Slipspace COM launcher

The Slipspace Communications (COM) Launcher is an experimental piece of technology invented by the UNSC and is the only technology that enables faster than light communications except for the Forerunner technology that synchronizes the Halo firing sequence.

A communications probe launched by an underground gauss accelerator and a Shaw-Fujikawa Translight Engine creates a Slipspace rift in high orbit so that the COM probe goes through Slipspace like a "bullet" on an ultra-precise trajectory. It rips through normal time-space, travels through Slipspace, and drops back into normal space at its pre-set coordinates. The probe can traverse as far and as fast as any UNSC ship.

The wide-spread use of the technology could revolutionize long-distance communication, as it is far faster than conventional radio communication. If each ship were outfitted with such a device, messages could be delivered without requiring a ship to travel through space to manually give the message. Unfortunately, the cost to build a Shaw-Fujikawa ultra-precise low-mass launcher is comparable that of a fleet of ships, and even a single probe is worth the value of a capital city on an Outer Colony.

As of 2551 February 20, it's believed that there were only three launchers: one each on Earth and Reach, and a secret one on Onyx (lost when Onyx Sentinels destroyed it after sending a message to Earth).

Halo Graphic Novel
According to page 122 of the Halo Graphic Novel, some sort of superluminal communications system was used by Catherine Halsey to contact Earth from on board the Gettysburg. This may have been the launcher from Reach or a fourth, secret launcher. However, the exact system used isn't specified. All that's clear is that it was fast. and that Halsey was willing to risk several lives to keep it secret.

Trivia

 * Beside the Slipspace COM Launchers, another method of supposedly faster-than-light communication referred to as a "Slipbeacon" is mentioned in Halo: Blood Line Issue 1.