Mass Relay

Mass Relays are mass transit devices scattered throughout the galaxy, usually located within star systems. They form an enormous network allowing interstellar travel. Hailed as one of the greatest achievements of the extinct Protheans, a mass relay can transport starships instantaneously to another relay within the network, allowing for journeys that would otherwise take years or even decades with only FTL drives.

Specifications
Mass relays consist of two fifteen-kilometer 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 being damaged. They are "cold" objects that don't emit heat or radiation, unlike starships, making them difficult to find if their position changes. Some relays, like the Charon Relay, are "gravitationally anchored" to planets; 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, reaching speeds of hundreds of millions of times the speed of light. 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 before propelling the ship across space.

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 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.

Origins
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 alien races 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, 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 Reaper harvests.

However, the Protheans did have a keen interest in the relays, and managed 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 to dark space, as well as the control center for all the relays, enabling the Reapers to sever travel between clusters. 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
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 too vital to risk. Prior to the events of Arrival, however, 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. This proves true when during Arrival, 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 its more than 300,000 inhabitants.

The Crucible
During the battle to retake Earth from the Reapers, the superweapon known as the Crucible is activated. When Commander Shepard interacts with the Catalyst, the AI that controls the Reapers, the Commander is given the option to destroy the Reapers, control the Reapers, or merge organic and synthetic life together. In all cases, the energy reaction required to achieve those options is transmitted through the mass relay network from the Citadel. If the control option is chosen, the Citadel survives intact, but the relays are severely damaged and the Reapers rebuild them. If the destroy or synthesis options are chosen, the Citadel and the relays are severely damaged, but eventually are rebuilt.

Identified Mass Relays

 * Alpha Relay - A mass relay with unique properties located in the Bahak system, on the edge of the galaxy. This served as the Reapers' "back door" in the event that the Citadel's hidden relay was unavailable to initiate a harvest.
 * Charon Relay - A mass relay in the Sol system. Before its activation in 2149, it was encased in a layer of ice and thought to be a satellite of Pluto.
 * Citadel - A space station which, unknown to its occupants, is also a mass relay. It is the gateway by which the Reapers normally emerge from dark space.
 * Conduit - A Prothean-built mass relay residing on Ilos. It is a one-way relay, with its destination terminus being the Relay Monument.
 * Harsa Relay - A relay in the batarian home system that links the Kite's Nest directly to the Exodus Cluster.
 * Kappa Iota Relay - A charted but rarely-used relay predating the Rachni Wars, abandoned due to the highly lethal radiation of the neutron star on the other side. The controversial TR-15 Letus probe was deployed through this relay.
 * Lowas Relay - Two researchers were arrested for landing on and attempting to tamper with the Lowas Relay without authorization in March of 2186.
 * Mactare Relay - A relay located in the Mactare system, home of the turian colony of Taetrus.
 * Mu Relay - A lost relay which had been displaced by a supernova. It is the only relay that links to the remote Pangaea Expanse cluster.
 * Omega 2 Relay - One of several mass relays in the Sahrabarik system.
 * Omega 4 Relay - One of several mass relays in the Sahrabarik system. Numerous explorers have tried to cross it, but none have ever returned. The mysterious Collectors can traverse it safely at will, however.
 * Pelion Relay - A relay that links the galaxy to the Nimbus Cluster.
 * Relay 314 - A dormant relay whose attempted activation by human explorers triggered the First Contact War.
 * Relay Monument - A Prothean-built miniature mass relay on the Citadel. It is the destination terminus of the Conduit.
 * Seinhand Relay - A relay mentioned in a relay traffic report intercepted by the Shadow Broker.
 * Shanxi-Theta Relay - A key relay in the First Contact War, near the human colony Shanxi.
 * Tasale Relay - A relay that links the galaxy to the Crescent Nebula.
 * Widow Relay - A relay that links the galaxy to the Serpent Nebula.

Trivia

 * 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).

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