Voyager 6
Voyager 6 was a deep-space probe launched from Earth by the National Aeronautics and Space Administration in the latter half of the 20th century. The sixth probe in the Voyager series, it was designed to collect data and transmit it back to Earth.
Technical information
Voyager 6 was equipped with an antenna and a ground test computer, for use in communicating with Earth. It was designed to transmit a signal relaying its readiness to transmit the information gathered, and to do so upon receipt of a code signal instructing the computer to transmit the data.
Disappearance
Voyager 6 ultimately disappeared into what was once called a black hole (see V'Ger), emerging on the other side of the galaxy, where it fell into the gravitational field of a planet populated by a race of living machines. The inhabitants found the probe to be one of their own kind, primitive, yet kindred. Discovering its simple, 20th century programming, which directed Voyager to collect all data possible and return that information to the creator, the machines interpreted it literally, and constructed a massive vessel around the probe in order to facilitate that directive. On its journey back to Earth, it amassed so much knowledge that it achieved consciousness itself, becoming a living thing. In the 2270's, the entity was detected by Starfleet en route to Earth, and intercepted by the USS Enterprise, whose crew was able to discover the fate of the long-lost probe. (Star Trek: The Motion Picture)
Background
A 1999 launch date was not referenced on screen, but it was included in the Star Trek Chronology. According to Decker's line in the movie, however, it was launched "more than 300 years ago". This suggests a launch date sometime prior to the 1970s. The actual launches of the first (and only) two Voyager probes took place in 1977. Despite Decker's statement, it is unknown what his background was in the history of NASA space exploration, suggesting that he was approximating its launch date.
Apocrypha
The William Shatner novel The Return, where Kirk and Picard join forces to lead an assault on the Borg homeworld and end the recent Borg/Romulan alliance once and for all presents the theory that a Borg transwarp conduit consumed the probe rather than a black hole, and that the planet seen by Spock was in fact, the Borg homeworld. The suggestion continues that the Borg assimilated the probe, yet the assimilation went "afoul" and changed Voyager 6 into a more sentient being.
Star Trek: Legacy presented the theory that the 20th century Borg civilization was a peaceful race. When V'Ger encountered them, they studied its programming, repaired the probe, and sent it on its task. When V'Ger returned to the Sol system it could not find its creator, but "a biological infestation." The probe returned to the Borg homeworld and joined with them, and its programming propagated throughout the Collective. Something of a civil war broke out. Massive amounts of knowledge, including the location of Earth, were lost in the resulting conflict, and the Borg of the 24th century were born.