The Astronomican is a psychic beacon located on Terra, which projects an astropathic beam 70,000 light years across the Milky Way Galaxy which human Navigators utilise to pilot the starships of the Imperium of Man through the otherwise unnavigable chaos of Warpspace. As the beam generated is psychic it exists within the psychic universe of the Warp. The beam can be psychically sensed from almost anywhere in Warpspace and is vital for faster-than-light travel since the mutant human Navigators use it as the only fixed reference point in the galaxy which is required for a starship to reach its intended destination when it travels through the Warp. Without it, the Imperium could not survive as an interstellar society as Warp travel would be impossible. It is uncertain how faster-than-light travel was done in the Dark Age of Technology before the Astronomican existed, though it can be safely assumed that a form of faster than light travel that is non-dependent on entering Warpspace and may be similar to Necron phase drive technology was used, or more likely that Warp jumps were simply unable to reach across as many light years as at present because of the lack of a fixed reference point. This is how the Tau's faster-than-light stardrive technology functions, as their vessels make shallow "Warp dives" across a much smaller length of realspace than current Imperial starships, and is probably how Mankind once operated as well.
At the very start of the Great Crusade in ca. 800.M30, the planets closest to Terra could be easily reached by blind jumps through the Immaterium. The Empyrean of that time had only recently calmed after the great turbulence of the Age of Strife following the birth of the Chaos God Slaanesh. However, it soon became clear that to travel further from Terra, the Imperium's Navigators would require a stable astronavigational reference point within the Immaterium to triangulate their current location against if longer Warp jumps were to be attempted with any hope of success.
Following the successful conclusion of the Unification Wars, the Emperor first ordered the construction of the Astronomican on Terra, which would enable his forces to expand the Great Crusade to the stars beyond Sol. Huge numbers of Tech-priests were brought from Mars to oversee the project and the majority of the Terran population were drafted to construct the towering machine-buildings needed to support this labour. At the time the Astronomican was the single largest artifice on Terra, and the entire device was merely a focus through which the Emperor could direct His fathomless psychic energies to generate a partly self-sustaining telepathic signal through the Empyrean (although few were aware of this fact). The psychic navigational beam the Astronomican generated was able to propagate through the Warp and those attuned to its unique frequencies and modulations, the Navigators, were able to use it as a beacon and pole star when plotting journeys through the Immaterium. By this beacon the Warp could be travelled at speeds and with a margin of safety that had been unprecedented, although the risk of course could never be fully mitigated. The Astronomican was an incalculable boon to both the Great Crusade and the fledgling interstellar domain of Mankind that it was creating. In a similar way, the Emperor was able to shut down the Astronomican or interrupt the beam at will. Only a handful of individuals knew that the great signal was actually powered by the Emperor's psychic powers, and they lived in fear that should He be disabled or killed, the galaxy would be plunged into a new Age of Strife. Such was the Astronomican's effect that even in that distant age of the Imperium's first founding some referred to it as the Divine Light, or the Emperor's Light, often without fully realising the literal truth of those words.
With His Imperial Webway Project only in its planning stages, the Emperor resorted to powering such a reference point Himself, using the focus chamber He had built earlier beneath the Imperial Palace to project a beam of unimaginable psychic power and range through the Warp. So colossal was the Emperor's psychic might that in the early days of the Great Crusade in the late 30th Millennium He could power the Astronomican while He was off-world leading the Imperial war effort personally. For all His might, however, the Emperor still had limitations, and the task of powering the Astronomican so that its beacon was perceptible over the whole of the territory of the growing Imperium of Man became more and more taxing, weakening Him to the point He was nearly choked to death by a powerful Ork Warlord on the world of Gorro, only to be saved in the nick of time by His Primarch son Horus. The ever-growing demand of the Astronomican upon the Emperor was one of the reasons He handed command of the Great Crusade over to the Warmaster Horus after the victorious Ullanor Crusade against the Orks and returned to Terra to complete His Webway Project, an initiative intended to make the Astronomican all but unnecessary as with an extension into the Webway, the starships of Mankind would be able to cross the galaxy in the blink of an eye without having deal with the deadly dangers of the Warp.
The Astra Telepathica
In addition to this invisible beacon among the stars, the forging of the Imperium amongst the stars would not have been possible without the creation and continuation of the Astra Telepathica. This special corps of inter-stellar communicators was created by the Emperor during the final months of the conquest of Terra, as the Emperor had foreseen their absolute future need. For the most part the Emperor did not favour the use of psychic talents in others. As a psyker himself, albeit one of unprecedented power and control, He was well aware of the dangers inherent with human contact with the Warp. Furthermore the Age of Strife on Old Earth had its own "witch-kings" and possessed psykers to attest to those dangers, some of whom the Emperor had personally slain. However, he was able to identify those strong enough to maintain control of their abilities and so with some prudence certain psykers were employed in a variety of special roles in the Imperium, and the Astropaths were to be such an exception.
The Horus Heresy
Just before the outbreak of the Horus Heresy in the early 31st Millennium, the Emperor of Mankind's Imperial Webway Project required more and more of the Emperor's psychic might as he sought to penetrate the corridors of the Eldar's Webway and claim it for Mankind, dangerously weakening the Astronomican's signal. Malcador the Sigillite then suggested a compromise: the beacon would still be guided by the Emperor Himself, but a large part of the required energy would be provided instead by the psychic and life energies of 10,000 sacrificial psykers, allowing the Emperor to pour His own psychic might into finishing His great project. The Emperor approved the notion only with the greatest of reluctance, and also made it clear that this would only be a temporary solution, for He had no intention of claiming the lives of thousands of men and women on a regular basis, indefinitely.
Tragically, the Emperor would never resume His former role as the active ruler of the Imperium because of the wounds He suffered at the hands of Horus during the Horus Heresy's climactic Battle of Terra. Although His mind would continue to calibrate the psychic beam's frequency following His physical internment in the Golden Throne, the Emperor would never be able to resume powering the beacon by Himself, and the temporary measure of having the Astronomican powered by the regular sacrifice of thousands of psykers became permanent. At the present time, the Astronomican is a complex piece of machinery powered by 10,000 specially-trained psykers, whose life-forces are exhausted in a matter of months maintaining the beacon. Their ranks must therefore be constantly replenished with fresh recruits drawn from the tithe of psykers brought by the Black Ships to Terra to be used by the Adeptus Astronomica.
It is the duty of the portion of the Adeptus Terra called the Adeptus Astronomica to maintain all aspects of the Astronomican, including training those Sanctioned Psykers who will power it. The "psychic light" of the Astronomican is beamed from Terra, from the Chamber of the Astronomican buried deep beneath the Himalazian (Himalayan) Mountains. The Astronomican is powered by the psykers trained by the Adepta, though it is the omnipotent will of the Emperor Himself that constantly directs this psychic energy across the depths of approximately 70,000 light years of galactic space. Although the Emperor does not provide the psychic energy of the beacon, only He has the psychic control required to handle such immense amounts of psychic energy and direct it across the galaxy.
The psykers who power the Astronomican are recruited into the Adeptus Astronomica through the division of the Adeptus Terra called the Adeptus Astra Telepathica. They are initiated into the Adeptus Astronomica as Acolytes, and are taught how to use their powers as well as the philosophy of the Adepta and the pivotal role it plays in the continued existence of the Imperium. Astronomica Acolytes learn the lore and technical arcana of the Astronomican. Some achieve a mystic state, gaining the status of "Chosen". It is these Chosen of the Astronomican who are eventually called to serve in the Chamber of the Astronomican. Here they fuel the Astronomican's beacon with their psychic energies; as their powers are drained by the Emperor to fuel the beacon, the life forces of the Chosen slowly fade and they die, their lives a necessary sacrifice for the continued good of the Imperium and the existence of Mankind in a hostile galaxy. This psychic power is then directed by the mind of the Emperor across the width of the approximately 70,000 thousand light year range of the beacon. Because Terra is situated in the galactic west, the Astronomican does not cover the extreme Eastern Fringe of the galaxy. Warp travel beyond the Astronomican's reach is severely limited -- thus, it is the Astronomican's reach that effectively determines the true borders of the Imperium.
Destroying the Astronomican would most likely halt or badly cripple all Imperial Warp travel within the galaxy, effectively bringing the Imperium of Man to its knees, both economically and militarily.
The mutant Navigators, as well as other psykers of any intelligent species, can sense the Astronomican. It is described as a sensation of silvery light and a chorus of many heavenly voices, beginning first with a solitary voice and progressively growing into an angelic choir. It is suggested that "pious Space Marines" who can hear the Astronomican possess the ability to fall into a meditative trance when they focus on it, perhaps as a result of their genetic descent from the Emperor through the gene-seed of their Primarchs. Captain Gabriel Angelos of the Blood Ravens Chapter of Astartes was known to occasionally enter a sort of battle trance when he "heard" the Astronomican while in battle, improving his prowess in combat for the duration of the trance.
The Chamber of the Astronomican is located deep beneath the Himalazian (Himalayan) Mountains of Terra. The original purpose of the device which would become known as the Astronomican has been lost to time; what is known is that it was built by the Emperor of Mankind Himself as a focus for His fathomless psychic power. Because of its vital role in the maintenance of the Imperium, the Astronomican has great religious significance for believers in the creed of the Imperial Cult and the Emperor's divinity and it is also sometimes metaphorically referred to as "the Light of the Emperor", "the Ray of Hope" or "the Golden Path."
Limitations of the Astronomican
Warp Storm activity can affect the overall range of the Astronomican but it generally has a range of approximately 70,000 light years. As the galaxy is about 100,000 light years in diameter and Terra is approximately 25,000 light years towards the galactic southwest, the Astronomican barely penetrates the Eastern Fringe of the galaxy; even less so in the wake of the arrival of the Warp-smothering Tyranid Hive Fleets.
The Astronomican's potent psychic beacon, unbeknownst to the Imperium's Adepts, is what is drawing the Tyranids to threaten the Milky Way Galaxy and all of the Imperium of Man. It is also drawing them, inexorably and irresistibly, towards Terra itself.
- Codex: Tyranids (5th Edition), p. 28
- Codex: Tyranids (3rd Edition)
- Dark Heresy: Sourcebook (RPG), pp. 6, 158, 221, 225, 247, 255, 279
- Deathwatch: Core Rulebook (RPG), pp. 7, 181, 285, 287, 290-291
- Horus Heresy: Collected Visions, pp. 9, 38, 188, 190, 252, 260, 351
- Imperial Amrour - The Horus Heresy: Betrayal - Book One, pg. 21
- Rogue Trader: Core Rulebook (RPG), pp. 7, 19, 48, 61, 155-156, 176, 183, 306, 310-311, 320
- Warhammer 40,000 Rulebook (6th Edition), 139, 146, 160, 169, 173, 197, 215, 403
- Warhammer 40,000 Rulebook (5th Edition)
- Warhammer 40,000 Rulebook (4th Edition)
- Warhammer 40,000 Rulebook (3rd Edition)
- Warhammer 40,000: Rogue Trader (1st Edition), pp. 25-26
- White Dwarf 140 (UK), "Space Fleet: Astronomican" Jervis Johnson, Andy Jones, Simon Forrest & Rick Priestley
- Blood Ravens: The Dawn of War Omnibus (Novel) by C.S. Goto