Installation 04

Installation 04, famously known among fans as Alpha Halo, was one of a series of gigantic ring worlds (the Halos from which the series gets the name) 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. It is the main setting of Halo: Combat Evolved, the first game in the Trilogy. After its destruction at the end of the Battle of Installation 04, The Ark started to construct a replacement Installation 04.

Background
The purpose of Installation 04, along with its brethren, was to preclude and thwart the parasitic lifeforms known as the Flood from consuming all sentient life in the galaxy. In the event of a Flood outbreak, one of the seven installations could be triggered to annihilate all sentient life within 25,000 light years. Once one ring is set off all remaining rings would activate. All seven rings could be fired at once if need be.

Installation 04, until its destruction by the Master Chief, 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 if the weather is intentionally irregular, or it follows a pattern such as snow being deliberately made at the control room to slow the Flood.

Ancient History
Installation 04 was built by the Forerunners over 100,000 years before the game Halo:CE (It was 101,217 years old in 2552 CE, but this is measured in "local time").

The Installation was activated sometime thereafter and all Forerunner and most of the Flood were killed. Installation 04 lay dormant for a millenia, with 343 Guilty Spark and his Sentinels ready to keep the Flood confined in the Flood Containment Facilities and keeping the infrastructure of the Halo in perfect working order.

Destruction
"Halo... It's finished..." "No, I think we're just getting started."

- Cortana talking to the Master Chief after the destruction of Installation 04.

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 through slipspace. It is assumed that the Covenant knew about the Ring as the Prophets all have a holographic representation of them on their headdresses and Cortana saying previously 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 Pillar of Autumn.

The Pillar of Autumn arrived around September 19, 2552 and was promptly shot down by the warding Covenant space forces. At around the same time, the Covenant ground forces on Installation 04 accidentally unleashed the Flood from captivity. The ensuing four-day battle 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 which destroyed 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. Installation 04 was the first Halo installation to be destroyed.

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 Threshold's atmosphere on entry.

The destruction of Installation 04 also triggered the activation of a portal leading to a Micro Dyson Sphere on Onyx.

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, by the Master Chief and 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 ring world 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. The ring world 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 Light years from Earth.

Speculation
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 (explicitly 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. It is probable that the Forerunner had chosen Threshold as the staging point for constructing Installation 04 in a fail-safe to attempt to isolate any major Flood outbreak. It may have been as simple as its proximity to the Gas Mine stations on Threshold, which the Forerunner may have used for power.

Survivors of Installation 04
After the destruction of Installation 04, Spartan 117 and Cortana, who were responsible for the ring's destruction, fled the installation in a Longsword. Shortly afterward, they reunited with five other survivors on a Pelican. The survivors of the Battle of Installation 04 were as follows:

On a Longsword

 * Master Chief Petty Officer John 117
 * UNSC AI Cortana

On a Pelican

 * Sergeant Major Avery J. Johnson
 * Corporal Locklear
 * Lieutenant Elias Haverson
 * Warrant Officer Sheila Polaski

Other

 * 343 Guilty Spark

Unknown
The following two Marines have been seen after having a part in the Battle of Installation 04, but it is unknown how they made it off Halo before the explosion. Note that there may have been other survivors.
 * Sergeant Pete Stacker
 * Private Chips Dubbo

It should also be noted that Spartan-II Linda-058 survived the destruction of Halo, even though she was in cryosleep the entire time.

The Survivors of the Human-Covenant War
However, not all these Marines survived the remaining war. Corporal Locklear was killed by a Slipspace crystal. It is theorized that this may have been suicide over his grief at the loss of Shelia Polaski, killed by plasma in a Slipspace bubble along with two Spartans. Lieutenant Haverson along with Admiral Whitcomb took a wounded Covenant flagship, Ascendant Justice, and waged a one-ship attack on the Covenant Fleet gathered around Unyielding Hierophant in order to distract them long enough to be caught in the explosion of the station, and when the Unyielding Hierophant was destroyed it destroyed about 500 other ships. Sergeant Johnson was killed during the Battle of the Ark, and Master Chief and Cortana were MIA after that same battle.

Pete Stacker and Chips Dubbo participated in the 21 Gun salute at the end of the war.

Trivia

 * You can see the destroyed Alpha Halo in one of the cutscenes of Halo 2.
 * Halo would require a massive amount of gravity to hold it into place; this is why it is orbiting Threshold.
 * Alpha Halo is where humanity first comes into contact with the Flood.
 * Installation 04, along with its successor, are the two most featured Halos in the trilogy.
 * Alpha Halo's architecture is similar to that of the Ark, if not nigh-identical.
 * Alpha Halo is the only known destroyed Forerunner installation. (However the Ark may not be completely destroyed)
 * Alpha Halo is the first installation to contain The Flood until the Covenant released it by mistake.
 * Johnson describes it as "God's own anti-son-of-a-bitch-machine" and a "giant hula-hoop".
 * This is the only Halo Installation that is shown in every single Halo game in the original trilogy.
 * The numbering of the rings concerning the IWHBYD ring sequence is the opposite to the the rings' actual installation numbers and should not be confused.
 * It is still unknown at the moment for any flood spores or even it's species to be still roaming around in the wreckage of the installations because not all of the broken ring parts are completely destroyed. But again might have been proven wrong due to that Cortana mentioned 'Just dusts and echoes'. Either way, it pretty much seems to be not bothered by any nonetheless.