Transphasic torpedoes were starship-mounted projectile weapons of unprecedented destructive potential. Developed along with ablative generators by Captain Kathryn Janeway and the crew of the USS Voyager in the late-24th century to defend against the Borg, a single torpedo was capable of destroying a Borg cube.

In 2404, Janeway, at this point an admiral, embarked on an illegal mission to change history by traveling back in time and securing Voyager's safe return to the Alpha Quadrant sixteen years early. Bringing the advanced technology with her, Admiral Janeway used a temporal rift to travel to 2378, and assisted the crew in the installation of the transphasic torpedoes. They proved extremely effective at combating the Borg vessels guarding a transwarp hub in grid 986, allowing the crew to destroy two cubes. Later, the crew used transphasic torpedoes to destroy the interspatial manifolds supporting the structure of the hub, causing the Borg's transwarp network to collapse. (VOY: "Endgame")

ApocryphaEdit

According to the Pocket Books novel Greater than the Sum, transphasic torpedoes were in fact kept by Starfleet as the weapon of last resort to be deployed to starships only when all else had failed against the Borg. They were the one and only thing Starfleet knew the Borg had not yet adapted to and for that reason wanted to keep this ace in the hole for as long as possible. As a result, during the subsequent Borg invasion, the Enterprise-E is the only Federation ship equipped with transphasic torpedoes, engaging in rapid hit-and-run missions to limit the Borg's opportunities to adapt to them.

The warhead technology of the torpedoes was also revealed in the novel. It is based on generating a destructive subspace compression pulse. Upon detonation the torpedo delivers the pulse in an asymmetric superposition of multiple phase states. Shields can only block one subcomponent of the pulse. The other subcomponents deliver the majority of the pulse to the target. Every torpedo has a different transphasic configuration, generated randomly by a dissonant feedback effect to prevent the Borg from predicting the configuration of the phase states.

In the novel Lost Souls, set in 2381, during a Borg invasion, the Borg finally adapted to the transphasic torpedoes when Starfleet was forced to send the torpedo specs to the entire fleet following a mass incursion of Borg cubes.

In the game Star Trek Online, transphasic torpedoes are not nearly as powerful as depicted in Star Trek: Voyager, but they do have a chance to penetrate a target's shields.