Halo Array

"Halo doesn't kill Flood, it kills their food! Humans, Covenant, whatever. We're all equally edible. The only way to stop the Flood is to starve them to death. And that's exactly what Halo is designed to do: wipe the galaxy clean of sentient life."

- Cortana

The Halo Array, or individually as Halo, alternatively known as the Sacred Rings by the Covenant, Fortress Worlds by their Forerunner creators, and Installations by the Monitors that run them, is a system of colossal ring-world superweapons.

The Halos were constructed by order of the Master Builder at the conclusion of the Human-Forerunner wars with the intent of being an ultimate weapon. Eventually, five were lost and the remaining seven were used by the Forerunners as a last resort when combating the parasitic Flood. When nothing else could be done, they activated the rings, killing all sentient life forms within three radii of the galactic center.

In addition to serving as weapons of last resort, the Halo installations are research facilities, mainly for the study of the Flood.

Background
The Halo Array is a network of ring-shaped artificial worlds created by the Forerunners in order to kill all sentient life within range of the array, virtually the entire galaxy. Rather than a weapon of war, these were used as a last resort against the Flood, a parasitic extra-galactic species that threatened to infest every sentient lifeform in the galaxy. By activating the rings, the Forerunners denied the Flood access to these lifeforms, effectively starving the species to virtual extinction, though it clung to life in a number of Forerunner facilities, including some Halo installations. The two known Halo installations, referred to as Alpha Halo and Delta Halo respectively, orbit gas giants, though it is not known whether this is the case of all the Halos, and Alpha Halo maintained an atmospheric mining facility until it was destroyed by the Covenant. Though separated by thousands of light-years, the twelve installations are networked together and capable of remote activation at the Ark, an installation outside the galaxy. Each Halo installation has a maximum effective range of 25,000 light-years. The methods utilized by the array to conduct this "mass sterilization protocol" involve the superluminal conveyance of a burst of cross-phased super-massive neutrinos. These particles are tuned to emit a harmonic frequency that destroys the nervous system of any life form within range. Lower organisms such as plants, fungi and bacteria are unaffected, along with inanimate structures. Precursor technology, however, is extremely susceptible to damage from the Halo effect.

The Halo Array in its entirety has been fired once in known history, approximately 100,000 years ago, by the Didact in order to stop the Flood from overwhelming the galaxy. Despite being the network's builders, the Forerunners were killed in this last suicidal attempt to save the rest of the universe from a worse fate. The array has nearly been activated three times since; Alpha Halo was almost activated by the Master Chief before being deactivated by Cortana, while Delta Halo was activated by Commander Miranda Keyes, forced by the Chieftain of the Jiralhanae Tartarus, and Sergeant Johnson was forced by the Prophet of Truth to activate the Ark. None of these attempts were successful. A replacement Halo for the destroyed Alpha Halo was activated outside of the array's range, eliminating the freed Flood, but did not activate the rest of the array.

Each Halo Installation is given a designation number, from 01 to 07, and is overseen by a Monitor. The Monitors are given control of the Installations' Sentinels, Constructors, and Enforcers, and are responsible for containing the Flood test subjects in their research facilities and protecting the Halo from intruders.

The Array also encompasses Installation 00, also known as "the Ark", from which the array can be remotely and safely activated while out of firing range. Inhabitants on the Ark are able to survive the effects. The Forerunners also built Shield Worlds as a means to escape the effect without having to leave the galaxy.

Construction
The Halos were constructed by the Forerunner Builders several hundred years after the Flood had first been encountered and driven from the Milky Way. The most extreme faction of Builders, which at the time controlled the Forerunner Council, claimed that more extreme measures should be used to protect the galaxy against a possible Flood resurgence. Although they faced strong opposition from the Prometheans, including the Didact, the faction won the approval of the Council and commenced work on the Array.

Twelve Halos were commissioned by the Master Builder, Faber, but only one of them survived to be used in the Forerunners' final plan. Another array of six installations remained under the Librarian's control at Installation 00.

Forerunner-Flood War
The first test of a Halo installation was conducted by 05-032 Mendicant Bias, who fired one of the weapons at Charum Hakkor on a low power setting. He then took the Halo with him as per his mission to assault the Gravemind, but both he and the ring vanished and were not seen for another 43 years.

The other eleven Halo installations were taken to the Forerunner Capital during the tribunal against Master Builder Faber, where it was to be decided whether to decommission them. However, in the midst of the trial, Mendicant Bias, turned rampant by the Gravemind, appeared with the twelfth Halo and assaulted the Capital. He attempted to seize control of the remaining Halos and fire them, but was only able to control five. One of those five was destroyed by the combined tidal forces of the Capital, the firepower of the Forerunner fleets, and the stress of a recent slipspace jump. Seven of the Halos were recalled to Installation 00 via slipspace jump, yet only one escaped before the portal collapsed.

Little else is known of their extensive history, though it is known that at least one was functional for 101, 217 local years. After exhausting every other strategic option, their creators had no choice but to activate the Halos' main weapons, terminating all sentient life in the galaxy.

Post Forerunner-Flood War
The seven Halos then remained relatively dormant for approximately one hundred thousand years, though at least one of them experienced a major Flood outbreak, and they might have experienced brief visits by other species. Eventually, one of them, Installation 04, was discovered by the theocratic alien superpower known as the Covenant. The Covenant, who revered the Forerunner as gods, believed that the Halo's main weapon was actually a source of "Divine Wind" meant to propel them on a path they called the "Great Journey". During their investigations on Installation 04, the Covenant encountered the Flood; they quickly, but only partially, contained the outbreak. Soon, some of the crew of the human vessel, led by Captain Jacob Keyes accidentally broke the containment, expecting to find a large Covenant weapons cache. A massive four-way battle ensued among the Covenant, the Sentinels led by 343 Guilty Spark, the Flood, and the humans. This finally culminated when SPARTAN John-117 detonated the fusion engines of the Pillar of Autumn on the surface of the installation, which compromised the structural integrity of the ring, and, due to the gravitational momentum and inertia still perpetuating the ring in orbit around Threshold, tore the ring apart. The remains of Installation 04 were scattered as debris in space. The explosion sent massive chunks of the ring careening into the moon Basis due to the constant bombardment of small objects from the ring as seen in the sky.

Later, the Covenant discovered a second Halo, Installation 05. On this Halo, the Flood had already been released and much of the ring had been compromised, including its monitor, 2401 Penitent Tangent. Soon, another four-way conflict began. The battle ended in the Flood manifesting a Gravemind and successfully escaping the ring aboard infected Covenant ships. The ring was nearly activated by Tartarus, the Chieftain of the Jiralhanae, but Commander Miranda Keyes pulled the Activation Index out of the installation's core at the last moment, causing all six functional installations to go into "standby mode", ready for remote activation from the Ark.

The human survivors returned to Earth and resumed their fight against the Covenant, who had uncovered a large structure on Earth, which they assumed to be the Ark. The Covenant landed the Forerunner Dreadnought in the center of the structure, activating it. However, the structure was soon revealed not to be the Ark, but a generator of a portal to Installation 00, a massive artificial world located outside the galaxy. Human and Covenant Separatist forces proceeded through the portal to the newly-discovered installation to prevent the Covenant from activating the remaining Halo rings remotely. Eventually, the Gravemind arrived at the Ark aboard High Charity, the Covenant's capital city, which had been turned into a Flood hive. The hive crashed onto the installation, releasing the Flood.

Soon, 343 Guilty Spark, John-117, and Arbiter Thel 'Vadam discovered that the Ark had been constructing a new Installation 04, to replace the one which had previously been destroyed. This new installation was only days from being complete. The humans decided to activate the new ring to stop the Flood, which in turn destroyed itself and damaged the Ark. It is unclear how the system was affected by this, but if the Ark was rendered inoperative, the Halo system would likely have been gravely damaged by the destruction of Installation 04 and its Monitor, 343 Guilty Spark.

There is a conflict with real life history regarding the firing record of the Array. It is insinuated that the Halo Array caused total extinction of all sentient life on Earth and throughout the Milky Way. As of now, there is no fossil record of such a complete extinction event. However, the Ross-Ziegler Blip, a tiny aberration in Earth's fossil records was not discovered until 2332, explaining the disparity.

Custodial
The Installations are designed to be run by advanced artificial intelligence constructs specially purposed by the Forerunners. The highest intelligence on each Installation is a single Monitor. The Monitor's task is to ensure that the installation's Constructors, Sentinels, and Enforcers repair, maintain, and defend the ring from damage, contain Flood specimens, and ensure that their own installation is ready to fire on demand, including running activation simulations.

In addition, each installation contains less intelligent constructs, known as Sentinels. The Sentinels serve virtually any purpose necessary to ensure that the Halo functions properly and are capable of combating small Flood outbreaks. Should a major Flood outbreak occur, heavier automatons, such as the Enforcers and Sentinel Majors will be created to assist the Sentinels. In the meantime, Constructors are also created to ensure that the Forerunner structures on the Installation are kept in optimal shape, and that they are not damaged by conflict or weather.

Save for the Monitor, all automatons can be constructed at specialized production facilities that float high in the Halo's atmosphere. They seem to contain almost limitless materials for constructing an almost indefinite number of automatons.

Environment
The Halo installations were also designed to be habitable and thus support a wide range of environments, habitats, ecosystems, and climates. Installation 04 and Installation 05 supported warm, temperate forests that were both deciduous and coniferous, swamps, and cold, snow covered tundra environments. Installation 00 had climate ranging all the way from tundra, to forest, to desert. It is unknown whether this range is shared by all the Halos, but the similarity of these environments and that of Installation 00 makes it likely.

While the terrain may appear to be naturally formed at first glance, it is actually artificially constructed. Strato-Sentinels extract raw materials from the source, process them in transit, and deposit building materials at the Installation. Four huge terraforming factories then move across the face of the installation to "skin" it with landmasses and bodies of water. These factories also hold in the ring's nitrogen-oxygen atmosphere, which is then leaked out to the surface, eventually pouring through the super-structure, and tugged in place by centripetal force. The installation's buildings are then built by Constructors and Sentinels. An intricate layer of metallic panels is laid down several meters below the surface of the terrain, upon which rock, soil, and eventually vegetation is added. Slopes, hills and mountains can be created by sculpting these panels. Some features, such as landslides, are the result of time, and have been formed naturally. Others, such as the myriad network of tunnels and cave systems that riddle the installation's internal structure, are travel conduits used by the ring's Sentinels and Enforcers. There are also some indigenous life-forms that live on the Halos; it is likely that the Forerunners brought these lifeforms to the installations.

Technology
Each Halo has several assets that are conditional to all installations. Each contains a Control Room located somewhere upon its inner surface, from which a Reclaimer must manually insert the installation's Activation Index to activate it. Each also possesses a Library, a large structure protected by an energy shield, where the Index is housed and protected by Sentinels. The index itself is a semi-solid holographic representation of the data needed to activate the ring, and can only be inserted successfully by a Reclaimer.

Installations also possess Cartographer facilities, holographic representations of the installation's inner passages and networks, to be used as a navigational reference for traversing the installation. Flood containment facilities are also standard, used by the installation's Monitor to conduct research on and observe the surviving Flood specimens; these are protected by Sentinels to prevent an outbreak.

Installations also contain advanced teleportation grids, allowing instantaneous transportation for a Monitor or Reclaimer to any place on the installation. These grids cannot be controlled by the Flood, though the Gravemind was able to use that of Installation 05 by controlling 2401 Penitent Tangent. For Sentinels, the installation is riddled with tunnel and cave networks for access and transportation. The Sentinel portals are connected to factories via tunnels.

The Halo rings achieve their "gravity" through centripetal force. As the ring spins, centripetal force pushes all spinning bodies away from the center of rotation. The ring is uniform, and because of this all points are the same distance from the center, therefore centripetal force applies evenly. This force also pushes objects on the surface of the ring away from the center of rotation, or pushes them down on the surface of the ring, translating into gravity. Each installation has a gravity of one standard G, which is achieved through some degree of gravitic manipulation. When the was detonated, it blasted a large mass from Installation 04; no longer a full loop, the centripetal force applied in much greater strength on the newly formed weak point, causing the ring to rip itself apart.

In the event of an emergency, a Halo can maneuver itself to avoid damage from a collision or weaponry. Thrusters are spaced around the ring, allowing it to move out of harm's way.

Each Halo is 10,000 km in diameter, similar to that of Earth, which is 12,756 km in diameter. However, the original twelve Halo rings were 30,000 km in diameter, meaning that one of the seven remaining rings is 30,000 km, while the other six are 10,000 km, unless the last remaining of the original twelve was destroyed or in some way damaged beyond repair, and a smaller 10,000 km ring was built to replace it. Each installation orbits a large, dense planet which serves as a gravitic anchor. For example, Installation 04 and Installation 05 orbited Threshold and Substance, respectively, which are both large gas giants.

Firing method
When activated, the Halo rings would wipe out all sentient life within three radii of the Milky Way's center by sending a burst of cross-phased supermassive neutrinos. This burst of super-massive neutrinos is carefully tuned to possess a harmonic frequency, which destroys the nervous system of any macroscopic organism that possesses one, even one as rudimentary as a notochord, as shown in the aftermath of a low-powered test firing of a Halo performed by the Master Builder on the San 'Shyuum world of Faun Hakkor.

This pulse is propelled across the Halo Array's full radius at superluminal speeds, covering that area and cleansing it of all affected life, though simpler life forms that do not possess a neural system, such as microbes, fungi, algae, mosses, and traditional plants are unaffected.

Function
The Halo Array is complex, but each ring can be activated individually by a Reclaimer, also activating the other installations, or they can all be activated simultaneously from Installation 00. In the event of a major Flood outbreak, an installation's Monitor will seek out a Reclaimer if available, whom they will enlist to aid them. It will teleport them to the installation's library and will assist the individual in retrieving the Index. The Monitor then stores the Index within its data arrays for safe transportation, lest the Reclaimer fall prey to the Flood before they can activate the installation. Once at the control room, the Reclaimer must insert the Index into the control panel in order to begin the activation sequence.

The main weapon is amplified by phase pulse generators, three of which are close to the control room, at least on Installation 04. Each installation has a maximum effective radius of 25,000 light-years and functions via form of lethal radiation. The pulse is designed to kill all sentient life in the installation's three-dimensional radius, with the array covering the entire galaxy. The pulse targets the nervous system of sentient life forms via the issuing of a harmonic frequency. The only known way to avoid the effects of these pulses is to escape to a Shield World or to escape to Installation 00, which is out of range of the array. Once activated, the other installations will fire simultaneously, eradicating all sentient life in the galaxy, thus starving and killing the Flood.

Trivia

 * The Halos are remarkably similar to the eponymous megastructure of Larry Niven's Ringworld series. However, Niven's ringworld encircles a large star, whereas the Halos merely orbit planets, being much smaller than the former. The Halos are also very similar to the Orbitals from Iain M. Banks' Culture series.
 * The Halo textures for both Halo: Combat Evolved and Halo 2 are the same, and are both symmetrical.
 * In a Halo: Combat Evolved prototype, Installation 04 had a section that was only partially constructed. This feature was dropped from the final game, though it may have inspired the second Installation 04.
 * On the screen where Captain Keyes taps his pipe on the opening scene of the Pillar of Autumn, TRAJECTORY FD: HALO is displayed by the hologram of Installation 04, although the UNSC was not aware of the Installation's nature at the time.
 * In 2011, the Halo Array was the number 1 most "Ridiculously Oversized Videogame Weapon" by PasteMagazine.com.

List of appearances

 * Halo: Combat Evolved
 * Halo 2
 * Halo 3
 * Halo Wars
 * Halo: Reach
 * Halo: The Fall of Reach
 * Halo: The Flood
 * Halo: First Strike
 * Halo: Ghosts of Onyx


 * Halo: Contact Harvest
 * Halo: The Cole Protocol
 * Halo: Evolutions - Essential Tales of the Halo Universe
 * The Mona Lisa
 * Halo: Cryptum
 * Halo Legends
 * Origins
 * Homecoming
 * Odd One Out