After Hynek's death in 1986, his colleague
Jacques Vallée extended Hynek's classification system by two steps, specifically close encounters of the fourth and fifth kinds, as published in Vallée's book ''Confrontations: A Scientist's Search for Alien Contact'' (1990). The
Mutual UFO Network (MUFON) 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 Vallée'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. While technically not an extension of the Vallée 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. ==See also==