Halo Array

The Halos, as humans call them based on intercepted Covenant transmissions (Alternatively known as the Sacred Rings by the Covenant or Fortress Worlds by their creators), are seven massive ring-shaped installations constructed by the Forerunner throughout the galaxy one hundred thousand years ago as a last resort when combatting the parasite known as the Flood. They are both research facilities and weapons of mass destruction. Each installation was constructed orbiting a gas giant planet, though it is unknown exactly why this was done. Once the Halos were activated, the Forerunners also designed a massive shelter called "Shield World" to rescue themselves from the destruction. The seven Halos are designed to be remote controlled from a facility called "The Ark"

Each Halo has a simple designation, consisting of "Installation (number)"

Custodial
The installations are designed to be run by advanced Artificial Intelligence constructs specially purposed by the Forerunner. The highest Intelligence on each Installation is a single Monitor. The Monitor's task is to ensure that the Installation runs smoothly, that the Flood stays imprisoned, and that the Installation is ready to fire on demand. They also simulate activation of the Halos regularly. At least one of the Halos has a sucessful firing record of 1.2 trillion simulated firing, and all seven of the Halos have been fired at least once before as part of the initial purge.

In addition, each Installation contains less intelligent constructs, the Sentinels. The Sentinels serve virtually any purpose necessary to ensure that the Halo works properly; they are also capable of combatting small flood outbreaks. Should a major Flood outbreak occour, heavier automations, 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 automations can be constructed at specialized Sentinel Production Facilities that float high in the Halo's atmosphere. These Facilities have never been seen (except briefly in the sky above the level Sacred Icon, one can be seen crashing as you exit near the end of the level, you then fight through the remains of it in Quarantine Zone), but they seem to contain limitless materials for constructing any number of automations.

Environment
Each Halo was designed with extraordinarily complex environmental systems and ecosystems. They contain oxygen atmospheres and gravity typical of an Earth-like planet. They also contain complex habitats which include thousands of types of flora as well as various primitive animal life. There are also what appear to be "oceans" on each Halo which house fish and other aquatic life.

The surface of each installation appears to have a "tropical" atmosphere of warm weather and forrested mountains and plains. There are naturally occouring weather patterns, including hurricanes and rainstorms. It is unclear how such a complex environmental system was created, let alone how it has remained active for a hundred thousand years.

Each Halo's size varies, but the diameter for most appears to be about 10,000 km. All currently presented evidence points to Installation 05 being smaller than this, although the validity of this claim can be disputed. The thickness of Installation 04, from one side to the other, is about 320 km, so the other Installations probably also waver around that value.

Technology
Each Halo has a Control Room located somewhere on the ring. The Control room serves as the central chamber for the Ring's activation, and serves to control all functions of the ring. It is the Installation's command center.

Each Installation contains a number of basic Structures, including a "Library" which contains an "Index." The Index is the key to the Installation's activation. They also conatin a "Silent Cartographer" which serves as a map to the Installation, as well as numerous Flood Containment Facilities and Flood Research Facilities. Every Halo has live Flood specemins that are kept for study. Any Flood outbreak is caused when these specemins somehow escape containment.

The Installations also contain advanced transportation grids for use by the Monitor. These Grids allow instantaneous transportation to any place on the ring for the Monitor and any other persons or things it chooses.

Function
The Halo system is complex, but the Halo's main weapons can be activated from any of the Halos, or from a control center known as The Ark.

In the event of a major flood crisis, the Installation's Monitor will seek out a "Reclaimer" who they will immediately transport to the Library. They must navigate to the Index Chamber, and then retrieve the Index. The Monitor then stores the Index for safe keeping, and they immediately proceed to the Control Center. Once at the Control Center, the Reclaimer must insert the Index into The Core of the Installation. Once this happens, the main weapon fires. The Main weapon is then amplified by the Phase Pulse Generators built into the ring, and the weapon expands in the form of a shock wave. The Shock wave of at least one Halo (Installation 04) has an effective radius of approximately 25,000 light years. The activation also triggers the other six installations, and their systematic activations cause a chain reaction which affects the entire galaxy.

This shock wave does not destroy planets or even structures, it destroys sentient life. Since the Flood can only replicate using Sentient life forms as hosts, the weapon essentially starves them to death, killing all flood in the Galaxy along with all sentient life.

The rings destroy all Flood hosts by breaking down or destroying Calcium structures in the body. Calcium is the primary material used by the Flood to create a combat or carrier form. This also kills the host. All sentient life forms, including sapient species and most animals, are vulnerable. Plants and non-calcium using animals (Such as insects and Drones) are not affected by the pulse because they are immune to the Flood.

History
The Halos were created a few hundred years after the Flood was discovered by the Forerunners, because their builders saw that their current isolation and containment procedures were not adequate to stop the spread of the Flood. Little is known of their extensive history, (At least one was functional for 101,217 years) but after exhausting every other tactical option, their creators had no choice but to activate the Halos' main weapon, and destroy all sentient life in the galaxy.

The seven halos then remained relatively dormant for one hundred thousand years (Though at least one of them experienced a major Flood outbreak) until one of them, Installation 04 (known amongst fans as Alpha Halo), was discovered by the theocratic alien superpower, the Covenant. The Covenant, who revered the Forerunner as gods, believed that the Halo's main weapon was actually a "divine wind" meant to propel them on a path they called the "Great Journey". This idea caused them to recklessly release the Flood of the installation, an act further complicated by the arrival of the human vessel UNSC Pillar of Autumn. A massive four way melee ensued between the Covenant, the Forerunner Constructs, the Flood, and the Humans. This finally culminated when the Human Spartan known as the Master Chief detonated the fusion engines of the Pillar of Autumn on the surface of the Installation, destroying it completely.

Later, a second Halo, Installation 05 (known amongst fans as Delta Halo), was discovered. On this Halo, the Flood had already been released somehow, and a similar conflict erupted. This conflict has yet to be resolved. However, the Installation was almost activated, and now the five other Halos stand on "Standby status".

Unknown Halos


Installation 04, and 05 are the only two Fortress Worlds yet discovered in the Halo Universe but when we look at the Monitor naming scheme, they offer more proof as to the existence of other Halo's.

Looking at the number in the Monitor's name, 343 Guilty Spark, 343 is seven to the third power. Seven being a number seen frequently in Bungie games and mythology. The number "2401", from 2401 Penitent Tangent the Monitor of Installation 05, is seven to the fourth power.

When the numbers of each Monitor are examined, a pattern emerges: each seems to be seven raised to the power of the installation number minus one; therefore this table to the right can be established.

You may also notice that Installation 07's Monitor's possible number contains 117, or the Master Chief's call number.

Astronomy
In another science fiction series called Ringworld author Larry Niven created "Ringworld" a ring world similiar to Halo. However Halo is much smaller than Ringworld. While the diameter of Niven's ringworld is close to the diameter of Earth's orbital of 300,000,000 km, the 10,000 km diameter of Installation 04 is much closer to the diameter of Earth itself, which is 12,756 km. Ringworld has a star similar to our sun in its center, and Halo is orbiting a planet, but does not encircle it.

Trivia

 * On Halo PC, on the menu, look closely at the Halo before it starts moving. somewhere on it can be seen the letters Gny Sgt Marcus, referring to Marcus R. Lehto.
 * Several multiplayer maps on both Halo and Halo 2 have what appears to be an Alien head made of clouds on part of the ring in the sky. This is a nod to the creatures from the Alien movie series.
 * Halo: Ghosts of Onyx describes the Halos as being the "sword" of the Forerunner, as Shield World was their "Shield." This description is mysteriously similar to the lines Cortana says on the Halo 3 Announcement Trailer; "I am your shield, I am your sword."

Related Links
Installation 04
 * Control Room
 * The Silent Cartographer
 * Alpha Base
 * The Library
 * Flood Containment Facilities

Installation 05
 * Control Room
 * The Library
 * Quarantine Zone
 * Sentinel Wall
 * The Temple
 * Flood Research Facilities
 * Relay Station
 * Sentinel Production Facility