In Traveller, there are a few rules but no detail. The rules are:
• A Jump takes one week, no matter the distance covered.
• The starship and its passengers are completely isolated from both other ships and people in Jump Space, and from other ships and people in Real Space.
• A starship cannot enter Jump Space within 100 diameters of a planet or a star.
• A starship in Jump Space passing within 100 diameters of a planet or star will be precipitated out of Jump Space into Real Space.
While we have few other concrete details, we can, of course, speculate:
A starship cannot enter Jump Space within 100 diameters of a planet or a star and a starship in Jump Space passing within 100 diameters of a planet or star will be precipitated out of Jump Space into Real Space.
The implication of this is that gravity wells and mass points interfere with both the transition from Real Space to Jump Space, and travel within Jump Space. This reminds me a lot of CJ Cherryh’s novel, “Tri-planetary”, and the triple mass-point of the title which allows starships to drop back into Real Space to both correct course and indulge in illicit trade.
I think it’s in GURPs: Traveller that we first see the idea of Jump Masking. This is where the relationship between the destination Main World and its stellar Primary become important when plotting Jumps. In part, this seems like a game mechanic to force player characters and their ships to spend a little more time in Real Space where they might fall prey to pirates or raiders (and so create tense and exciting game sessions), and in part this seems to be a logical extension of the 100 diameter rule. For instance, our sun – a standard G Class star - has a diameter of about 1.4 million kilometres. This means that a ship has to be at least 140 million kilometres from our sun to Jump, or to exit Jump. As Earth orbits our sun at a mean 149 million kilometres, then ships with line of sight to Earth would, in theory, only have to worry about Earth’s 100 diameter zone.
Though, even if the Main World is outside the 100 diameter limit of the primary star, there will be times of the year when the Primary will be between the Main World and the world any incoming ship is jumping from. In that case, it is likely that there will be an inbound Jump Zone at, say, the destination system zenith and an outbound Jump Zone at the system nadir.
The starship and its passengers are completely isolated from both other ships and people in Jump Space, and from other ships and people in Real Space.
This is one of the distinctive features of Traveller Jump Space in that once the ship has Jumped, it is pretty much self contained within its Jump Bubble. The Jump Bubble extends some little way beyond the exterior surface of the ship so that it is, in theory, possible for other matter to travel along with the ship. CJ Cherryh’s Chanur novels are, to me, the most evocative of Traveller-esque fiction written by someone who has probably never heard of Traveller. In one of the novels, the characters Jump while passing through a dust/gravel cloud near an asteroid belt. When they drop out of Jump Space, there is consternation from local System Traffic control as they release a cloud of near-light speed gravel from their Jump field and send it sleeting across the destination system.
A Jump takes one week, no matter the distance covered.
This is the biggy and one of the constants of Traveller. Jump Drives are rated by the distance, in parsecs, they can move a ship in a single Jump. A Jump takes one week and can range from one to six parsecs. But, how does it work?
We are told that as the Jump Drive activates, it creates a bubble (the Jump Bubble) around the ship. Simultaneously, it tears a hole in three-dimensional space and draws the Jump Bubble into Jump Space. The hole seals behind the departing ship and, to an observer, the ship vanishes. A week later, the Jump Drive tears a second hole in three-dimensional space and expels the Jump Bubble into Real Space. The Jump Bubble collapses during this process and the release of energy creates what is known as a Jump Flash. This Flash is detectable out to several million kilometres so if one wishes to come and go quietly, one needs to drop down out of Jump Space way out in the Kuiper Belt or the Oort Cloud, away from the Zenith and Nadir Jump Zones, and come insystem on a very low deltaV approach.
I postulated a theory that Jump Drives actually access higher energy states. The topography of these energy states directly maps over the topography of Real Space, but each higher energy state is smaller than the next lower. This curves space.
Gravity is a constant, and as such gravity wells created by stars, planets and planetoidal masses penetrate through the stacked energy states. This means that mass points will interrupt travel through higher energy states and precipitate the starship back into Real Space.
As each higher energy state is smaller than the one below it, distances between points are shorter and so travel for the same amount of time will take one further at a higher energy state than at a lower energy state. On the other hand, it takes more fuel to attain the higher energy state which is why a Jump-6 ship needs to have 60% of its mass as fuel, when a Jump-1 ship only requires 10% of its mass as fuel.
Jump Flash is partially caused by the drop down of the starship into normal space, and partially by the leakage of energy from the higher energy states into Real Space.
It is believed that Human-built Jump Drives are not very efficient as Droyne ships do not give off Jump Flashes when they drop down into Real Space. Which makes one wonder what the Droyne know of Jump Space and higher energy states that Humans are yet to discover.