Installation 04

Installation 04 is one of a series of gigantic ring worlds (the Halos from which the series gets the name) apparently left behind by the enigmatic, technologically-advanced, and apparently extinct Forerunners. The Covenant reverentially describe Installation 04 as either the First Holy Ring, or the First Sacred Ring. 04 is also known colloquially among Halo fans as Alpha Halo.

Details
Planet of Orbit: Threshold

Monitor: 343 Guilty Spark

Status: Destroyed

Background
The purpose of Installation 04, along with its sister installations, was to prevent the parasitic lifeforms known as the Flood from consuming the Galaxy's sentient life. In the event of a galaxy-wide Flood outbreak, the seven installations could be triggered to annihilate all sentient life, thereby preventing the Flood from taking control of the galaxy.

Installation 04, until destroyed, was controlled by the Forerunner artificial intelligence construct (Monitor), 343 Guilty Spark.

Ancient History
Installation 04 was built by the Forerunners approximately at 98,665 BC (It was 101,217 years old in 2552 AD). It is unclear why the gas giant Threshold was specifically chosen to be the planet which Installation 04 orbited, but it is supposed that the large number of Forerunner machines already operational in Threshold (namely the network of Gas Mines), may have played a role in their decision. It could also be because it was the only planet in the system that had enough gravity to hold halo in place.

The Installation was activated sometime thereafter, and all Forerunner and Flood (save those in captivity in the Halos) were killed. Installation 04 lay dormant for millenia, with 343 Guilty Spark and his Sentinels keeping the Flood confined in the Flood Containment Facilities and keeping the infrastructure of the Halo in perfect working order.

Destruction
This all changed around August of 2552, when the Covenant discovered the location of Installation 04 after the Battle of Sigma Octanus IV. The Covenant sent the Fleet of Particular Justice to secure the Halo in preparation for the arrival of the holy city High Charity. After the Battle of Reach, a single UNSC cruiser, the, arrived at the Installation around September 19, 2552 and was promptly shot down by warding Covenant space forces. At around the same time, the Covenant ground forces on 04 accidentally unleashed the Flood from captivity. The ensuing four-day melee that took place all over the ring was called the Battle of Installation 04. Humans, Flood, and Covenant fought massive battles against one another as the Forerunner Constructs tried to control the outbreak by activating the Halo. In the end, a single human called the Master Chief detonated the Autumn's fusion reactors, destroying the Halo.

Only a handful of humans and Covenant survived the battle. 343 Guilty Spark floated through space until he met a group of Covenant on Threshold. Inspired by his words, they began a heretic rebellion against the greater Covenant. Installation 04's destruction is seen as the first step in the chain of events that led to the Civil War of the Covenant.

There is now a large field of debris where Installation 04 was, though a few large chunks of the ring remain. Some smaller pieces of the Installation 04 have landed on Basis and others have burnt up in re-entry heat within Threshold's atmosphere.

Despite the enormous amount of damage, places of the ringworld seem to remain inhabitable somehow, possibly due to the Forerunner technology that built it.

Astronomy
The "Ringworld" of Halo is much smaller than Larry Niven's 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 Halo 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.



Besides the very beautifully sculpted landscape on Halo's surface, the sky is also worthy of attention. Depending on the viewpoint, you can see the star, a moon (called Basis), a gas giant (called Threshold), and the band of Halo itself, which looks like a street into the sky.

In the course of Halo 1, neither Threshold nor Basis is reachable, all action takes place on or in the Halo. In Halo 2 there is a multiplayer level on Basis, and there is a campaign level in Threshold's atmosphere. That level is set on a Forerunner gas mining facility.