Halo (megastructure)

Halo rings are eponymous fictional megastructures and superweapons in the Halo science fiction franchise. 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 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 Iain M. Banks' Orbital 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.

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 do not exactly match up with Bishop's idea. 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. 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.

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