Ease of travel between worlds is one of the cornerstones of our unified society and the right to travel is one of the primary laws of humanity. As such, interplanetary travel is easily affordable for anyone. The costs of space travel are recouped in cargo taxes and fees - people are cheap, luggage is expensive.
The primary type of space travel is through use of the A-S drive. The drive surrounds the ship with a field that causes it to slip into another 'layer' of space. The ship can then emerge some distance away. Since time in the underspace corresponds to distance travelled and staying inside underspace too long is dangerous, the maximum safe range of a jump is about two minutes or two-hundred light years.
Due to the nature of the A-S drive and the size of the machinery involved, a ship needs to be within a certain ( admittedly wide ) size limit.
While with the proper precautions A-S drive travel is safe for anyone in any condition, about ten percent of the population experience dizziness, nausea, or fainting during jumps. The effect can be lessened or sometimes neutralised if the passenger is in an interior chamber of the ship rather than by a window.
The main problems with underspace travel are the possibilities of emerging inside or too close to a star or solid body - given the speeds involved, it's hard to aim accurately - or staying in the underspace too long. Both of these problems are mimimised by the hub beacons.
Hub beacons have a signal range of about twenty light years - that is, a ship in underspace can detect a beacon that is in real space twenty light years away.
While the risk of emerging inside a solid object is near zero between systems, generally a ship is aiming to appear inside a star system. On inhabited planets, a beacon can either orbit as a satellite or be attached to a building. A building with a beacon is called a 'lighthouse'.
There are 'shipping lanes' of hub beacons networked between the inhabited worlds. Beacons are spaced at ten light year intervals so that a ship can slip into and emerge from underspace like a swimmer coming up for breath on every stroke. The beacons also relay traffic information so that ships travelling the same lane can avoid hitting each other.
The shipping lanes are also used for communication between worlds, as signals can travel the underspace.
Despite having A-S technology for two thousand years, the underspace is still not understood. It operates by different physical laws. It generally appears to be swirling mist, but attempts to bring samples into real space invariably fail - the samples dissipate as soon as they enter real space. No life has been discovered, though the possibility exists that it wouldn't be recogniseable to us.
The A-S field provides a certain amount of protection against the underspace, but it is still unsafe to remain in the underspace for more than two minutes. Longer than that and ships - or just part of the crew - vanish. Crew vanishing tends to happen by section - it will not be a random assortment of people from throughout the ship, it will just be the fore section, or such.
A physical pilot is needed in the underspace - drone and controlled ships vanish.
Once in a while someone tries to set up a planetary bridge so that a person could walk from one planet to the other. Attempts have invariably ended in chunks of countryside vanishing into the underspace.
The Dead Zone
The Dead Zone is a thirty light year sphere around the star system that corresponds to the closest point between our universe and the Yogzarthu universe. Within the zone, use of the A-S drive and A-S-based communication is forbidden lest the Yogzarthu pick up the signals and find their way back.
The Yogzarthu cannot enter our universe without a signal to follow, and presumeably the reverse is true as well. The reasons why this is have not been studied due to the risk involved.
With the advent of stasis technology, it became possible to put a crew into stasis and send them to a planet at sublight speeds. This was only done once, with the Second Mahi Expedition to the Dead Zone Planet.
The Aquaans had only one ship - the Waayaun. The Waayaun was a generational ship that could only travel at sublight speeds. It was enormous, full of refining and manufacturing technology, hydroponics and artificial greenhouses, and at least eight back-ups for every system. Despite its size, it could be landed on planets.
The Yogzarthu had spacecraft - the 'space whales', enormous creatures adapted to live in space. Whether they were a natural species or created by the Yogzarthu is unknown. The space whales were cybernetically augmented, though did not seem to have had faster-than-light capability. During the war, the Yogzarthu duplicated and adapted A-S drives to their ships.
Qohathoth writings mention ships and imply their space travel used their vibrational science, though how a sound-based propulsion system functioned in a vacuum is unknown.
Qohathoth writings mention the tsilok-Shilau achieved faster-than-light travel using a substance they called 'dark matter', which somehow twisted space around them. It is thought this was a variation of the A-S drive.