Aerial objects or optical phenomena seen by an observer who has no explanation for the sighting, even though it may later be rationalized.

Brief History

Although unexplained aerial phenomena have been reported for millennia, the first documented sighting of unidentified flying objects deemed to be aircraft occurred on June 24, 1947. Veteran pilot Kenneth Arnold was flying a private plane over the state of Washington when he observed what appeared to be nine intensely bright objects traveling at extraordinarily high speeds toward Mt. Rainier. He later reported that the objects “flew like a saucer would if you skipped it across the water.” News reports picked up the story, headlining the objects as “flying saucers,” a misnomer that remained in use until the U.S. Air Force introduced the term “UFO” six years later. Due to the publicity generated by Arnold’s sighting, a rash of similar sightings soon broke out over the North American continent.

As stories of flying saucers and potential alien invasions continued to grow, the U.S. Air Force began taking an interest because of possible risks to national security. Commencing in 1948, it began compiling a file of UFO reports termed Project Blue Book. When radar was able to supplement visual sightings of UFOs over Washington, D.C., in July, 1952, the U.S. government established a panel of scientists, under the auspices of the Central Intelligence Agency (CIA), to investigate the information in Project Blue Book. Although its report was classified at the time, later declassification revealed its conclusion that 90 percent of the sightings could be explained as misapprehended natural phenomena such as astronomical bodies, meteorological effects, aircraft, balloons, and birds seen under unusual conditions.

Continuing criticism of the Air Force for allegedly covering up important UFO information or bungling investigations led to the formation of a second panel of scientists in February, 1966, to determine if UFO phenomena warranted further investigation. After several years of intensively studying the most puzzling cases, the team of experts, led by renowned physicist E. U. Condon, published its results as “A Scientific Study of UFOs.” Known as the Condon Report, this investigation concluded that, due to a lack of compelling evidence, further extensive study of UFOs could not be justified and any expectation that science would be advanced thereby was misconstrued. Based on the recommendations of the Condon Report, the Air Force officially terminated Project Blue Book in 1969, the same year the report was released; by this time the Air Force had amassed records on 12,618 sightings or related mysterious events.

Because the Condon Report left a number of sightings admittedly unexplained, however, several scientists, principally James McDonald, a University of Arizona meteorologist, and J. Allen Hynek, a Northwestern University astronomer, remained unconvinced by its conclusions. Extremely critical of the cavalier manner in which puzzling cases were explained away and inexplicable sightings summarily dismissed, they believed that the conclusions of the Condon Report were unwarranted and premature. According to Hynek, the residual core of unexplained sightings (about 5 percent) remaining after natural explanations and hoaxes were eliminated constituted some extraordinary new phenomena which should receive serious scientific attention. Although neither McDonald nor Hynek claimed that the unexplained sightings proved the existence of extraterrestrial visitors, they did believe that some novel natural phenomenon was involved which, under proper investigation, could significantly advance the frontiers of science.

Types of UFO Reports

Sightings may be classified into four broad categories: visual sightings in daylight, visual sightings at night, radar sightings, and close encounters or physical evidence. The final category may be subdivided into three types of close encounters. Close encounters of the first type are those in which an observer reports a close-at-hand experience without tangible physical effects; close encounters of the second type, where there are physical effects on objects or people but no contact with the occupants of a UFO; and close encounters of the third type, in which live entities are sighted or contact is made with UFO occupants.

A classic case of a daylight sighting occurred in the early afternoon of January 7, 1948, in Louisville, Kentucky. The State Highway Patrol, responding to calls of residents who reported a strange object moving west at high speeds, alerted officials at nearby Godman Air Force base. The object was soon spotted, but could not be identified. Captain Thomas Mantell, approaching the base in a P-51 plane, saw the object and climbed to 25,000 feet in pursuit. He radioed the control tower that he was chasing a large metallic object moving at 180 miles per hour. Soon after, radio contact with Captain Mantell was lost; an hour later, his crashed plane was found. Although there were rumors at the time that he had been shot down by an alien aircraft, later investigation proved that he had been chasing a 100-foot Skyhook balloon, a classified project then being tested by the U.S. Navy. It was concluded that because the airplane had no oxygen equipment, the captain had lost consciousness at 25,000 feet; without his control, the plane eventually spiraled into a fatal dive.

A classic nighttime sighting is the case of the Lubbock Lights. On the evening of August 25, 1951, three professors in the town of Lubbock, Texas, were observing meteors when they noticed fifteen to twenty lights passing silently overhead from north to south. The lights were yellowish-white with a diffuse glow, moving through a 30-degree arc in about one second. There were no reference points to gauge size and distance, and the professors initially assumed that the objects were flying at an altitude of at least one mile, implying that they were of immense size and moving incredibly fast. The sightings were reported to a news service, which released a sensationalist article with an accompanying, probably faked, photograph completely different from what the professors had observed. Over the next several weeks, the professors made additional sightings and even attempted to obtain the objects’ true height by coordinating observations from widely separated observers in the surrounding countryside. Although this experiment failed due to a lack of data (the lights did not appear to the observers), a rancher thirty miles from Lubbock inadvertently solved the mystery when, during his third observation of the lights, some flew low enough for him to recognize them as birds flying south. When one emitted a cry, he identified the familiar call of the plover. Flying overhead for their yearly migration, their oily white undersides reflect the city lights beneath them as a shiny glow. Their glowing breasts created an illusion of large, rapidly moving objects at high altitude.

Although radar images of UFOs are believed by some to be incontrovertible proof of alien spacecraft, several effects can produce false radar echos. These include electronic interference and reflections from ionized layers or cumulus clouds. The most famous case of radar phantoms was the “invasion” of Washington, D.C., on the nights of July 19 and July 26, 1952. At 11:40 p.m. on July 19, a group of seven unidentified, erratically moving targets appeared on the radarscope at the Washington National Airport. Although visual sightings from ground and air were attempted, no strange aircraft were spotted. The next day, the press reported that a fleet of flying saucers had invaded Washington, intensifying the UFO mania sweeping the nation that summer. The entire process occurred again one week later, but this time the Air Force dispatched fighter planes to search out the invaders. No visual counterpart to the radar images could be found. Eventually the facts were sorted out, and it was determined that the radar images had been caused by weather. A severe drought and unrelenting heat wave had produced intensely hot days followed by rapid cooling at night, creating temperature inversions with abnormal distributions of moisture; these conditions readily generate false radar images. Although such images had been observed many times before, it was probably the flying saucer mania then sweeping the United States which allowed these events to escalate to the status of a UFO “invasion.”

Close Encounters

One of the most famous examples of a close encounter of the third kind is the UFO sighting and alleged abduction story of Barney and Betty Hill on the night of September 19, 1961. About 11:00 p.m., while driving through the White Mountains of New Hampshire, the couple saw a large, disk-shaped object in the sky which they stopped to observe. They had no further memories of the event, but upon returning home at 5:00 a.m. they could not account for two hours of their trip. Ten days later, Betty began having repeated nightmares in which she and her husband were taken aboard a UFO and subjected to physical examinations, and Barney suffered from insomnia and severe anxiety. Because the feelings and dreams were so vivid and so frightening, the Hills sought psychiatric help, eventually coming under the care of Dr. Benjamin Simon, a prominent psychiatrist specializing in hypnotic therapy. After six months of treatment by time-regression hypnosis, what appeared to be memories of the two lost hours emerged. Hypnosis seemed to confirm Betty’s dreams, in which the Hills were abducted by the occupants of an alien spacecraft, undressed, and thoroughly examined by a group of humanoids who even collected skin and hair samples.

In Simon’s professional opinion, the abduction story was a hallucination, transmitted so thoroughly from Betty to Barney by recounting her dreams that Barney later remembered it as real. Simon was also convinced, however, that the Hills had indeed experienced something unusual and frightening that night. Some real, but unknown, physical stimulus apparently triggered a psychological response: the terrifying fantasy that produced the repressed memories. Although many natural explanations, such as freak plasma emissions from high-voltage lines, have been proposed to explain this sighting, a lack of collaborating data keeps this case firmly ensconced in the “unknown” category.

Because, in a close encounter experience, there is little chance of mistakenly identifying common objects as UFOs, such cases are extremely important in attempting to understand UFO phenomena. There are only three possible explanations for close encounter reports: hoaxes and practical jokes, hallucinations or psychotic aberrations, or real experiences, accurately and truthfully reported. Unfortunately, modern science has no foolproof method for evaluating the truth or falsity of witnesses’ claims; current investigative techniques, including truth serum and hypnosis, have limitations. Until new, more reliable scientific methods evolve, there will always be an element of uncertainty associated with even the most reliable of testimonies.

Photographs of UFOs

Most of the photographs of alleged UFOs offer no compelling evidence that these are alien spacecraft under intelligent control. Some common problems that cause serious researchers to reject the majority of these photographs are poor quality of the images; suspicious circumstances associated with taking the photographs, suggesting fraud; the lack of original negatives; incompatibility of witnesses’ stories with the photographic image; and inconsistencies in the photograph, suggesting a double exposure or montage.

Several techniques can be used to check whether a photographic image is genuine. The color contouring method generates a colored map of brightness variations in an image; a tiny model, being relatively close to the camera, would appear brighter than a UFO in the sky—the closer the object, the brighter the image. The edge enhancement technique suppresses all bright and dark regions to a common shade, greatly exaggerating the boundaries between tones; a thin wire used to suspend a model may thereby be made visible. By digitizing an image into pixels, the approximate distance of an image may be gauged because the closer the object, the sharper the pixel. Other photographic verification techniques include subjecting negatives to microscopic analysis and granulation tests.

IFOs (Identified Flying Objects)

The J. Allen Hynek Center for UFO Studies (CUFOS) keeps detailed records on most UFO reports. Their files contain simple, ordinary explanations for 92 percent of all sightings. The remaining cases cannot be identified due to insufficient data.

Many daytime sightings can be explained by research balloons, such as the Skyhook balloon. These giant plastic bags, hundreds of feet in diameter and laden with heavy instruments, are launched from Air Force bases to collect meteorological information about the upper atmosphere (above 30,000 feet) where, because of their size and reflectivity, they are clearly visible from earth. Other daytime sightings include lenticular shaped clouds and the rare but eerie phenomenon of ball lightening.

Nighttime sightings include Venus, the brightest planet, bright stars or planets seen through atmospheric turbulence produced by low-flying military jets, aerial jet refueling operations, and satellite debris reentering the lower atmosphere.

Because radar is an electronic device, radar sightings of UFOs are mistakenly believed to be infallible proof of physical objects invading the skies. In point of fact, interpreting radar signals requires considerable technical training and experience, leaving open the possibility that human operators can misunderstand phantom echoes. Under normal circumstances, radar reflections do not confuse an experienced operator, but unusual meteorological conditions, such as bubbles of warm air surrounded by cool air, can create erratically moving radar phantoms.

Occasionally, objects alleged to have been left by alien UFOs or to have been recovered from a crash site are presented to researchers for analysis. Eliminating blatant hoaxes, which have included small mummified humanoids and “little green men,” the supposed objects of extraterrestrial origin have all proven to be pieces of mundane earth objects or meteorites. Some common objects submitted for examination by genuinely puzzled citizens have include mangled parts of batteries and old radios, corroded lead pipe, tangles of wire, and chunks of aluminum. No fragment ever studied by reliable analysts shows any evidence of having been grown or produced on an alien world.

Abduction by Aliens

Following the publicity generated by the Betty and Barney Hill case, millions of Americans have reported being abducted by aliens; many also have reported being subjected to physical examinations as part of some ongoing genetic study. Women have claimed to have had eggs or embryos removed or to have had a fertilized embryo implanted in their wombs; men have reported having sperm extracted. The veracity of these claims is supposedly supported by the similarity of the alleged abductees’ stories, as well as by scratches and cuts purportedly verifying that painful, invasive examinations were indeed performed. It is, however, not surprising that abduction stories are remarkably similar, given that accounts of UFO abductions are pervasive in American popular culture, appearing in novels, movies, television shows, and comic books.

The detailed testimony provided by alleged abductees under regressive hypnosis offers no verification of their claims since hypnosis is not a truth serum. People can willfully lie while under hypnosis, or provide plausible details derived from unconscious fantasies to please the hypnotist. Forgotten knowledge stored in the subconscious mind may be recalled as a pseudomemory under hypnosis. The stories of Betty and Barney Hill, for example, may have included subconsciously stored information from the movies and television programs about alien invaders that were popular at the time.

Lie detector tests used to assess the truthfulness of alleged abductees’ stories are also subject to interpretation. Although useful in criminal investigations, the results of lie detector tests are not considered reliable enough to be admitted in courts of law as definitive evidence of truth.

The Nature of Scientific Evidence

Most scientists would agree that life is probably found in abundance on planetary systems throughout the galaxy and that intelligent life has evolved on many of these planets. Although based on known science, this is still speculation and provides not one iota of support to the oft-stated corollary that intelligent aliens are visiting Earth. When contemplating UFO phenomena, scientists must first consider the facts they are attempting to explain, then ask whether the alien spacecraft hypothesis is a better explanation than ordinary explanations already available. In the study of UFO phenomena, it is crucial to carefully distinguish an observed fact (evidence) from an interpretation of the fact, which is not evidence no matter how reasonable the inference seems.

Hypotheses about the facts may be conceived, but to be considered science, the hypothesis must lead to predictions which can be tested through experimentation. When more than one hypothesis can explain the same observation, science employs Occam’s razor to decide between them. Occam’s razor, named after the fourteenth century English philosopher William of Occam, states that lacking sufficient experimental evidence for making a choice among competing hypotheses, the simplest hypothesis is the one most likely to be correct. Extraterrestrial visitation as an explanation for UFOs is a hypothesis more complex than warranted to explain the extant observations. More and better data will be required to justify accepting UFO phenomena as evidence of alien visitation.

The alien visitation hypothesis also requires abandoning well-tested laws of science without clear evidence to justify it. The stars in the galaxy are so far apart that alien visitors, even traveling close to the speed of light, would invest an enormous amount of time, not to mention the expenditure of energy required, just to get here.

In conclusion, although humans are most likely not the only intelligent life in the universe, and although extraterrestrial beings may visit Earth sometime in the future, at this time there is no compelling evidence that earth has been or is being visited by aliens. In science, extraordinary claims require extraordinary evidence. The extraterrestrial hypothesis is an extraordinary claim unsupported even by ordinary evidence, much less extraordinary evidence. Therefore, the extraterrestrial hypothesis is currently implausible and insupportable.


  • Achenbach, Joel. Captured by Aliens: The Search for Life and Truth in a Very Large Universe. New York: Simon & Schuster, 1999. Focuses on anomalies outside the mainstream of science as a cultural phenomenon; a large portion deals with UFOs and alien abduction accounts.
  • Clark, Jerome, ed. The UFO Encyclopedia: The Phenomenon from the Beginning. 2 vols. Detroit: Omnigraphics, 1998. Comprehensive coverage of individual UFO sightings and phenomenon, well researched and referenced. Although written from a position of general belief in UFOs, it gives equal time to scientific explanations for observed phenomena.
  • Curran, Douglas. In Advance of the Landing: Folk Concepts of Outer Space. Expanded and updated edition. New York: Abbeville Press, 2001. Places turn-of-the-millennium UFO beliefs in cultural perspective.
  • Good, Timothy. Above Top Secret: The Worldwide UFO Cover-up. New York: William Morrow, 1988. Based on the premise that UFOs have not been satisfactorily explained and that the U.S. government is involved in a massive cover-up, this work purports to uncover reports revealing that mysterious phenomena beyond current knowledge are involved and that the government is concerned.
  • Hynek, J. Allen. The UFO Experience: A Scientific Inquiry. Reprint. New York: Marlowe, 1999. Written shortly after the release of the Condon Report and first published in 1972, this work objectively examines the data on unexplained cases and proposes a means whereby a process of scientific verification could be established. One of the best books ever published on UFOs, it is readily accessible to both scientists and general readers.
  • Randles, Jenny, and Peter Warrington. Science and the UFOs. New York: Basil Blackwell, 1985. This work attempts to show that a hard, cohesive core of evidence does exist among the numerous unexplained UFO cases, but several different phenomena, both physical and psychological, are probably involved.
  • Sagan, Carl, and Thornton Page, eds. UFOs: A Scientific Debate. New York: W. W. Norton, 1972. A thorough examination by authorities representing the entire range of responsible opinion. These experts, from fields ranging from astronomy to psychiatry, discuss virtually all aspects of UFOs; their information is presented, without bias, for scientific scrutiny to illuminate the relevant issues.
  • Story, Ronald D. UFOs and the Limits of Science. New York: William Morrow, 1981. Traces the history of UFOs, examines “hard data” including photographs, and details the ten best-known cases. Concludes by using UFO phenomena in discussing the nature of scientific evidence.

Accident investigation

Air Force, U.S.



Weather conditions

Unidentified Flying Objects, whatever their source, have been seen and photographed around the world. This UFO appeared over Bulawayo, Southern Rhodesia (now Zimbabwe) on December 29, 1953.

(Hulton Archive)