The Adeptus Astronomica has the duty of maintaining the Astronomican for the Imperium of Man, the psychic beacon in the Warp directed by the mind of the Emperor that provides guidance for the mutant Navigators that pilot all Imperial starships in the galaxy. They recruit new members through the auspices of the allied Adeptus Astra Telepathica.
The Astronomica is largely composed of psykers; its non-psyker members are hereditary servants. The organisation is based in Terra's Forbidden Fortress, where even the Adeptus Arbites' Judges and Inquisitors must receive an invitation to lawfully enter. The Adepta's head is the Master of the Astronomican, who represents the organisation as a High Lord of Terra and a member of their Senatorum Imperialis.
Before expanding the Great Crusade to the stars beyond the Sol Sector in the late 30th Millennium, the Emperor of Mankind first ordered the construction of the Astronomican on Terra. The purpose of this colossal endeavour was that the entire device was to serve as a focus through which the Emperor could direct His fathomless psychic energies to generate a partly self-sustaining navigational beacon for the commercial and military starships of the Imperium. The psychic navigational beam generated by the Astronomican was able to cut through the Warp across the galaxy and those attuned to its frequencies and modulations, the sanctioned human mutants known as Navigators, were able to use it as a beacon and pole star when plotting journeys through the Immaterium. Originally, the Astronomican was powered solely by the Emperor, but this duty was eventually shifted to a great choir of 10,000 psykers instead, heralding the birth of the organisation that would become the Adeptus Astronomica.
In addition to this invaluable beacon among the stars, the forging of the Imperium during the Great Crusade would not have been possible without the creation and skills of the Astra Telepathica. This special corps of interstellar telepathic communicators was created by the Emperor during the final months of the Unification Wars, as the Emperor had foreseen the absolute future need for a means to communicate across the depths of space if a new interstellar human empire was to be created. As a psyker Himself, the Emperor understood the use of psychic talents in others, and was well aware of the dangers inherent in any contact with the Warp.
The Age of Strife on Old Earth had been afflicted by many "witch-kings" and daemon-possessed psykers whose existence embodied those dangers, some of whom the Emperor had personally slain. However, He was able to identify those psykers with minds and hearts strong enough to resist the temptations of the Warp. These men and women were then employed in a variety of special roles in the newborn Imperium. Amongst these so-called "Sanctioned Psykers" were the Astropaths, "astro-telepaths," capable of telepathically communicating with others of their kind over vast interstellar distances through phantasmal visions, empathic transmission and oenological vision allegory rather than words. This was an essential talent for the success of the Imperium due to the vast distances that separated its inhabited worlds. The only practical means for the Imperium to maintain any semblance of unity across interstellar distances was to make use of Astropaths, but such communication could prove as temperamental as the Warp itself.
In the present-day Imperium of the late 41st Millennium, capable psykers not talented or disciplined enough for use by the Adeptus Astra Telepathica are handed over to the Adeptus Astronomica for a different kind of training. The task of the Adeptus Astronomica is to train the young psykers they receive from the Adeptus Astra Telepathica, so that they can serve in the Chamber of the Astronomican. The whole organisation is basically a training institute run by a class of older members known as Instructors. Some of the Instructors are given the title High Instructor and specialise in a certain area of teaching. The day-to-day administration of the Astronomica, such as maintenance of its facilities, are taken care of by a body of administrative functionaries and hereditary servants.
Young psykers recruited from the Adeptus Astra Telepathica are initiated into the Astronomica as Acolytes. They are taught not just how to use and control their psychic powers but are introduced to the Lore of the Astronomican. They learn the value of their lives, study philosophy and are gradually brought to a deeper understanding of the universe and the nature of the Warp. Those who achieve this mystic state become the Chosen and are eventually called to serve in the Chamber of the Astronomican beneath the Himalazian (Himalayan) Mountains in the Imperial Palace on Terra alongside 10,000 other Chosen. Their psychic energies will be used by the mind of the Emperor in the Warp to power the Astronomican as He directs its psychic beacon across the galaxy. In only a few short solar months each Chosen will die, their psychic and life energies completely drained by the sheer intensity of the demands the Emperor's mind will place upon them. As such, new Chosen must constantly be culled from the populations of the Black Ships of the Adeptus Astra Telepathica.
Due to their duties, members of the Adeptus Astronomica are rarely seen off of Terra, though the Instructors and High Instructors as well as the non-psychic functionaries who are responsible for the continued maintenance of the Astronomican are allowed to travel freely. Similar to the Tech-adepts of the Adeptus Mechanicus, the Adeptus Astronomica is considered a form of religious order in the 41st Millennium and its members wear clothing similar to other devotees of the Imperial Cult, including blue, hooded robes.
The status of Chosen is considered a great honour among the Imperium's Sanctioned Psykers. To fall short of attaining this goal is not a failure; rather they go on to become Instructors or are absorbed as administrative functionaries in the Forbidden Fortress. Those Adepts of the Adeptus Astronomica who do achieve Chosen status are considered above the Instructors and even the Master of the Astronomican in rank. Their heads are shaven and they wear yellow robes and the scarlet badge of the Chosen. The rest of their lives are spent in prayer and contemplation until they are called to serve in the Chamber of the Astronomican.
No Victory Without Toll
Only the Emperor's mind is powerful enough to survive directing the raw forces supplied by the servants of the Adeptus Astronomica to shape the psychic beacon of the Astronomican. The same survivability does not hold true for those members of the Adeptus Astronomica themselves, for their fate is a sad one. The efforts of generating so much mental energy soon destroys them, leeching their souls and reducing them to empty husks. Many die every day, but they are not the only psykers who are asked to make the ultimate sacrifice, for the Emperor cannot eat as men eat, or drink or breathe air. His life has long since passed beyond the point where such things can sustain Him and the only viable sustenance for the Emperor is human life force –- souls -– and He has an insatiable appetite. Not just any human will suffice for the Emperor's table, for the donor must have psychic powers.
Therefore, the Imperium is scoured by the vast flotillas of the Black Ships in a tireless search for emergent psykers. During their long, crowded journey back to Terra, some of the psykers will be found to have the strength of mind to be recruited into the Adeptus Astra Telepathica or become trained by the Adeptus Astronomica, but many more will serve their Emperor in a more gruesome way. They are given wholly to the weird machinery that surrounds the Master of Mankind and their souls are siphoned, slowly and agonisingly, to feed His mighty spirit. Many hundreds, even thousands, must die in this way every day for the Emperor, the Imperium, and all of Humanity to survive. The Emperor's dream of a new age of enlightenment, a time when Mankind was freed from superstition and ignorance, has turned into something far different. His Imperium has long endured, but only by virtue of an oppressive and necessarily harsh rule carried on in His name. It is an age of tyrants and of unreason, an era of stagnation and intolerance. His people have regressed into religious obfuscation while the Emperor sits immobile, His thoughts unknown; for the creature whose will extends over a million worlds is unable to lift a shrivelled finger for himself.
- Codex Imperialis (2nd Edition), pg. 94
- Dark Heresy: Core Rulebook (RPG), pg. 247
- Horus Heresy: Collected Visions, pp. 190, 252, 352
- Warhammer 40,000: Rogue Trader (1st Edition) by Rick Priestley, pp. 133, 139-140
- Warhammer 40,000: Rulebook (Digital Edition) (7th Edition), pp. 275, 279, 320, 379
- Warhammer 40,000: Rulebook (6th Edition), pg. 139
- White Dwarf 140 (US), "Space Fleet: Additional Background, Rules, Data Cards & Counters - Adeptus Astronomica," Jervis Johnson, Andy Jones, Simon Forrest & Rick Priestley, pp. 46-75