Galactic Barrier
The Galactic Barrier is an energy field composed of negative energy surrounding the rim of the Milky Way Galaxy. It appears as a purple/pink band of energy. No form of transmission can penetrate the barrier. (TOS: "By Any Other Name") Warp travel through the barrier causes extreme sensory distortions. (TOS: "Is There in Truth No Beauty?") The barrier has been encountered on several occasions by Earth and Starfleet vessels.
The first encounter involving an Earth vessel occurred in 2065 with the SS Valiant. 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 USS Enterprise under the command of Captain Kirk discovered the recorder-marker ejected from the Valiant. Decoding its message, they retraced the Valiant's path, being almost destroyed in the process. At that time the barrier did not register at all on the Enterprise's sensors, although the ship's deflectors did react to it.
Entering the barrier was 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, (TOS: "By Any Other Name") 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. 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 is known to have happened at least three times, with the best known example being Mitchell. (TOS: "Where No Man Has Gone Before")
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 Enterprise's sensors registered the barrier as negative energy, but of such magnitude that all instruments were off the dials.
The option to flood the Enterprise's 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. (TOS: "By Any Other Name")
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 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. (TOS: "Is There in Truth No Beauty?")
In the latter two occasions no one was affected in the same manner as Mitchell was just three years earlier.
