Mass relay

Mass relays were mass transit devices scattered across the Milky Way, usually positioned within particular star systems, that together formed a galaxy-wide network for rapid interstellar travel. A relay could hurl a starship instantaneously to another relay in the network, collapsing journeys that would otherwise have taken years or centuries with ordinary faster-than-light drives into a matter of days or hours. Long hailed as the greatest achievement of the extinct Protheans, the relays were the spine of galactic commerce and the chief reason the scattered species of the galaxy could function as a single community centered on the Citadel.

Structure and operation#

A mass relay consisted of two curved metal arms roughly fifteen kilometers long surrounding a set of revolving, gyroscopic rings about five kilometers across. At the heart of these rings sat a massive, blue-glowing core of element zero. The relays were fashioned from an unknown but extraordinarily resilient material, the same substance from which the Citadel was built, and were protected by a quantum shield that locked their structure at the subatomic level, rendering them nearly impervious to harm. A relay could survive the wake of a supernova without damage. They were cold objects, emitting neither heat nor radiation, which made them difficult to locate if their position had shifted over the ages. Some, like the Charon Relay, were gravitationally anchored to celestial bodies, while others drifted in open space and had to be carefully tracked.

A relay functioned by creating a virtually mass-free corridor of space-time between itself and another relay, propelling a vessel across distances that would take centuries even at faster-than-light speeds. Before transit, a ship's pilot supplied the relay with the amount of mass to be moved, after which the vessel was guided into the approach corridor. The relay then aligned itself with its destination counterpart and flung the ship across the void. Vessels emerged with no fixed exit point, scattering around the destination relay with positional drift of many thousands of kilometers being common, which made mining or trapping a relay impractical.

Primary and secondary relays#

The network divided into two kinds of relay. Primary relays could propel a ship across thousands of light years but connected only to a single partner relay. Secondary relays could link to any other relay across shorter distances, generally only a few hundred light years. After the Rachni Wars, the spacefaring species of Council space refused to open a primary relay without first knowing where it led, fearing they might stumble upon another powerful and hostile species as they had the rachni. This caution caused friction when the turians encountered human pioneers who, ignorant of the Citadel Council's prohibition, were attempting to open any relay they could find. That clash eventually escalated into the First Contact War.

Many relays lay dormant for reasons that were never fully understood, though they could be reactivated with relative ease. A Prothean data cache discovered on Mars led humanity to a relay encased in ice and orbiting Pluto, long mistaken for a moon named Charon, which gave the Charon Relay its name.

True origin#

Despite common belief, the mass relays were not built by the Protheans. As the Reaper Sovereign revealed, the Protheans were merely one of many sentient species to discover the relays and the Citadel and exploit them. When the Reapers exterminated the Protheans, the asari were the next race to find the network thousands of years later. By drawing civilizations to use the relays and the Citadel, the Reapers ensured those civilizations evolved along predictable paths, accelerating their advancement, shortening the interval between harvests, and hastening the process of galactic extinction.

The Protheans nevertheless took a keen interest in the relays and managed to unravel the secret of their operation. This knowledge allowed them to construct the Conduit, a one-way prototype relay on Ilos, before they were destroyed. It was later discovered that the Citadel itself was an enormous, inactive mass relay leading directly to dark space, as well as the master control hub for the entire network, allowing the Reapers to sever travel between clusters the instant they re-entered the galaxy.

Destruction and the Reaper War#

The galactic community believed the relays to be indestructible, and no serious attempt had ever been made to damage one, since they were too vital to risk. The researcher Dr. Amanda Kenson and her team calculated that a sufficiently large mass hitting a relay with enough force could overwhelm it, and that the resulting release of energy from such a massive engine would rival a supernova. Following a rescue attempt for Kenson that became an effort to delay the Reaper invasion, an asteroid was deliberately steered into the Alpha Relay in the Bahak system. The impact tore the relay apart in an explosion that annihilated the Bahak system and killed more than three hundred thousand inhabitants.

During the final battle to retake Earth from the Reapers, the relay network served as the conduit for the Crucible. When Commander Shepard reached the Catalyst and chose to destroy the Reapers, control them, or merge organic and synthetic life, the resulting energy was transmitted through the relay network from the Citadel, overloading the relays' element zero cores and altering every point of space for hundreds of light years around them. In the aftermath the relays were rebuilt by the galaxy's survivors. Where Shepard's forces had been insufficient, the relays suffered heavier damage and even exploded while transmitting the Crucible's energy, leaving the survivors a far harder task of reconstruction.