The great gas giant of Jupiter is a major Forge World of the Adeptus Mechanicus and the fifth world of the Sol System in the Segmentum Solar. Jupiter is the site of the massive manufactoria and starship construction facilities of the Mechanicus' ancient orbital Jovian Shipyards, which date back at least to the time of Old Night.
Jupiter is a gas giant orbited by a complex web of dozens of smaller satellite moons. It is almost a solar system in miniature, with the gas giant at its core rather than the blazing orb of a sun. The cloud of satellites and Trojan asteroids surrounding it are full of human colonies, manufactoria and Mechanicus forges, powered by drinking in the radiation surging from the mammoth planet andfeeding on the mineral riches that in centuries of exploitation had yet to be fully exhausted. Jupiter is Terra's shipyard, and its sky was forever filled with vessels. Centred on Ganymede and a dozen other smaller moons, spacedocks and fabricators worked ceaselessly to construct everything from single-crew Raven interceptors up to the gargantuan hulls of mighty Emperor-class Battleships.
The early history of Jupiter remains largely lost to Imperial savants. During the Age of Strife, Jupiter's moons were occupied by cruel xenos overlords of an unknown race. Following the Treaty of Mars, when the Red Planet was first incorporated into the burgeoning Imperium, the Great Crusade finally began in earnest. The rest of the Solar System was the first region of space to be conquered by the Emperor and his newly rearmed and re-equipped Legiones Astartes. Alien invaders were flushed from the moons of Saturn and Jupiter and their wretched human slaves repatriated to Terra. The Jovian Shipyards and the Jovian Void Clans were utilised throughout the Great Crusade to build a fleet of vessels for the Armada Imperialis, the precursor of the Imperial Navy.
In the late 41st Millennium, the Jovian Shipyards still play a vital role in the construction of the fleets of the Imperial Navy in combination with the Ring of Iron of Mars itself. The space docks and orbital workshops that circle Jupiter, like an artificial ring of more moons, are home to the millions of technomat and drone Servitors that work under the supervision of the Mechanicus' Artisan and Rune Priest Tech-priests to build the Imperial Navy's warships. Each vessel is a vast undertaking, often taking solar decades or even centuries to complete for the largest and most powerful warships. Many of those who work on the construction of an Imperial Navy warship will live and die during the process, never seeing the magnificent starship they struggled to bring to life as a mobile cathedral dedicated to the greater glory of the God-Emperor. Each of the Imperium's warship designs is constructed by only a single Jovian orbital shipyard; for instance, the Emperor-class Battleship, as well as Tyrant and Dominator-class Cruisers, are typical of the Imperial warships produced at the Jovian Shipyards and are each produced by a completely different orbital manufactoria.
The Jovian Shipyards are also responsible for the manufacture and production of the Plasma Drives that provide power to all of a starship's systems and the construction of a vessel's vital launch bay components and landing bays, which are equipped with dedicated ports for launching Attack Craft and magnetic guidance coils for steering them safely back aboard after a mission. To further enhance their vessels, the Jovian Shipyards also possess the capabilities to manufacture their own pattern of Nova Cannon, which replaces the standard explosive shells with Vortex Warheads. The resultant rifts into the Warp have been known to rend target vessels cleanly in half.
- Ananke - Ananke is a moon of Jupiter known to have had Mechanicum habitats on its surface during the Horus Heresy era.
- Callisto - One of the greater Galilean moons of Jupiter that was also known to have had Mechanicum habitats in orbit above it by the time the Horus Heresy began.
- Europa - One of the greater Galilean moons of Jupiter, this geo-engineered Ocean Moon was known to have had Mechanicum habitats on its surface during the Horus Heresy era.
- Ganymede - One of the greater Galilean moons of Jupiter that was known to have had Mechanicum habitats on its surface during the Horus Heresy era and serves as the central hub of the massive complex that makes up the Jovian Shipyards.
- Io - One of the greater Galilean moons of Jupiter that was known to have had Mechanicum habitats on its surface during the Horus Heresy era.
- Iocaste - One of Jupiter's minor moons known to have had Mechanicum habitats on its surface during the Horus Heresy era.
- Thule - At the time of the opening years of the Horus Heresy, the asteroid moon of Thule had orbited the shipyards of Jupiter for six millennia. Suspended high above the gaseous surface of its patron planet, it dwelled innocuously beyond the greater Galilean moons: Callisto, Ganymede, Europa and Io. It was an ugly chunk of rock, its gravity so weak that its form was misshapen and mutated. The Mechanicum had hollowed it out using massive boring machines and filled its interior with energy and life-support-carrying conduits, as well as vast tunnels and chambers. The dead core of Thule had become a giant manufactorum of forge temples and compressors, a massive gravity engine its beating heart. This construction extended from the surface via metal tendrils that supported blister domes, clinging like limpets to the rock, and pneumatic lifter arrays. But Thule was no mere misshapen asteroid. It was an orbital shipyard of Jupiter and the place where the Word Bearers' Apocalypse-class Battleship, the Furious Abyss, was clandestinely built by Dark Mechanicum elements. Following its completion and subsequent launch, these same elements destroyed Thule so that there were no witnesses that could verify the vessel's existence. The ragged clump of its remains still orbited far out at the edges of the planetary system for millennia to come.
- Kadal - Kadal was a satellite munitions fortress and one of the primary reserves of fleet ordnance in the Sol System. A lump of ultra-hard nickel-iron ore, it had taken the Mechanicum two solar decades to excavate, and another decade to graft the fortress structures within. Nova shells, torpedo warheads, macro shells, propellant and explosive precursors, all lay in vibration and temperature-controlled dark at the core of the rock. When a warship needed rearming it would approach and hold anchor away from Kadal. Barges would then shuttle the munitions load out and return. Nothing and no one approached closer than several thousand kilometres; anything that tried was greeted by Kadal's formidable defences. When dealing with enough destructive material to crack open a moon, there were no acceptable risks. During the later years of the Horus Heresy at the opening phase of the Solar War, the Alpha Legion instigated an anti-Imperial revolt and Kadal was subsequently destroyed.
- Saros Station - Located in a geosychronous orbit above Jupiter's Great Red Spot was a vast spindle, which appeared like a massive crystal chandelier, bathed in the constant crimson glow. Saros was a resort platform where the Jovian elite could find respite and diversion from the works of the shipyards and manufactoria. It was said that only the Venus orbitals could surpass Saros Station for its luxury. It was outfitted with avenues of gold and silver, acres of null-gee gardens and auditoriums; and the finest opera house outside the Imperial Palace.
- The Horus Heresy - Book One: Betrayal (Forge World Series) by Alan Bligh, pg. 18
- Battle for the Abyss (Novel) by Ben Counter
- Nemesis (Novel) by James Swallow, pp. 83, 87
- Betrayer (Novel) by Aaron Dembski-Bowden
- Praetorian of Dorn (Novel) by John French, Part 2, Ch. 3; Part 4, Ch. 1