Halos are fictional megastructures and superweapons in the Halo video game series. They are referred to as "Installations" by their AI monitors, and are collectively referred to as "the Array" by the installations' creators, the Forerunners. The series' alien antagonists, the Covenant, refer to the structures as the "Sacred Rings", believing them to form part of a greater religious prophecy known as "The Great Journey". According to Halo's fiction, the Forerunners built the rings to contain and study the Flood, an infectious alien parasite. The rings also act together as a weapon of last resort; when fired, the rings kill any sentient life capable of falling prey to the Flood, starving the parasite of its food. The installations are at the crux of the plot progression for the Halo series.
The Halos are massive ringworlds, which feature their own wildlife and weather. The constructs resemble Larry Niven's Ringworld concept in shape and design. The structure that Halo: Combat Evolved takes place on was initially to be a hollowed-out planet, but was changed to its ring design later in development; a staff member provided "Halo" as the name for both the ring and the video game after names such as Red Shift were suggested.
Design and development 
The term "megastructure" refers to artificial structures where one of three dimensions is 100 kilometers (62 mi) or larger. The first use of a ring-shaped megastructure in fiction was Larry Niven's novel Ringworld (1970). Niven described his design as an intermediate step between Dyson Spheres and planets - a ring with a radius of more than 93,000,000 miles (150,000,000 km) and a width of 1,000,000 miles (1,600,000 km); these are dimensions far exceeding the ringworlds found in the Halo series, which feature radii of 5,000 miles (8,000 km) The Halos are closer in proportion to the Bishop Ring (habitat), an actual proposed space habitat first explained by Forrest Bishop, though the proportions of the Halos don't exactly match up with Bishop's idea, and don't sit at Lagrange Points, as Bishop proposed. As seen in the games, Halo installations feature a metallic exterior, with the interior of the ring filled with an atmosphere, water, plant life, and animal life. A massive wall on the sides of the structure, combined with the centrifugal force produced by the ring's rotation, keep the environment from leaking into space. What appear to be docking ports and windows dot the exterior surface, suggesting that a fraction of the ring structure itself is hollow and used for maintenance, living, and power generation.
Before the title for game developer Bungie's next project was announced and development of the game that would become Halo was in its early stages, the megastructure that Halo: Combat Evolved took place on was a massive, hollowed-out planet called "Solipsis". The planet became a Dyson Sphere, and then a Dyson Ring. Some Bungie staffers felt the change to a ringworld was "ripping off Larry Niven", according to Bungie artist Paul Russel. Bungie employee Frank O'Connor wrote in a post on Bungie.net that "the specific accusation that we swiped the idea of a ring-shaped planet wholesale is not accurate," explaining that Bungie used a ringworld because "it's cool and therefore the type of thing a Forerunner civilization would build."
At the time, the game was known as Blam!, but Bungie had always expected to replace the working title with something better (Blam! was only used after studio co-founder Jason Jones could not bring himself to tell his mother their next project was dubbed Monkey Nuts.) Titles such as The Crystal Palace, Hard Vacuum, Star Maker, Star Shield, and The Santa Machine were suggested. Russel suggested calling it Project: Halo because of the ring. Despite concerns that the title seemed too religious or lacked action, the name stuck. In turn, "Halo" became the ring's name as well.
Combat Evolved's Halo was intended to be populated with large animal life, collectively known as Fauna. The Fauna included "pseudo-dinosaurs" and mammals, as well as a Chocobo-like creature—the "Blind Wolf"—that players could ride. The animals were removed for technical and conceptual reasons; there were difficulties in getting herd and behavior action to work, and under pressure to complete the game's more central aspects, the animals were dropped. Bungie also felt that the desolate ring heightened the sense of Halo's mystery, and made the appearance of the parasitic Flood more terrifying and unexpected.
Scientific analysis 
Physicist Kevin R. Grazier, Ph.D, posited in a 2006 essay the composition and problems associated with a Halo installation. The complete Halos seen in Halo: Combat Evolved and Halo 2 orbit gas giants similar to Jupiter, though much larger; the bodies exhibit characteristics of both a jovian planet and a small star. In each system, there are five points where a body of negligible mass would remain stationary to the two much larger bodies in the system, the gas giant and its moon. These areas, known as Lagrange points, are classified by stability; while bodies at 60° angles to the gas giant would remain in the same location relative to the other objects in the system, the other three Lagrange points are meta-stable, having the tendency to be unstable in one direction. As the Halos are located at point L1, the installations must actively correct its orbit. The apparent gravity of the Halo installations is close to Earth normal. A Halo would have to spin with a tangental speed of 7 kilometers (4.3 mi) per second to match Earth's gravity, translating to 19.25 rotations in a day.
Aside from its unstable position, Halos would have to contend with thousands of meteor and micrometeor impacts which would destabilize or destroy the ring; there is no evidence in the games that the installations project an energy shield to prevent this occurrence. Because of the magnetic environment around the gas giant, a Halo would be exposed to high levels of radiation. Earth is protected from such radiation by charged particles created by the planet's magnetic field. Grazier posits that huge conductive cables could run the circumference of a Halo; when an electrical current was run through these cables, a protective magnetic environment could be created to sustain life.
In the games, spectroscopic analysis of the ring's composition proved "inconclusive", implying that the Halos are constructed of an unknown material (unobtainium). Were a Halo to be constructed using conventional materials a light steel alloy would be most feasible. Assuming that the ring structure is 50% empty space, a 5000 km ring composed of steel alloy at an average density of 7.7 grams (0.27 oz) per 1 cubic centimeter (0.061 cu in) would result in a total mass of 1.7x1017 kg. The amount of material required to build such a ring would be akin to the total material available in the asteroid belt.
Installation 03 
Installation 03, also referred to as Gamma Halo, appears in Halo 4, the first title in the Reclaimer Trilogy by 343 Industries. Whilst no gameplay takes place on the installation, an extremely dense asteroid field surrounding the installation is the site of the UNSC scientific research base Ivanoff. It is here that UNSC scientists are conducting experiments on the Forerunner artifact called The Composer, which has the ability to convert biological forms, specifically humans, into AIs. Once the game's antagonist, The Didact, activates the device, the UNSC base is left uninhabited. No further information regarding the fate of the installation was revealed.
Installation 04 
Installation 04, also referred to as Alpha Halo, appears in Halo: Combat Evolved. The majority of gameplay takes place in areas on this installation, and its exploration drives the story. The ring is managed by an artificial intelligence known as 343 Guilty Spark, and is located in the Soell system, dominated by a gas giant known as Threshold. Halo orbits Threshold's only satellite, an extremely large moon known as Basis. A group of humans aboard the ship Pillar of Autumn crash-land on the ring after being pursued by the alien Covenant. The ring holds religious significance to the aliens, while the humans believe it is a weapon that could turn the tide of the war against the Covenant in their favor. In reality, the ring is home to a virulent parasite called the Flood, which are accidentally released by the Covenant and threaten to infest the galaxy. The human soldier Master Chief eventually detonates the Pillar of Autumn's reactors in order to destabilize the ring and cause it to break up, preventing the spread of the Flood and the activation of the Halo network, which would kill all sentient life as a fail-safe to starve the Flood. The Ark was alerted to its destruction and proceeded to create Installation 04B, which, too, was destroyed by Master Chief. During the game's events, Guilty Spark alludes to a previous firing of the network, which Bungie's director of cinematics Joseph Staten said occurred around 100,000 years previous to the events of the game in the year 2552.
Installation 05 
During the events of Halo 2, the Covenant and humans discover a second ringworld, Installation 05, or Delta Halo. It was monitored by 2401 Penitent Tangent, who completely ignored Flood warnings and was captured by their leader, the Gravemind. The Covenant leadership wants to activate the installation, believing it is the key to their salvation. At the same time, the Flood, led by an intelligence known as the Gravemind, lay siege to the Covenant's city-ship, High Charity. After 343 Guilty Spark informs Halo's true purpose to the Arbiter, a Covenant holy warrior, of the danger that the Halos truly represent, a group of humans and Covenant Elites prevent the firing of the ring. The unexpected shutdown activates a fail-safe protocol, priming the remaining Halo installations for remote activation from a location known as The Ark. In Halo 4, it is revealed that the UNSC has created an oversight base on the Installation (or around it), as they did with Installation 03.
The Ark 
The Ark, also referred to as Installation 00, is located outside the Milky Way galaxy and serves as the construction and control station for the Halo weapon system. During Halo 3, the Covenant discover a portal on Earth that leads to the Ark and are pursued by the humans and a breakaway faction of Covenant opposed to activating the rings. Gravemind, having hijacked High Charity, also crash-lands on the installation. The remote firing of the rings is halted by Master Chief and the Arbiter. In order to end the threat of the Flood, Master Chief decides to activate Installation 04B under construction in The Ark, the replacement for the Halo that he destroyed in Halo: Combat Evolved. Unknown to everyone but 343 Guilty Spark, a premature firing would destroy the installation; the monitor attempted to defend 'his' ring but was defeated by Master Chief, who proceeded to fire the weapon. The firing tears apart the incomplete Halo and severely damages The Ark as Master Chief, Cortana, and the Arbiter try to escape through the Portal, which closes as they enter, leaving Master Chief and Cortana drifting in space while the Arbiter returns to Earth successfully, forming the setting of Halo 4.
According to Greg Bear's Forerunner Trilogy, a "Greater Ark" served as the location for the construction of larger 30,000 km rings.
See also 
- Grazier (2006), 39–40.
- Hiatt (1999), 94–96.
- Gillen (2000), 45.
- Grazier (2006), 41.
- McLaughlin (2007), 1.
- Jarrard, et al (2008).
- Perry (2006), 6.
- Trautmann (2004), ix.
- Trautmann (2004), 73.
- Toyama (2001), 61.
- Preston (2000), 19.
- Lehto, et al (2002).
- Grazier (2006), 44–45.
- Grazier (2006), 46.
- Grazier (2006), 49.
- Grazier (2006), 47.
- Grazier (2006), 48.
- Grazier (2006), 42.
- Trautmann (2004), 77.
- Grazier (2006), 43.
- Trautmann (2004), viii.
- Barrat (2007), 2.
- McLees, et al (2006).
- "Cortana: That's what I thought he said. The Prophet of Regret is planning to activate Halo! / Master Chief: Are you sure? / Prophet of Regret: I shall light this sacred ring, release its cleansing flame, and burn a path into the divine beyond! / Cortana: Pretty much."—Bungie. Halo 2. (Microsoft Game Studios). Level/area: Regret. (2004)
- Barrat (2007), 3.
- "343 Guilty Spark: Fail-safe protocol: in the event of unexpected shut-down, the entire system will move to standby status. All installations are now ready for remote activation. / Keyes: Remote activation? From here? / 343 Guilty Spark: Don't be ridiculous. [...] the Ark, of course."—Bungie. Halo 2. (Microsoft). Level/area: The Great Journey. (2004)
Hiatt, Jesse (November 1999). "Halo; the closest thing to the real thing". Computer Gaming World: 94–96.
Jarrard, Brian; Luke Smith (2008-08-21). "Bungie Podcast: 08/21/08; With Paul Russel and Jerome Simpson". Bungie.net. Retrieved 2008-08-27.
Perry, Douglass C (2006-05-17). "The Influence of Literature and Myth in Videogames". IGN. pp. 1–6. Retrieved 2009-03-19.
Preston, Jim (August 2000). "Scoop!; Halo". PC Gamer: 19.
Toyama, Kevin (May 2001). "Cover Story: Holy Halo". Next Generation Magazine: 61.