Threshold

Threshold is a gas giant, and the seventh and largest planet in the Soell system. It was the gravity anchor for Installation 04, which was situated at the L1 Lagrange point between Threshold and its largest natural satellite, Basis.

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

Prehistory
Over 100,000 years ago, Threshold was a part of the sizeable and extensive Forerunner ecumene. Wishing to harvest the gas giant's rare and valuable gases, the Forerunners used Threshold as a mining colony. They built numerous automated gas mining complexes in the upper atmosphere of the planet, tethered to several band orbitals, used for harvesting the hydrogen, helium, and methane gas of the planet.

Upon the discovery of the Flood, the Forerunners refitted some of the gas mines to serve as a Flood research facilities. These facilities proved only one thing: more extreme measures were needed to combat the Flood and to keep them from spreading. At the end of their war against the Flood, the Forerunners chose Threshold as a staging point for one of the Halo superweapons: Installation 04. The Halo was placed on the L1 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.

Human-Covenant War
In 2552, the Fleet of Particular Justice followed the to Threshold, and, in a series of events, Installation 04 was destroyed. Many pieces of debris from Installation 04 were pulled into Threshold's gravity well, or its moon Basis, and destroyed. In the skirmish over Threshold after the ring's destruction, Cortana was able to jump into slipspace with the Covenant Ascendant Justice while in Threshold's atmosphere.

During the battle, the Covenant dispatched an artifact retrieval group led by Sesa 'Refumee to one of the gas mining platforms in Threshold's atmosphere. After Halo's destruction, the installation's monitor, 343 Guilty Spark, traveled to the gas mine and edified 'Refumee on the true nature of the Halos. Convinced that the Covenant religion was false, 'Refumee and his troops rebelled against the Prophets and were branded heretics as a result. The newly-appointed Arbiter, Thel 'Vadamee, and a Covenant Special Operations contingent were dispatched to silence the heretics; at the conclusion of the resulting conflict, the gas mine was dropped 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. The Office of Naval Intelligence began experiments with the Flood on the ship before the Flood escaped and the Mona Lisa was destroyed by a nuclear missile fired by. Also in the system at the time was a single Covenant ship apparently left behind to monitor or guard the Halo's remains.

Post-Covenant War
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. One of Threshold's gas mines is the location of a War Games simulation, a favorite among Spartan fireteam leaders.

Composition and atmosphere
Threshold is an immense gas giant with a diameter of 214604 km, and twelve satellites, several of which are Forerunner installations. Threshold is the largest planet in the Soell system, and the seventh planet in Soell's orbit. Threshold is purplish-orange in color, and contains many rare and valuable gases. Threshold's atmosphere is made up of white ammonia clouds, amber-colored ammonium hydrosulfide clouds, phosphorus compounds, ice crystals and other gases. The clouds are arranged in thin, tendril-like formations and resemble sinuous ribbons. The planet's winds blow at hundreds of kilometers an hour. Threshold is known to have storms frequently rage in its atmosphere similar to the storms that occur on the planet Jupiter. The planet has a core of liquid hydrogen.

The pressure inside the gas giant is extremely powerful, and effectively able to crush any object that delves too deep into the planet's atmosphere. Threshold has a powerful magnetosphere that is easily capable of disrupting the magnetic fields of plasma weaponry such as plasma torpedoes. The temperature of the outer atmosphere of Threshold is around three hundred degrees Celsius. Further down, in the planet's denser atmosphere, temperatures reach over seventeen hundred degrees Celsius.

Satellites
Threshold is orbited by twelve satellites. Threshold's largest natural satellite, Basis, is in fact the size of a huge terrestrial planet. Basis' diameter, 23,848 kilometers, is nearly double that of Earth; the other satellites of Threshold 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. Several small asteroids also orbit Threshold, near Installation 04.

The most notable of Threshold's orbiting installations was Installation 04, situated at the L1 Lagrange point between Threshold and Basis. Threshold's atmosphere is home to an extensive network of Forerunner gas mining platforms and refinery stations, hanging from a band-like low-orbital suspension grid in the upper atmosphere. Although, by the mid-26th century, only a handful of these mines and refineries remain operational. The Forerunner space station designated Deep-Space Anomaly 0198 by the UNSC is also in high orbit over the gas giant.

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, Halo 2 and most other visual media, Threshold's surface texture resembles that of a terrestrial world more than a conventional gas giant. The planet's appearance is revised in Halo 2: Anniversary to more accurately reflect its nature, with its face now covered in latitudinal cloud bands characteristic of gas giants.
 * 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.
 * Microsoft's Windows 10 operating system is codenamed Threshold, after the planet in the Halo franchise. Threshold joins Project Spartan and Cortana as references to the Halo franchise in the Windows 10 release.

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
 * Terminals
 * Halo: Escalation
 * Halo 2: Anniversary
 * Terminals
 * Halo 5: Guardians