Astropaths are psykers who are responsible for carrying out all of the superluminal communications that tie the Imperium of Man together using their telepathic abilities. Chosen from the multitudes of psykers born across the vast breadth of the Imperium, these individuals are detected and interred by one of the fearsome Black Ships of the Adeptus Astra Telepathica and brought to Terra due to their great power and their potential use to the Imperium. The fate of the vast majority of these psykers is to fuel the insatiable fires of the Astronomican so that the Imperium might be held together for another day. Of those allowed to live, a tiny fraction are judged strong enough to undergo training and go on to serve the Imperium in a staggering array of capacities as a Sanctioned Psyker, from Inquisitor to Imperial Guard Battle Psyker.
Those chosen to become Astropaths undergo the ritual of Soul Binding, in which the body and soul are scoured clean of the taint of the Warp by the searing purity of the Emperor of Mankind's beneficence. After months of preparation through fasting, prayer and ritual preparation, the chosen psykers are brought into the very depth of the Imperial Palace in procession of a hundred at a time, there to undergo a ritual that will either kill them, drive them insane, or bind them for all eternity to the Emperor. So intense is the ritual that the supplicants' sensory organs are almost totally overloaded - leaving them blinded by the experience - with many suffering further nerve damage, incurring loss of smell, touch, or hearing.
Being soul-bonded with the Emperor is necessary, however, as it greatly heightens an Astropath's already formidable psychic powers and gives them the ability for which they were chosen—to send telepathic messages through space instantaneously across large interstellar distances, thereby becoming the main network of interstellar communications in the Imperium. Most Imperial starships have an Astropath on board and most Imperial planetary governments have access to Astropaths.
Without the Astropaths' unique ability, long-distance communication in the Imperium would be impossible. However, their powers are still susceptible to the local conditions of the Warp (such as Warp Storms) and as such, their messages can be greatly delayed or lost. Due to the strenuous nature of their job and the result of their soul-bonding with the Emperor, many Astropaths are physically frail and can die quite young through mental exhaustion or overexposure to the energies of the Warp.
Just as travel within the Imperium is a complicated and inexact science, so too is the business of exchanging messages between the many and varied planets that make up the Imperium. Planetary communications systems such as vox-casters, hardwired telegraph and telephony lines and the more advanced vox-communicators suffice to pass messages amongst the nations of a world, yet have almost no use beyond the bounds of the planet’s surface. Such devices require many Terran years for their signal to reach even the nearest planet of a star system and have no surety of even being detected when they arrive. The perils of travel ensure that human or Servitor messengers are just as unreliable and potentially as slow as radio or other energy wave communications.
The Imperium is forced to rely upon communication by psychic, or astropathic means. Astropaths communicate with symbols and iconic images, projecting these messages through vast distances of space by means of psychic power drawn from the Warp. This process is usually exhausting and requires ritual and focus in order to keep the psyker in the right frame of mind. These can take a wide variety of forms, such as use of the Emperor's Tarot, vision quests, automatic writing, trances, séances and the like. The Gaolist Astropaths of Hredin for example, spend many years etching their messages onto painstakingly illuminated sheets of iron and then destroy the work of art upon a massive grinding wheel when they are ready to transmit the information. The pain of annihilating a much-loved labour is said to produce psychic messages of unparalleled clarity.
These messages are received by fellow Astropaths in various ways. Some appear as vague and troubling dreams, whilst others appear as visions or mystic portents. Others appear within whatever ritual method or divination technique the receiving psyker happens to practise. Thus warning of an Ork invasion might appear as a glistening imperfection in fish entrails, a looming cloud of smoke, bleeding orifices or a worrying combination of runes or sigils within a holographic matrix. Astropathic messages must not only be transmitted from one Astropath to another but decoded at the other end. Each Astropath employs slightly different symbols and each has a preferred style or “flavour”. Some messages take weeks of poring over tomes of augurs and symbolism before they can be reconstructed, though the best Astropaths can do this word for word. Some remain a mystery forever. Some messages are received by Astropaths at entirely the wrong end of the galaxy and must be passed on to others who are nearer the place in question.
Some messages simply do not get to their intended recipient or are drastically misinterpreted along the way. In addition, there are too few Astropaths. Most worlds, especially those with small populations or on the fringes of the Imperium, have no Astropaths at all, and must rely on the infrequent visits of passing Chartist ships or Administratum census-takers to make contact with the outside galaxy at all. For this reason the Adeptus Terra cannot react quickly to every event in the Imperium, even when an event occurs that is great enough to attract the notice of the vast and ponderous bureaucracy. On most worlds, the Imperium feels very far away.
- Codex Imperialis (1st Edition), by Rick Priestley, pp. 36–37
- Rogue Trader Core Rulebook (RPG), pp. 48, 154, 158, 162-167, 168-170, 171-172
- Warhammer 40,000: Rogue Trader (1st Edition), pp. 133, 147-148
- Warhammer 40,000 Rulebook (4th Edition), pp. 94, 123
- Warhammer 40,000 Rulebook (5th Edition), pp. 104, 124
- Dark Heresy Core Rulebook (RPG), pp. 246–264