In ufology, a close encounter is an event in which a person witnesses an unidentified flying object. This terminology and the system of classification behind it was started by astronomer and UFO researcher J. Allen Hynek, and was first suggested in his 1972 book The UFO Experience: A Scientific Inquiry. He introduced the first three kinds of encounters; more sub-types of close encounters were later added by others, but these additional categories are not universally accepted by UFO researchers, mainly because they depart from the scientific rigor that Hynek aimed to bring to ufology.
Sightings more than 500 feet (150 m) from the witness are classified as "Daylight Discs," "Nocturnal Lights," or "Radar/Visual Reports." Sightings within about 500 feet are subclassified as various types of "close encounter." Hynek and others argued a claimed close encounter must occur within about 500 feet 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 a 1977 film, Close Encounters of the Third Kind, which is named after the third level of the scale. Posters for the film recited the three levels of the scale, and Hynek himself makes a cameo appearance near the end of the film.
UFOs seen in the daytime, generally having discoidal or oval shapes.
UFO reports that seem to have radar confirmation. Though seeming to offer harder evidence of physical reality, the vagaries of radar propagation often make the ascription ambiguous.
Close Encounters of the First kind
Visual sightings of an unidentified flying object seemingly less than 500 feet away that show an appreciable angular extension and considerable detail.
Close Encounters of the Second kind
A UFO event in which a physical effect is alleged. This can be interference in the functioning of a vehicle or electronic device; animals reacting; a physiological effect such as paralysis or heat and discomfort in the witness; or some physical trace like impressions in the ground, scorched or otherwise affected vegetation, or a chemical trace.
Close Encounters of the Third kind
UFO encounters in which an animated creature is present. These include humanoids, robots, and humans who seem to be occupants or pilots of a UFO.
The UFO researcher Ted Bloecher proposed six subtypes for the close encounters of the third kind in the Hynek's scale.
A: An entity is observed only inside the UFO.
B: An entity is observed inside and outside the UFO.
C: An entity is observed near to a UFO, but not going in or out.
D: 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.
E: An entity is observed, but no UFOs are seen and no UFO activity has been reported in the area at that time.
F: No entity or UFOs are observed, but the subject experiences some kind of "intelligent communication".
Subtypes D, E, and F may be unrelated to the UFO phenomenon.
Extensions of Hynek's scale
Close Encounters of the Fourth kind
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 erstwhile associate Jacques Vallee argued in the Journal of Scientific Exploration that a CE4 should be described as "cases when witnesses experienced a transformation of their sense of reality", so as to also include non-abduction cases where absurd, hallucinatory or dreamlike events are associated with UFO encounters.
Close Encounters of the Fifth kind
A UFO event that involves direct communication between aliens and humans. This type of close encounter was named by Steven M. Greer's CSETI group and are described as bilateral contact experiences through conscious, voluntary and proactive human-initiated cooperative communication with extraterrestrial intelligence. This is very similar to some "contactees" of the 1950s who claimed regular communication with benevolent aliens.
Close Encounters of the Sixth kind
Death of a human or animal associated with a UFO sighting.
Close Encounters of the Seventh kind
The creation of a human/alien hybrid, either by sexual reproduction or by artificial scientific methods.