Installation 04

Installation 04 was 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 described Installation 04 as either the First Holy Ring, or the First Sacred Ring.

04 is also known colloquially among Halo fans as Alpha Halo. It is the main setting of the first Halo game, Halo: Combat Evolved. After its destruction at the end of the Battle of Installation 04, a second Installation 04 was constructed to replace it.

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.

Climate
The climate of Installation 04 is very similar to Earth's climate. There have been hurricanes over the oceans on the ring. The weather is, however, extremely unpredictable - rain and snowfall can occur at random, leading some to wonder it the weather is intentionally irregular, or whether there is some indiscernable pattern.

Ancient History
Installation 04 was built by the Forerunners at approximately 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. Another common theory is that it was placed there mathematically in order to cover the whole of the galaxy with the spread of the Halos' main weapon. Located in a region that lacked any nearby habitable planets, it is probable that the Forerunner had chosen Threshold as the staging point for constructing Installation 04 in a failsafe to attempt to isolate any major Flood outbreak. Although it may have been as simple as because of the Gas Mine stations local to Threshold, which the Forerunner may have used for power.

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
The Covenant discovered the location of Installation 04, after the Covenant Fleet of Particular Justice followed the single UNSC cruiser, the, which was fleeing from the Battle of Reach. It is assumed that the Covenant knew about the Ring before as the Prophets all have a holographic representation of them on their headdresses and Cortana saying that she used the coordinates received by the beam in The Fall of Reach. Due to the Covenant's faster Slipspace capabilities they arrived before the Autumn. The Pillar of Autumn arrived 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, SPARTAN-117 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 Covenant Civil War.

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 ring-world seem to remain habitable somehow, possibly due to the Forerunner technology that built it.

Replacement
Main Article: Installation 04 (II)

Shortly after the destruction of the Halo, the Ark began to automatically construct a new Halo to replace Installation 04. This exact copy of the first Halo ring was nearing completion only a few months after the first ring was destroyed, but was subsequently also destroyed when it was fired before it was ready.

Astronomy
See full article: The Halos

The "Ringworld" of Halo is much smaller than Larry Niven's Ringworld, by a magnitude of 3,000. 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.

Circumstantial evidence from Bungie indicates that Alpha Halo may be located in the Iota Horologii System, only 50 lightyears from Earth.