From what I understand of Jump Drives -- which, let's be honest, are based upon vague assumptions about physics and lots of handwaving* -- they work by generating a field of some sort which propels the ship into a higher-energy state (or dimension) and it is the presence of a gravity well** which collapses the Jump Field and returns the ship to normal space.***
Sensibly, this means the reverse is also true: a safe, reliable jump field cannot be generated within a gravity well of any sufficient strength. This is backed by the rules, which states that jumps cannot be made at all within 10 diameters of any planet or solar mass, and that safe jumps cannot occur within 100 diameters of same. There are even charts detailing how long, on average, a ship of X thrust must travel in order to reach a safe jump point.
Which makes it all the more irritating to discover that there are no handy rules for determining where they jump into the system in the first place.
Since there must be a gravity well to interrupt the jump field, the ship needs to arrive somewhere more concrete than a vague reference to "in system." Are they close to the mainworld, or is it on the other side of the solar system? Is the gas giant along their path? Or are they circling that tiny, lone planetoid with the elliptical orbit deep in B.F.E.?
"Getting there," I am told, "is half the fun," and so in the spirit of this fun I offer a more complicated process for arriving in-system. Why more complicated? Because Traveller.
1. Find the system to which they're going.
I wish to state for the record that there's no way I could run a Traveller game without the internet. The resources it's offered me have been extraordinary. One of these resources is the Interactive Atlas of the Imperium.
Let's say that the PCs are jumping into, say, Wonstar (0538/Five Sisters, Spinward Marches). A quick click on that link will give you access to system data and planetary UWP (including stellar type!) Another click on the system detail link will give you the planets in the system and their orbits.
2. Determine the position of the planets.
I don't keep track of precisely what day it is on the Imperial calendar (Bad Traveller GM! Bad!) because I would rather, you know, have fun than worry about how many hours were spent in jump space and therefore what day it is. So I just cheat and enter the date of the game we are currently playing. Since we play weekly, the dates naturally change, and I have yet to worry about them hopping back and forth between two systems. If that becomes a habit, I would naturally need to keep a closer eye on things for sake of consistency, but until that happens I'll just fake it.
3. Roll to determine which planetary mass collapsed the jump field.
Roll, or have one of the PCs (preferably the pilot or astrogator) 2d6-1 and having that result be the planet to which they are within 100 diameters. In case of an empty orbit, choose the closest planet based upon how well the astrogator plotted the jump route.
Normally, plotting a jump route is an Easy (+4 to the roll) task, minus the number of parsecs in the jump -- but this is just to get to the system. If the astrogator wishes to plot a more precise course that takes orbital mechanics into account, then the task becomes Average (+0) and the degree of success is applied to the results of the roll above, thereby getting the ship closer to its destination. An astrogator wishing to aim for a precise planet will take a penalty: -2 if aiming for a large mass such as a gas giant, -4 if aiming for a medium-sized planet (4 through 10 on the size table), and -6 if aiming for anything smaller.
4. Plug in their starting slot, destination slot, and thrust.
Et voila, you have transit time! As well as plenty of opportunities to get into trouble with pirates, customs ships, wreckage, navigational hazards, and Eris knows what else.
Leaving the system uses a different set of numbers, which I shall save for another post.
*This is not at all meant disparagingly. I myself occupy the back corner where pseudoscientific jargon happens to back into "Well, that just might be barely plausible."
** How is it that gravity somehow propagates into this higher dimension? There's a plot seed here.
*** Unless you are of the opinion that the field shuts down because all the jump fuel is exhausted.**** This strikes me as terribly unwise -- the interstellar equivalent of "Sure, we can make it to the next gas station" and "Well, when the car stops moving our trip must be finished."
**** Which isn't to say this isn't an option; I expect certain deep-space refueling facilities used by the military can only be reached this way. I'm just saying I wouldn't do this as my first choice, and certainly not on a regular basis.
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