Galactic barrier
The galactic barrier was an energy field composed of negative energy surrounding the rim of the Milky Way Galaxy. Invisible to both the naked eye and visual recording equipment at a distance, at close range the barrier shone with a purple- to pink-colored glow. No form of transmission was known to be capable of penetrating the barrier. () Warp travel through the barrier caused extreme sensory distortions. () The barrier had been encountered on several occasions by Earth and Starfleet vessels. () The Stilph inhabited a region just inside the barrier. ()

History
The first encounter involving an Earth vessel occurred in 2065 with the . The ship was swept into the barrier by a magnetic storm. Subsequent events resulted in the destruction of the Valiant and the loss of all hands, leaving the barrier's formal discovery to wait two hundred more years.

In 2265, the  under the command of  James T. Kirk discovered the disaster recorder ejected from the Valiant. Decoding its message, they retraced the Valiants path, being almost destroyed in the process. At that time the barrier did not register at all on the Enterprises sensors, although the ship's deflectors did react to it. ()

Entering the barrier is extremely hazardous. Shields could not block all the energies of the barrier; the leakage was sufficient to heat up the hull to over 2,000 degrees, () damage several systems and disable the warp drive of the Enterprise. Also passing the shields and hull were strange electric shocks to the nervous systems of the Human crew members with high esper ratings. In most cases this caused death from brain damage, as was the case for nine Enterprise crewmembers. Individuals with the highest ESP ratings such as Dr. Elizabeth Dehner's 089 and Lieutenant Commander Gary Mitchell's 091 survived the shock and were altered as a result. They developed a multitude of psionic powers at a geometric rate. They began to view their former friends and shipmates as lower lifeforms. This transformation was known to have happened at least three times, with the best-known example being Mitchell. ()

In 2268, Kelvans from the Andromeda Galaxy entered the Milky Way Galaxy through the barrier. Their advanced multigenerational ships were damaged by the barrier and subsequently destroyed. The Kelvan survivors of one ship required a new vessel for the journey home. They hijacked the USS Enterprise and refitted it to survive the barrier and the intergalactic journey. Presumably, due to improvements in sensor technology, this time the Enterprises sensors registered the barrier as negative energy, but of such magnitude that all instruments were off the dials. 

The option to flood the Enterprises warp nacelles with positive energy through control valves was available to Kirk, but not taken, during the passage through the barrier. Combination of that positive energy and the negative energy of the barrier would have destroyed the ship. ()

Later that year, the Enterprise left the Milky Way Galaxy when Larry Marvick, driven mad by the sight of Medusan Ambassador Kollos, sought to escape his tormenting visions. During that excursion the ship was stranded inside a space-time continuum void deep within the barrier. No sensor data or reference points were available to exit the void in the right direction. Fortunately, Kollos was able to help pilot the ship out. ()

During the s investigation of a subspace rift left by the Dark Matter Anomaly in 3190, Cleveland Booker experienced an energy surge while unsuccessfully attempting to jump the ship via the spore drive. It was subsequently found that he had been exposed to particles unique to the galactic barrier, indicating that the DMA originated outside the galaxy. ()

Later that same year, the decision was made to cross the galactic barrier to initiate first contact with the unknown Species 10-C after it was discovered that the DMA couldn't be destroyed. By this point, the energies within the galactic barrier had grown strong enough that shields were actually weakened by passing through, forcing the crew to find and hide in bubbles of stable space to enact repairs before continuing their travels. ()

Appendices

Background information 
The script of "Where No Man Has Gone Before" likens the galactic barrier to the aurora borealis.

The composition of the barrier was somewhat controversial, based on the analyses during both encounters, as it was initially totally unknown. Negative (as in having no detectable) energy, radiation and density,  but later on in  it is directly and clearly stated to be in fact negative energy; not just negative (in) energy. This is evident from the plan to destroy the ship by venting positive energy inside the barrier to cause an explosion.

In Star Trek: Star Charts, on page 11, the barrier is depicted as a toroidal shape that surrounds the entire galactic disc. It's not situated at the rim as directly stated on screen in , but only thickest at the rim; over 10,000 light years thick, starting out from a distance of 40,000 light years from the center of the galaxy, outward. Closer to the UFP's neighborhood of the galaxy, the barrier thins down to a thousand or so light years above and below. Even a large number of visible stars in the outer parts of the galactic disc are situated inside the barrier in the Star Trek: Star Charts cross-sectional view of the galaxy. 

Furthermore, the Valiant, the Enterprise, the Kelvans etc. didn't exit, enter or try to exit the galaxy throughout the rim, but from the closer and thinner above and below parts of the barrier. Visual evidence presented in the episodes would however contradict this notion. Star Trek: Star Charts also states that the neurogenic effects of the barrier are unpredictable. The energy field of the barrier joins up with the Great Barrier at the center of the galaxy. It is stated to be a distinct but related energy field. Star Trek: Star Charts would however seem to contradict itself on page 13, as the  is stated to have started its journey home from a distance of approximately 50,000 light years from the galactic center. This would have placed it well within the barrier based on the earlier depiction on page 11.

A further contradiction occurs in  which mentions a Starfleet vessel, the , travelling to the Lesser Magellanic Cloud for a five-year mission of exploration. Assuming this refers to the galaxy also known as the Small Magellanic Cloud, that would mean the Leondegrance left the Milky Way, with no reference to it encountering the barrier.

Apocrypha 
Greg Cox's Star Trek: The Next Generation - Q Continuum novels state that the galactic barrier was in fact created by the Q Continuum 500,000 years ago as a method of preventing the powerful being known only as "0" from returning to the Milky Way Galaxy, from which he was banished for his destruction of the Tkon Empire. Even in 2374, the galactic barrier is still extremely dangerous for starships to travel through. These novels also explain that the telepathic powers that occur as a side effect of exposure to the barrier in telepaths are caused by the power of the Q that exist in the barrier, while those who develop a god complex are 'infected' by the presence of 0, who constantly attempts to affect beings on the other side to give himself access to the galaxy once again. During an attack by the Calamarain, the  was forced to hide in the barrier, but Lieutenant Reginald Barclay managed to come up with the idea of using the bio-neural gel packs to absorb the psychic energy from the barrier to enhance their shields, providing some improved protection from the barrier's psychic influence. As the Enterprise departed the barrier, Betazoid scientist Lem Faal managed to briefly open an artificial wormhole leading to the other side. However, as this resulted in 0 briefly returning to our galaxy and only being defeated by the alliance of Q and the Calamarain, it is unlikely that the Federation will ever use this technology to begin exploration in other galaxies.

In Q-Squared, a disembodied Q is the one who takes over Dr. Elizabeth Dehner and Lieutenant Commander Gary Mitchell.

In the novel The Wounded Sky the galactic barrier was a temporary phenomenon that no longer existed in 2275. It was the leading wavefront of a "megabubble" that was created by an exploding "metastar" in one of the Milky Way Galaxy's satellite globular clusters.

In the Star Trek: Stargazer series of books, the barrier can be easily traveled through in 2333 due to improvements in shield technology. There are a number of star systems outside the barrier, including one populated by the descendants of the SS Valiant crew, who all have minor telepathic and telekinetic abilities. In the novel , a member of the Nuyyad race goes on a rampage after becoming god-like in power from exposure to the barrier. He is eventually killed; however, he continues to exist as an energy lifeform and returns to the barrier.

In the novel Captain's Glory, it is theorized that the barrier was created by the first humanoid race 4 billion years ago to protect the Milky Way Galaxy from the Totality, a dark matter-based lifeform.

According to the game , the library computer entry on the planet killer mentions a school of thought which believes that the galactic energy barrier may have been created by some ancient race as a safeguard in order to keep things such as the planet killers out of the galaxy. The entry also states that if this is true, then the barrier is not very effective.

According to the novel The Higher Frontier, Humans are not a naturally psychic race, as they lack a paracortex or similar structure in their brains. Instead, every Human esper is bonded with a Spectre, a non-corporeal entity that provides its host with psychic ability. The galactic barrier awakened and supercharged Mitchell and Dehner's Spectres, driving them insane in the process. The galactic barrier apparently has no effect on naturally telepathic species, which explains why Spock wasn't affected.

In the comic , the Romulans established a secret base near the galactic barrier in 2280 in order to study its effects. They learned that the barrier's power allowed it to give psychic abilities to Humanoids. Romulan Star Empire scientists eventually discovered a means of leeching the barrier of its power and bestowing it on their test subjects. Their base of operations and their experiment was found out about by Vulcan spy Xon whose true nature was discovered which resulted in his capture. The Enterprise was involved in a rescue attempt and during the skirmish, the Romulan ship was lost in the barrier where its scientific discovery was lost to the Empire.°

Related topics 
 Beyond the Galactic Edge, Humanity's Quest for Infinity

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