returned to him. (Clark, 398)
 * During the military investigation of green fireballs in New Mexico, UFOs were photographed by a tracking camera over White Sands Proving Grounds on April 27, 1949. The final report in 1951 on the green fireball investigation claimed there was insufficient data to determine anything. However, documents later uncovered by Dr. Bruce Maccabee indicate that triangulation was accomplished. The data reduction and photographs showed four objects about 30 feet in diameter flying in formation at high speed at an altitude of about 30 miles. Maccabee says this result was apparently suppressed from the final report.
 * Project Blue Book director Edward J. Ruppelt reported that, in 1952, a U.S. Air Force pilot fired his jet's machine guns at a UFO, and that the official report which should have been sent to Blue Book was quashed.
 * Astronaut Gordon Cooper reported suppression of a flying saucer movie filmed in high clarity by two Edwards AFB range photographers on May 3, 1957. Cooper said he viewed developed negatives of the object, clearly showing a dish-like object with a dome on top and something like holes or ports in the dome. The photographers and another witness, when later interviewed by Dr. James McDonald, confirmed the story. Cooper said military authorities then picked up the film and neither he nor the photographers ever heard what happened to it. The incident was also reported in a few newspapers, such as the Los Angeles Times. The official explanation, however, was that the photographers had filmed a weather balloon distorted by hot desert air.
 * On January 22, 1958, when NICAP director Donald Keyhoe appeared on CBS television, his statements on UFOs were pre-censored by the Air Force. During the show when Keyhoe tried to depart from the censored script to "reveal something that has never been disclosed before," CBS cut the sound, later stating Keyhoe was about to violate "predetermined security standards" and about to say something he wasn't "authorized to release."  What Keyhoe was about to reveal were four publicly unknown military studies concluding UFOs were interplanetary (including the 1948 Project Sign Estimate of the Situation and Blue Book's 1952 engineering analysis of UFO motion). (Good, 286–287; Dolan 293–295)
 * In February 1960, NICAP revealed that the USAF inspector general had issued a directive to Air Force commands declaring UFOs to be "serious business" to the national defense. The directive was confirmed by an Air Force spokesperson.  NICAP board member and first CIA director Rear Admiral Roscoe Hillenkoetter then sent the directive with a cover letter to the Senate Science and Astronautics Committee stating that, "It is time for the truth to be brought out in open Congressional hearings.  ...behind the scenes, high-ranking Air Force officers are soberly concerned about the UFO's.  But through official secrecy and ridicule, many citizens are led to believe the unknown flying objects are nonsense.  ...to hide the facts, the Air Force has silenced its personnel" through the issuance of a regulation.
 * Astronomer Jacques Vallee reported that in 1961 he witnessed the destruction of the tracking tapes of unknown objects orbiting the Earth. (However, Vallee indicated that this didn't happen because of government pressure but because the senior astronomers involved didn't want to deal with the implications.)
 * In 1965, Rex Heflin took four Polaroid photos of a hat-shaped object. Two years later (1967), two men posing as NORAD agents confiscated three prints. Just as mysteriously, the photos were returned to his mailbox in 1993.
 * A March 1, 1967 memo directed to all USAF divisions, from USAF Lt. General Hewitt Wheless, Assistant Vice Chief of Staff, stated that unverified information indicated that unknown individuals, impersonating USAF officers and other military personnel, had been harassing civilian UFO witnesses, warning them not to talk, and also confiscating film, referring specifically to the Heflin incident. AFOSI was to be notified if any personnel were to become aware of any other incidents. (Document in Fawcett & Greenwood, 236).
 * John Callahan, former Division Chief of the Accidents and Investigations Branch of the FAA, Washington D.C., also a Disclosure Project witness, said that following a 1986 encounter of a Japanese airlines 747 with a giant UFO over Alaska, recorded by air and ground radar, the FAA conducted an investigation. Callahan held a briefing a few days later for President Reagan's Scientific Study Group, the FBI, and CIA.  After the briefing, one of the CIA agents told everybody they "were never there and this never happened," adding they were fearful of public panic.
 * In 1996, the CIA revealed an instance from 1964 where two CIA agents posed as USAF representatives in order to recover a film canister from a Corona spy satellite that had accidentally come down in Venezuela. The event was then publicly dismissed as an unsuccessful NASA space experiment.

Famous hoaxes

 * The Maury Island Incident
 * The Ummo affair, a decades-long series of detailed letters and documents allegedly from extraterrestrials. The total length of the documents is at least 1000 pages, and some estimate that further undiscovered documents may total nearly 4000 pages. A Jose Luis Jordan Pena came forward in the early nineties claiming responsibility for the phenomenon, and most consider there to be little reason to challenge his claims.
 * George Adamski over the space of two decades made various claims about his meetings with telepathic aliens from nearby planets. He claimed that photographs of the far side of the moon taken by a Soviet orbital probe in 1959 were fake, and that there were cities, trees and snow-capped mountains on the far side of the moon.
 * In 1987/1988 Ed Walters perpetrated a hoax in Gulf Breeze, Florida. Walters claimed at first having seen a small UFO flying near his home, and then in a second incident seeing the same UFO and a small alien being standing by his back door after being alerted by his dog. Several photographs were taken of the craft, but none of the being. Three years later, in 1990, after the Walters family had moved, the new residents discovered a model of a UFO poorly hidden in the attic that bore an undeniable resemblance to the craft in Walters' photographs. Various witnesses and detractors came forward after the local Pensacola newspaper printed a story about the discovered model, and some investigators now consider the sightings to be a hoax. In addition, a six-figure television miniseries and book deal were nearly struck with Walters.
 * Warren William (Billy) Smith, A popular writer and confessed hoaxster.

UFOs in popular culture
UFOs constitute a widespread international cultural phenomenon of the last half-century. Gallup polls rank UFOs near the top of lists for subjects of widespread recognition. In 1973, a survey found that 95 percent of the public reported having heard of UFOs, whereas only 92 percent had heard of US President Gerald Ford in a 1977 poll taken just nine months after he left the White House. (Bullard, 141) A 1996 Gallup poll reported that 71 percent of the United States population believed that the government was covering up information regarding UFOs. A 2002 Roper poll for the Sci Fi channel found similar results, but with more people believing UFOs were extraterrestrial craft. In that latest poll, 56 percent thought UFOs were real craft and 48 percent that aliens had visited the Earth. Again, about 70 percent felt the government was not sharing everything it knew about UFOs or extraterrestrial life. Another effect of the flying saucer type of UFO sightings has been Earth-made flying saucer craft in space fiction, for example the Earth-made craft Starship C-57D in Forbidden Planet, and the saucer part of the USS Enterprise in Star Trek And many others. For an excellent analysis of the interrelationship between popular culture and UFOs consult the research by psychologist Armando Simon, especially his contribution in Richard Haines' book, UFO Phenomena and the Behavioral Scientist.

Use in film and television

 * See List of major UFO film and television shows

Use in Music

 * See Ufoetry

General

 * Thomas E. Bullard, "UFOs: Lost in the Myths", pages 141–191 in "UFOs, the Military, and the Early Cold War Era", pages 82–121 in "UFOs and Abductions: Challenging the Borders of Knowledge" David M. Jacobs, editor; 2000, University Press of Kansas, ISBN 0-7006-1032-4
 * Jerome Clark, The UFO Book: Encyclopedia of the Extraterrestrial, 1998, Visible Ink Press, ISBN 1-57859-029-9. Many classic cases and UFO history provided in great detail; highly documented.
 * Curran, Douglas. In Advance of the Landing: Folk Concepts of Outer Space. (revised edition), Abbeville Press, 2001. ISBN 0-7892-0708-7. Non-sensational but fair treatment of contemporary UFO legend and lore in N. America, including the so-called "contactee cults." The author traveled the United States with his camera and tape recorder and directly interviewed many individuals.
 * Hall, Richard H., editor. The UFO Evidence: Volume 1. 1964, NICAP, reissued 1997, Barnes & Noble Books, ISBN 0-7607-0627-1. Well-organized, exhaustive summary and    analysis of 746 unexplained NICAP cases out of 5000 total cases — a classic.
 * Hall, Richard H. The UFO Evidence: A Thirty-Year Report. Scarecrow Press, 2001. ISBN 0-8108-3881-8. Another exhaustive case study, more recent UFO reports.
 * Hendry, Alan. The UFO Handbook: A Guide to Investigating, Evaluating, and Reporting UFO Sightings. New York: Doubleday & Co., 1979. ISBN 0-385-14348-6. Skeptical but balanced    analysis of 1300 CUFOS UFO cases.
 * Hynek, J. Allen. The UFO Experience: A scientific inquiry. Henry Regnery Co., 1972.
 * Hynek, J. Allen. The Hynek UFO Report. New York: Barnes & Noble Books, 1997. ISBN 0-7607-0429-5.    Analysis of 640 high-quality cases through 1969 by UFO legend Hynek.
 * Rose, Bill and Buttler, Tony. Flying Saucer Aircraft (Secret Projects). Leicester, UK: Midland Publishing, 2006. ISBN 1-85780-233-0.
 * Sagan, Carl & Page. Thornton, editors. UFOs: A Scientific Debate. \Cornell University Press, 1996. ISBN 0-7607-0192-2. Pro and con articles by scientists, mostly to the skeptical side.
 * Sheaffer, Robert The UFO Verdict: Examining the Evidence, 1986, Prometheus Books ISBN 0-87975-338-2
 * Sheaffer, Robert UFO Sightings: The Evidence, 1998, Prometheus Books, ISBN 1-57392-213-7 (revised edition of The UFO Verdict)
 * Sturrock, Peter A. (1999). The UFO Enigma: A New Review of the Physical Evidence. New York: Warner Books. ISBN 0-446-52565-0
 * Canada's Unidentified Flying Objects: The Search for the Unknown, a virtual museum exhibition at Library and Archives Canada
 * Canada's Unidentified Flying Objects: The Search for the Unknown, a virtual museum exhibition at Library and Archives Canada

Skepticism

 * Philip Plait (2002). Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing "Hoax". John Wiley & Sons, ISBN 0-471-40976-6. (Chapter 20: Misidentified Flying Objects: UFOs and Illusions of the Mind and Eye.)
 * Ian Ridpath "Astronomical Causes of UFOs"
 * Michael A. Seeds. (1995). Horizons: Exploring the Universe, Wadsworth Publishing, ISBN 0-534-24889-6 and ISBN 0-534-24890-X. (Appendix A)

Psychology

 * Carl G. Jung, "Flying Saucers: A Modern Myth of Things Seen in the Skies" (translated by R.F.C. Hull); 1979, Princeton University Press, ISBN 0-691-01822-7
 * Armando Simon,A Nonreactive, Quantitative Study of Mass Behavior with Emphasis on the Cinema as Behavior Catalyst," Psychological Reports, 1981, 48, 775-785.
 * Richard Haines"UFO Phenomena and the Behavioral Scientist." Metuchen: Scarecrow Press, 1979.
 * Armando Simon "UFOs: Testing for the Existence of Air Force Censorship." Psychology, 1976, 13, 3-5.
 * Armando Simon "Psychology and the UFOs." The Skeptical Inquirer. 1984, 8, 355-367.

Histories

 * Richard M. Dolan, UFOs and the National Security State: An Unclassified History, Volume One: 1941–1973, 2000, Keyhole Publishing, ISBN 0-9666885-0-3. Dolan is a professional historian.
 * Downes, Jonathan Rising of the Moon. 2nd ed. Bangor: Xiphos, 2005.
 * Lawrence Fawcett & Barry J. Greenwood, The UFO Cover-Up (Originally Clear Intent), 1992, Fireside Books (Simon & Schuster), ISBN 0-671-76555-8. Many UFO documents.
 * Timothy Good, Above Top Secret, 1988, William Morrow & Co., ISBN 0-688-09202-0. Many UFO documents.
 * Timothy Good, Need to Know: UFOs, the Military, and Intelligence, 2007, Pegasus Books, ISBN 978-1-933648-38-5. Update of Above Top Secret with new cases and documents
 * Bruce Maccabee, UFO FBI Connection, 2000, Llewellyn Publications, ISBN 1-56718-493-6
 * Kevin Randle, Project Blue Book Exposed, 1997, Marlowe & Company, ISBN 1-56924-746-3
 * Edward J. Ruppelt, The Report On Unidentified Flying Objects, 1956, Doubleday & Co. online. A UFO classic by insider Ruppelt, the first head of the USAF Project Blue Book
 * LeRoy F. Pea, Government Involvement in the UFO Coverup, or earlier title History of UFO Crash/Retrievals, 1988, PEA RESEARCH.

Technology

 * Paul R. Hill, Unconventional Flying Objects: a scientific    analysis, 1995, Hampton Roads Publishing Co., ISBN 1-57174-027-9.     Analysis of UFO technology by pioneering NACA/NASA aerospace engineer.
 * James M. McCampbell, Ufology: A Major Breakthrough in the Scientific Understanding of Unidentified Flying Objects, 1973, 1976, Celestial Arts, ISBN 0-89087-144-2 full-text online. Another    analysis by former NASA and nuclear engineer.
 * James M. McCampbell, Physical effects of UFOs upon people, 1986, paper.
 * Antonio F. Rullán, Odors from UFOs: Deducing Odorant Chemistry and Causation from Available Data, 2000, preliminary paper.
 * Jack Sarfatti, "Super Cosmos", 2005 (Authorhouse)
 * See also the
 * See also the