Threshold

Threshold is the seventh and largest planet in the Soell star system. It was the gravity anchor for Installation 04, which was situated at the Lagrange point between Threshold and its largest natural satellite, Basis.

Threshold is an immense gas giant with a diameter of 214,604 km (133,348 mi), and twelve satellites, several of which are Forerunner installations. Threshold's largest moon, Basis, has a diameter of 23,848 kilometers; the others are significantly smaller, but are nonetheless host to various Forerunner facilities. Although none of Threshold's satellites have been extensively surveyed, all but two of them appear to contain Forerunner installations, or are entirely artificial. The outposts functions are unknown, but they were likely used to provide maintenance for Installation 04.

Threshold's atmosphere is made up of white ammonia clouds, amber-colored ammonium hydrosulfide clouds, phosphorus compounds, ice crystals and other gases. Threshold is known to have storms in its atmosphere similar to the storms that occur on the planet Jupiter. The planet has a core of liquid hydrogen. Threshold's atmosphere was home to several Forerunner gas mining platforms, hanging from a high-altitude suspension grid in the upper atmosphere.

Threshold was a neutral location during the Human-Covenant War. Its Earth Survey Catalog number is B1008-AG.

History
Over 100,000 years ago, Threshold was a part of the sizable and extensive Forerunner empire. They built at least one automated, viable gas mine complex in the upper atmosphere of the planet, likely harvesting the hydrogen gas of the planet. The hydrogen was probably used as transmutation material for construction, though this is not confirmed.

Several hundred or thousand years later, upon the discovery of the Flood, the Forerunners refitted the gas mines to serve as Flood research facilities. These facilities proved only one thing: more extreme measures were needed to combat the Flood and to keep them from spreading. The Forerunners then chose the planet as a staging point for one of their massive fortress worlds, known as Halo and numbered as Installation 04. The Halo was placed on the Lagrange point between Threshold and Basis and was designed to destroy all sentient life in the galaxy, thereby halting the spread of the Flood, due to starvation.

In 2552, the Covenant were looking for an artifact from Sigma Octanus IV to uncover this planet as the location of the "Sacred Ring", in order to start the Great Journey. The Fleet of Particular Justice followed the there, and, in a series of events, Installation 04 was destroyed. In the Skirmish over Threshold after the ring's destruction, Cortana was able to jump into slipspace with the Covenant flagship Ascendant Justice while in Threshold's atmosphere.

The Heretics then took refuge on the gas mine (they had been based at a makeshift camp on Basis until this point) before the Arbiter, Thel 'Vadamee, and his Special Ops troops dropped it into Threshold's depths. High Charity and its fleet briefly visited Threshold as well, until they jumped to Installation 05.

At one point in October, the human prison ship Mona Lisa ended up picking up Covenant survivors over Threshold including one infected by the Flood. ONI began experiments with the Flood on the ship before the Flood escaped and the Mona Lisa was destroyed by a nuclear missile fired by the. Also in the system at the time was a single Covenant ship apparently left behind to monitor or guard the Halo's remains, which were gradually being pulled into Threshold and destroyed.

After the Human-Covenant War, a 500 light-second exclusion zone was established around Threshold, with both Sangheili and human warships patrolling the region as part of the post-war treaty between those species. Uncontrolled Sentinels, volatile Halo fragments, micro-debris, malfunctioning Forerunner artifacts, and the ever-present danger of the Flood made even surreptitious exploration and scavenging too dangerous for most.

Size
Threshold is an immense gas giant, far larger than Jupiter from the Sol System. Its moon, Basis, is in fact the size of a huge rock planet. Basis' diameter is nearly double that of the Earth.

Trivia

 * Threshold is described as purple in the book, Halo: The Flood, though throughout Halo: Combat Evolved and Halo 2, it is only shown as orange. Also, the planet is depicted as being light blue in the motion comic adaptation of The Mona Lisa. This may be caused by the position of the system's sun at different times or the proximity to the gas giant, or may simply be an oversight.
 * In Halo: Combat Evolved, the Assault Rifle's compass arrow points at the gas giant. This suggests that Threshold's atmosphere contains a large amount of iron particles, or that the assault rifles use a digital compass and that the Alpha Halo happens to be situated so that Threshold is aligned with the rifle's north during the events of the game.

List of appearances

 * Halo: The Fall of Reach
 * Halo: Combat Evolved
 * Halo: The Flood
 * Halo: First Strike
 * Halo 2
 * Halo: Evolutions - Essential Tales of the Halo Universe
 * The Mona Lisa
 * Halo: Reach
 * Halo: Combat Evolved Anniversary
 * Halo: Escalation
 * Halo 2 Anniversary
 * Terminals
 * Halo 5: Guardians (Simulation only)