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A mass relay

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.


See also: Codex/Technology#Mass Relays


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.

A starship performing a flyby along a mass relay's length seconds before going FTL

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.

An asari matriarch once suggested that the asari should build new mass relays of their own, but it is unknown if modern galactic civilization actually has the capacity to do so.


Exiting a mass relay's transit corridor

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.

Alpha Relay[]

See also: Codex/Arrival: The Reapers' Secrets#The Alpha Relay

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.

The Crucible[]

Three galaxy-spanning choices, one method of propagation

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[]

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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.

See Also[]


  • The mass relays were originally known as "phase gates"; a relic of this can be seen in the Codex under Systems Alliance: Military Doctrine, as well as within concept art.
  • 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.[1]