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 Slipspace Drive 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 the laws of known human physics and drops back into normal space at some very distant coordinates. The probe actually navigates through Slipspace and traverses as far and as fast as any UNSC ship.

The technology would revolutionize long-distance communication, 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, especially when the ship is further delayed by the Cole Protocol. Unfortunately, the cost to build a Shaw-Fujikawa ultra-precise low-mass launcher is comparable to a fleet of ships, and even a single probe is worth the value of a capital city on an outlying 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 (presumably lost when the planet was dissolved.

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 Gettysburg; this may have been the launcher from Reach, or a fourth one. 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.