In ufology, a close encounter is an event in which a person witnesses an unidentified flying object (UFO) at relatively close range, where the possibility of mis-identification is presumably greatly reduced. This terminology and the system of classification behind it were first suggested in astronomer and UFO researcher J. Allen Hynek's book The UFO Experience: A Scientific Inquiry (1972). Categories beyond Hynek's original three have been added by others but have not gained universal acceptance, mainly because they lack the scientific rigor that Hynek aimed to bring to ufology. Distant sightings more than 500 ft from the witness are classified as daylight discs, nocturnal lights, or radar/visual reports. Sightings within about 500 ft are sub-classified as various types of close encounters. Hynek and others argued that a claimed close encounter must occur within about 500 ft to greatly reduce or eliminate the possibility of misidentifying conventional aircraft or other known phenomena. Hynek's scale became well known after being referenced in the classic sci-fi film Close Encounters of the Third Kind (1977), which is named after the third level of the scale. Promotional posters for the film featured the three levels of the scale, and Hynek himself makes a cameo appearance near the end of the film.

Hynek's scale

Hynek devised a six-fold classification for UFO sightings. The six levels are arranged according to increasing proximity:

Bloecher subtypes

UFO researcher Ted Bloecher proposed six sub-types for the close encounters of the third kind in Hynek's scale: 1. Aboard: an entity is observed only inside the UFO. 2. Both: an entity is observed inside and outside the UFO. 3. Close: an entity is observed near to a UFO, but not going in or out. 4. Direct: an entity is observed—no UFOs are seen by the observer, but UFO activity has been reported in the area at about the same time. 5. Excluded: an entity is observed, but no UFOs are seen and no UFO activity has been reported in the area at that time. 6. Frequence: no entity or UFOs are observed, but the subject experiences some sort of "intelligent communication".

Extensions of Hynek's scale

After Hynek's death in 1986, his colleague Jacques Vallee extended Hynek's classification system by two steps, specifically close encounters of the fourth and fifth kinds, as published in Vallee's book Confrontations: A Scientist's Search for Alien Contact (1990). The Mutual UFO Network immediately adopted the extensions to the classification scale and has used them ever since.

Close encounters of the fourth kind (CE4)

A close encounter of the fourth kind is a UFO event in which a human is abducted by a UFO or its occupants. This type was not included in Hynek's original close encounters scale. Hynek's former associate Jacques Vallée argued in the Journal of Scientific Exploration that the fourth kind should refer to "cases when witnesses experienced a transformation of their sense of reality", to also include non-abduction cases where absurd, hallucinatory or dreamlike events are associated with UFO encounters. The film The Fourth Kind (2009) makes reference to this category.

Close encounters of the fifth kind (CE5)

As stated in Vallee's Confrontations (1990), a close encounter of the fifth kind is where an alien abductee receives some manner of physical effect from their close encounter, typically either injury or healing. Several years after Vallee's classification updates, some preferred that a close encounter of the fifth kind instead refer to human-initiated contact with extraterrestrial life forms or advanced interstellar civilizations, claiming direct communication between aliens and humans. This alternate interpretation of what a close encounter of the fifth kind (ce5) should represent has been contributed to Steven M. Greer. While technically not an extension of the Vallee scale that measures result-oriented data, this replacement of the originally coined CE5 classification has become popular in marketing human-initiated contact events. In a CE5 event, individuals or groups use specific protocols to establish communication or interaction with extraterrestrial beings. These protocols primarily involve the use of contact meditation and use of sounds or signals. Close encounters of the fifth kind is also referred to as human initiated close encounter.