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. It is the main setting of the first Halo game, Halo: Combat Evolved.

Details
Planet of Orbit: Threshold in the Soell system (location unknown) Replacement located within Installation 00

Monitor: 343 Guilty Spark

Status: Destroyed (though some charred pieces still remain) Replacement ring also 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.

Climate
The climate of Installation 04 is very similar to Earth's climate. There have been hurricanes over the oceans on the ring. One anomaly is the warm rain that fell one night.

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

Replacement
In Halo 3, Installation 04 is rebuilt, by an automatic function in The Ark that replaces a Halo when it is destroyed. When the new Installation 04 was released into space it was incomplete. John-117 and the Arbiter decided to activate the incomplete installation to eradicate the Flood army that was rapidly spreading across the Ark. Guilty Spark, who had arrived and allied with the two, expected such a reacreation, and became almost extatic at the thought of finally activating the ring.

The Master Chief and the Arbiter were forced to make a detour to the, now long consumed, High Charity, in order to recover Cortana, and the Halo's activation Index. John then detonated the city's damaged plasma reactor to destroy the hive and the Flood Gravemind in order to slow the rapidly growing legions of flood.

John, Cortana and the Arbiter landed in the snowy canyon that housed the Core in order to activate the ring. After eliminating the area's surviving Flood, who were trying to rebuild the Gravemind on the ring, the three meet with Sergeant Johnson and enter the control room.

Guilty Spark informs Johnson that the ring will require several more days to become fully operational, and that a premature firing will destroy both the new ring and the ark. Due to the gravity of the situation, Johnson disregards Spark's warnings and prepares to download Cortana into the Core. Spark, outraged by Johnson's reckless behavior, fully submits to his insanity, and attacks him, mortally wounding him. John and the Arbiter attempt to stop the insane construct, but are both subdued by Spark's energy weapon. He then goes on to lecture the chief on how even though he is decended from his creators, "...this ring is mine."

SPARTAN-117 then battles with Spark, but finds that he is unable to penetrate the monitor's shielding. With the last of his strength, Johnson attacks 343GS with his Spartan Laser, damaging the neurotic monitor's casing, thus allowing John to destroy him. Johnson hands the Spartan Cortana's data chip, pleading the Master Chief to leave him and activate the ring, before dying. Cortana broadcasts herself to the console of the control room, beginning the activation sequence.

As Guilty Spark perdicted, the ring was not ready to fire, and began it disintegrate. John and the Arbiter escaped through the first Phase Pulse Generator room, and then to Johnson's Warthog. The vibration of the activation began to destroy the ring, causing explosive reactions across the installation. John and the Arbiter barely managed to reach the UNSC Frigate, UNSC Forward Unto Dawn, before the ring detonates and "[does] a number on The Ark", according to Cortana.

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.