Mass Relays are mass transit devices scattered across the Milky Way, usually located within certain star systems. They form an enormous galaxy-wide network enabling rapid interstellar travel. Hailed as one of the greatest achievements of the now extinct Protheans, a mass relay can transport starships instantaneously to another relay within the network, allowing journeys that would otherwise take years or even centuries with only FTL drives to occur in only a handful of days, or even hours.
Mass relays consist of two fifteen-kilometer (or nine-mile) long curved metal arms surrounding a set of revolving, gyroscopic rings five-kilometers across. These rings contain a massive, blue-glowing core of element zero. The relays are made of an unknown but incredibly resilient material, the same material that the Citadel is built from, and are protected by a quantum shield that renders them nearly impervious to damage by locking their structure in place at the subatomic level. They are even capable of surviving a supernova's wake without any damaging effects. They are "cold" objects that don't emit any form of heat or radiation, unlike starships, making them difficult to find if their position has changed over time. Some relays, like the Charon Relay, are "gravitationally anchored" to celestial bodies; others appear to be out in space and are carefully tracked.
Mass relays function by creating a virtually mass-free "corridor" of space-time between each other. This can propel a starship across enormous distances that would take centuries to traverse, even at FTL speeds. Before a vessel can travel, the relay must be given the amount of mass to transit by the ship's pilot before it is moved into the approach corridor. When a relay is activated, it aligns itself with the corresponding relay in the network before propelling the ship across space. At the other end, vessels have no specific exit points, emerging randomly around the relay with positional "drift" of many thousands of kilometers being common. It would thus take an impractical amount of time to mine or trap a relay effectively.
There are two kinds of mass relay, primary and secondary. Primary relays can propel a ship thousands of light years, but only link to one other relay, its "partner". Secondary relays can link to any other relay over shorter distances, only a few hundred light years. After the Rachni Wars, space faring species in Council Space won't open a primary relay without knowing where it links to, in case they run into another powerful and hostile species like the rachni. This caused a rift when the turians found human pioneers, ignorant of this Citadel Council prohibition, trying to open any mass relay they could find while exploring the relay network, eventually leading to the First Contact War.
Many mass relays are currently dormant for unknown reasons, though they can be easily reactivated. A Prothean data cache found on Mars led humans to a mass relay encased in ice and orbiting Pluto, previously thought to be a moon called Charon, which the Charon Relay was eventually named after.
A piece of Prothean artwork depicting a mass relay, the Relay Monument, can be found on the Citadel Presidium. It has been interpreted as either a symbol of Prothean vanity, expressing the relays as their means to build a galaxy-wide empire, or possibly as a symbol of galactic unity, which the relay network also embodies. Tali'Zorah nar Rayya claims that after their long voyage through the galaxy, the quarians have also come to appreciate the aesthetic value of the mass relays.
The mass relays, despite common belief, were created not by the Protheans but by the Reapers. As stated by Sovereign, the Protheans were merely one of many sentient species to find the relays and the Citadel and take advantage of them. When the Reapers wiped out the Protheans, the asari were the next race to find the relays thousands of years later. According to Sovereign, by using the relays and the Citadel, galactic civilizations evolve along the paths the Reapers desire. In addition, the relays serve to accelerate the rate at which those civilizations advance, shortening the time between harvests and streamlining the process of galactic extinction.
However, the Protheans did have a keen interest in the relays, and did manage to crack the secret behind their operation. This enabled them to build the Conduit before they were destroyed by the Reapers.
Commander Shepard also discovers that the Citadel itself is an enormous, inactive mass relay leading directly to dark space, as well as the control center for all mass relays, enabling the Reapers to sever travel between clusters the moment they re-enter the galaxy. Its reactivation is, fittingly, more complex than that of an ordinary mass relay, requiring either a coordinated effort by the keepers or manual intervention by a Reaper.
Mass relays are believed to be indestructible by galactic society, but no known attempts have been made to actually damage or destroy a relay because they are the only means of long distance space travel and thus are too vital to risk.
Dr. Amanda Kenson and her research team calculated that if a large enough mass impacts a relay with enough force, the relay should not be able to withstand it. The consequences of destroying a mass relay are immense: as a huge mass effect engine manipulating massive quantities of energy, a relay could produce an explosion of supernova proportions.
Following a botched rescue attempt for Dr. Kenson that turns into an attempt to delay the Reaper invasion, a large asteroid is purposely steered into the Bahak system's Alpha Relay. The resulting impact tears apart the relay, causing an explosion which annihilates the Bahak system and kills more than 300,000 inhabitants.
During the battle to retake Earth from the Reapers, the superweapon known as the Crucible may be activated. When Commander Shepard interacts with the Catalyst, the AI that controls the Reapers, the Commander is given the option to destroy the Reapers, take control of them, merge organic and synthetic life together, or refuse to act altogether.
In all cases save the last, the energy reaction required to achieve these options is transmitted through the mass relay network from the Citadel. The relays' element zero cores overload from the massive amounts of energy involved. Every point in space for hundreds of lightyears from the relays is affected by the Crucible’s energy, effectively changing the entire galaxy.
The relays are rebuilt by the survivors whichever non-refusal decision is taken. If the control option is chosen, the Reapers themselves assume the responsibility.
If Shepard didn't rally enough of the galaxy's forces for the final fight, the relays suffer more extensive damage. They explode upon transmitting the Crucible's energy, and subsequent rebuilding afterward proves more difficult for the survivors.
Known Mass Relays
|Relay Name||Location (Cluster, System)||Local Points of Interest||Notes|
|Alpha Relay||Viper Nebula, Bahak||Aratoht||The oldest known mass relay in the galaxy. Features unique properties that enable it to expand its range and connect with numerous other relays over long distances. Destroyed, along with the Bahak system, by the Systems Alliance in 2185 in an effort to delay the Reapers' invasion.|
|Arcturus Prime Relay||Unknown, Unknown||May refer to either the Charon Relay in the Local Cluster or the relay in the Arcturus Stream that connects with the Exodus Cluster.|
|Charon Relay||Local Cluster, Sol||Earth||Encased in ice until it was discovered and reactivated by humanity in 2149 CE.|
|Citadel||Serpent Nebula, Widow||A large space station that serves as the seat of the Citadel Council. Unbeknownst to its occupants, it is also an enormous mass relay that the connects directly to the Reaper lair in dark space.|
|Conduit||Local Cluster, Sol||A local mass relay constructed in the city of London during the Reaper occupation of Earth in 2186 to facilitate the transport of victims to the Citadel, which at that time had been moved to the planet's orbit.|
|Conduit||Pangaea Expanse, Refuge||A Prothean-built prototype relay on Ilos. Connects to the Relay Monument on the Citadel, but the trip is only one way.|
|Harsa Relay||Kite's Nest, Harsa||Adek, Camala, Erszbat, Khar'shan|
|Kappa Iota Relay||Unknown, Unknown||A charted but rarely-used relay predating the Rachni Wars. Previously abandoned due to the highly lethal radiation of Turix, the neutron star on the other side. The controversial TR-15 Letus probe was deployed through this relay.|
|Lowas Relay||Unknown, Unknown||Two researchers were arrested for landing on and attempting to tamper with the Lowas Relay without authorization in March of 2186.|
|Mactare Relay||Unknown, Mactare||Taetrus||Connects with the Trebia system.|
|Mu Relay||Hawking Eta, Unknown||Displaced by a supernova 4,000 years ago and enveloped in a cloud of hot gas and dust, making it impossible to detect and seemingly lost forever. The Mu Relay links to several other relays and is the only relay that links to the remote Pangaea Expanse cluster.|
|Omega 2 Relay||Omega Nebula, Sahrabarik||Lorek, Omega, Thunawanuro||One of several mass relays in the Sahrabarik system.|
|Omega 4 Relay||Omega Nebula, Sahrabarik||One of several mass relays in the Sahrabarik system. A mysterious relay seemingly navigable only by the Collectors; no non-Collector vessel has ever returned after using it. Leads to the Tartarus Debris Field and Collector Base.|
|Pelion Relay||Nimbus Cluster, Pelion||Lesuss|
|Rachni Relay||Ninmah Cluster, Maskim Xul||Suen|
|Relay 202||Arcturus Stream, Arcturus||Arcturus Station, Benning||One of multiple mass relays in proximity of Arcturus. Leads into contested space and has an unsavory reputation. Scott Ryder was assigned to an outpost monitoring the relay.|
|Relay 314||Unknown, Unknown||Unauthorized activation by humanity in 2157 CE led to turian retaliation, triggering the First Contact War.|
|Relay Monument||Serpent Nebula, Widow||Located on the Citadel. Mistakenly believed to be a work of art, it is actually a miniature, prototype relay built by the Protheans. Serves as the destination terminus for the Conduit on Ilos, but a return trip is not possible.|
|Seinhand Relay||Unknown, Unknown||Said to have seen a 43% decrease in traffic per a relay traffic report intercepted by the Shadow Broker.|
|Shanxi-Theta Relay||Unknown, Unknown||Shanxi||A key relay in the First Contact War.|
|Sol Relay||Unknown, Unknown||May refer to either the Charon Relay in the Local Cluster or the relay in the Arcturus Stream that connects with the Charon Relay.|
|Tasale Relay||Crescent Nebula, Tasale||Illium|
|Widow Relay||Serpent Nebula, Widow||Bekenstein, Citadel||One of multiple mass relays in proximity of the Citadel.|
|Unknown||Aethon Cluster, Aru||Irune, Oma Ker|
|Unknown||Annos Basin, Pranas||Sur'Kesh|
|Unknown||Apien Crest, Trebia||Digeris, Palaven|
|Unknown||Arcturus Stream, Arcturus||Arcturus Station, Benning||One of multiple mass relays in proximity of Arcturus.|
|Unknown||Argos Rho, Hydra||Intai'sei, Pinnacle Station|
|Unknown||Armstrong Nebula, Unknown|
|Unknown||Artemis Tau, Unknown||Therum|
|Unknown||Athena Nebula, Parnitha||Lusia, Niacal, Thessia|
|Unknown||Attican Beta, Hercules||Feros|
|Unknown||Caleston Rift, Balor||Arvuna, Caleston|
|Unknown||Eagle Nebula, Imir||Anhur, Korlus, Jarrahe Station, Wrill|
|Unknown||Exodus Cluster, Utopia||Eden Prime, Terra Nova|
|Unknown||Far Rim, Dholen||Haestrom|
|Unknown||Gemini Sigma, Han|
|Unknown||Hades Gamma, Antaeus|
|Unknown||Hades Nexus, Hekate||Trident|
|Unknown||Hawking Eta, Chandrasekhar||The description for Corang mentions a relay in the Century system of Hawking Eta, but no such relay is shown on the Galaxy Map.|
|Unknown||Horse Head Nebula, Pax||Noveria|
|Unknown||Hourglass Nebula, Osun|
|Unknown||Ismar Frontier, Aquila||Zorya|
|Unknown||Kepler Verge, Newton||Ontarom|
|Unknown||Krogan DMZ, Aralakh||Tuchanka|
|Unknown||Maroon Sea, Unknown|
|Unknown||Minos Wasteland, Fortis||Gellix, Invictus|
|Unknown||Nubian Expanse, Dakka||Yamm|
|Unknown||Pangaea Expanse, Refuge||Ilos||Connects with the Mu Relay.|
|Unknown||Perseus Veil, Tikkun||Rannoch|
|Unknown||Petra Nebula, Vetus||Elysium, Grissom Academy|
|Unknown||Pylos Nebula, Nariph|
|Unknown||Rosetta Nebula, Enoch||2175 Aeia||Formerly on the list of forbidden relays that led to uncharted space.|
|Unknown||Sentry Omega, Hoc||Virmire|
|Unknown||Shadow Sea, Iera||Horizon|
|Unknown||Sigurd's Cradle, Skepsis||2181 Despoina, Sanctum, Watson|
|Unknown||Silean Nebula, Kypladon||Cyone, Dekuuna, Nevos|
|Unknown||Styx Theta, Unknown|
|Unknown||The Phoenix Massing, Tassrah||Aite, Ekuna, Heretic Station|
|Unknown||The Shrike Abyssal, Xe Cha||Heshtok, Talis Fia|
|Unknown||Titan Nebula, Haskins|
|Unknown||Valhallan Threshold, Raheel-Leyya||Garvug|
|Unknown||Voyager Cluster, Unknown|
|Unknown||Unknown, Turix||A relay located about 18,000 light years from Earth, orbiting the neutron star Turix. Connects to the Kappa Iota Relay. Destination of the TR-15 Letus probe.|
|Unknown||Unknown, Unknown||Tartarus Debris Field, Collector Base||Connects with the Omega 4 Relay.|
- Cerberus Daily News
- The mass relays were originally known as "phase gates"; a relic of this can be seen in the Codex under Systems Alliance: Military Doctrine.
- The mass relays evolved from more monolithic designs to their final "tuning fork" shape (as described in The Art of Mass Effect).
- According to Drew Karpyshyn, one of the rough ideas for ending the Shepard trilogy of games involved luring the Reapers through the relays then detonating the entire network, at the cost of isolating the galactic community from each other. While elements of this can be seen in Mass Effect 3, issues encountered include working out the details and wondering how to proceed with this in the next few games.