"It is the great paradox of the Imperium that the hungering dark beyond our worlds is also its only hope of salvation. Well, one of the great paradoxes. There are a few.”
–Boehm Melasc, Navis Nobilite princeling
Travel in the 41st Millennium is the province of an elite few, even when the trip is from one district of a hive city to another. "Most Askellians live and die without going far from their home or station of work. To move between worlds, then, is an almost unimaginable privilege for those used to a planet’s surface". The voidborn and the crew of those vessels that ply the distances between stars live in this state constantly, but for them, the situation is reversed, and they can see almost nothing but the corridors of their native ship. The passengers which a ship’s crew might ferry are considerably more capable of appreciating the marvels of space travel than the crew themselves, if only for having something against which to contrast it.
And yet, for all its wondrousness and convenience, space travel is often viewed with dread and superstitious awe. The vessels in the Imperium’s service, from lowly transports to mighty battleships, rely upon technologies more subject to rote operation and prayer than any sense of comprehension; the failure of any one system can quickly spell doom for the entire ship-borne population. Furthermore, while travel within a system is relatively safe—concerned only with asteroids, gravity riptides, piracy, and equipment failure—true interstellar travel requires traversing the perilous currents of the Warp.
To Walk Between Worlds
Travelling between different stars is not a casual endeavour. Once a ship translates into the Immaterium, it is able to make incredible journeys at unimaginable speeds, but given the vastness of interstellar distances, even this may take a long time. The amount of time a journey takes within the Warp depends on a number of factors, but the most crucial of these is the reliability of the route that the vessel traverses. Some routes have been carefully mapped by many centuries of traffic, while others are chaotic messes, ignored or avoided by most.
Within the Askellon Sector, Warp routes are typically placed into one of four categories. The Grand Processional is a single route of incredible size that runs swiftly and reliably between the sector’s most prominent worlds. Tributary paths are much more common and connect more systems, but run more slowly and with reduced stability. Lesser routes cobweb across Askellian space in networks of tenuous and barely charted paths, and are used only if no other passage is known. Uncharted paths are just that, perhaps the first time any vessel has attempted using the route or one that has not been employed for so long that no one knows its status. Their only constant in these routes is in the high uncertainty of the duration (and possibly, also the point of emergence).
Of special note is of course the Askellon Sector's infamous Pandaemonium. This Warp storm could erupt at any moment, dimming the blessed light of the Astronomican or bedevilling any vessel the Unending Storm captures in its ethereal talons. Other factors that could affect a journey might be unexpected Warp storms, favourable Warp currents, devout pilgrims who constantly chant prayers to calm the Immaterium, or nascent Psyker (DHII) eruptions that can roil the Sea of Souls.
A Realm Within the Void
It would be a mistake to think of a spaceship as a mere vehicle, a simple conveyance for getting from one world to another. Even the smallest is crewed by hundreds of toiling deckhands, while the complement of a Warp-capable craft can number in the tens of thousands. Although the crew of a space ship is bound to attend their duties, with even the elite bridge officers being occupied by vital tasks that dominate their time, the necessities of human life inevitably turn a ship into something resembling a city. In the case of the largest battleships and bulk cruisers, this can be the case in fact, where the crew in the depths of the ship might never know for sure whether they live and work in a planetbound hive or if they are travelling through the void.
Travel Within a System
Travel though realspace to other planets or destinations within a system is often much more reliable than through the Warp, so long as the vessel’s plasma drives maintain thrust. Though durations can vary, typically voyaging from one planet to another is a matter of a few days, and venturing outwards to the far edges of a system (typically needed before a ship can safely translate from realspace into the Immaterium) can take roughly a week. While usually not as perilous as travel through the Warp, voyages within a system can offer threats of other natures. Though there are likely few dangers from daemonic incursions, attacks from pirates and xenos raiders are much more possible. Asteroids, comets, and other stellar debris can act as cover—from behind which all manner of enemies can lay in wait—and careful captains always keep an eye on their augur arrays for signs of predators.
Impossible Odysseys
To travel between the stars in the 41 st Millennium is to abandon one’s notions of what is possible. The sheer scale of an Imperial starship already boggles the minds of many humans, but the distances crossed in an interstellar journey are almost beyond comprehension, involving measurements so large as to be meaningless in reference to anything a human could directly experience. But most of all, the bounds of the possible are tested by the journey through the Warp, a realm where madness is the only law. Warp-travelling vessels are ostensibly protected by Gellar Fields, ancient technological devices that project a bubble of reality around the ship to guard it in this hellish realm. However, even when the Gellar Field is functioning flawlessly, the mood on a ship during Warp travel is often restive and filled with apprehension.
The ship’s chapels are invariably packed during such journeys, and even the most minor incidents of bad luck are often treated as a sign of impending doom. "When the Gellar Field wavers, even for the merest fraction of an instant, things become much worse. At best, insanity begins to take hold on those onboard, filling their dreams and waking hours alike with visions of horror and despair. Ominous whispers might fill the air, or lights flicker despite a steady flow of power. Given the volatile mood the crew is likely to possess, this is a problem in itself, but it pales before a true breach of the Gellar Field, which allows the daemonic inhabitants of the Warp access to the ship or the souls of the crew."