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Locating Jump Gates

This article originally appeared in the August 2012 issue of Freelance Traveller.

Mongoose Traveller uses a fairly arbitrary and relative system to determine where planets are in a system, relative to each other and the system’s sun. This works fairly well when you use the standard gravitic drives and are prepared to ‘hand wave’ away the exact distances travelled – giving a rough idea of how long it will take to get from point A to point B is enough. However, when you use reaction drives in the Babylon 5 setting, it can become critical – making sure you have enough fuel and time to get from point A, through a Jump Gate, and to point B in another system can be tricky to work out when you have no other points of reference. Hopefully, this article will help to sort out a few of those points of reference.

Jump Gate Positions

Jump Gates are large structures consisting of three, four (most common), or five, ten-km-long vortex arrays arranged around a central point where they generate a hole or gate connecting normal space with hyperspace. The vortex arrays are not connected to each other, so rely on small station-keeping thrusters to maintain their alignment. Because of this, they are always found at Lagrange points oriented on a single planet within a system, where they can maintain their position and orientation with the minimum of effort. Similarly, because of their size, they need a Lagrange point that is large enough to accommodate the whole structure – this is why they are never constructed at Lagrange points oriented on small planets or moons.

Lagrange points are five points around a planet or moon where the gravitational forces of the sun and other planets in that system roughly cancel out, meaning that anything in those positions will orbit around the sun but will stay (more or less) in the same spot, without being pulled into the sun or another planet. They are normally identified as “L1” through “L5”. The relative locations of the Lagrange points are shown in the diagram to the right.

All of the jump gates in the explored galaxy fall into two main types: those built by the original gate-builders, which are between 6,000 and 10,000 years old; and those built by the younger races, which are less than 1,000 years old (about when the Minbari discovered hyperspace travel and the gate system). The original, older gates are found in many systems and are located at any of the Lagrange points, but all usually around a planet in the outer zone – leading many scholars to believe that they were built primarily as a means for the builders to explore the galaxy.

Of the gates built by the younger races, those in the race’s home world system are often located in the outer zone for security reasons, as it gives any defences several days to prepare for an invasion (indeed, when Earth finally gained jump gate technology it shut down the Centauri-built gate in Earth orbit and built a new one near Jupiter). Gates built by the younger races in colony systems are often built around one of the planets in the habitable zone, where ships going to and from the inhabited planets will have less distance to travel.

L3 jump gates are rare and are usually older gates built in uninhabited systems. The younger races prefer not to build gates at this point as they are too far away from any inhabited planets and, if in the same orbit as an inhabited world, have the star between the planet and the gate – from a security point of view, this gives hostile ships a big advantage.

L4/L5 jump gates are quite common, and are usually older gates, although due to the nature of these points, occasional asteroids or dust clouds are found nearby. Many of the younger races prefer this position when building a gate in a colony system as it is reasonably close to the planet, but far enough away to give warning if hostile ships come through.

L1/L2 points are fairly common in inhabited colony systems as they provide a fast transit from the jump gate to the planet, which maximises the potential for trade in the region.

Most planets will have at least one, if not several, space stations in orbit around the nearest planet to the jump gate – as transfer points between ships coming from hyperspace and ships coming up from the planet or other planets in the system. Some planets even have space stations in retrograde orbits (i.e., opposite to the normal rotation of the planet) so that the station remains facing the jump gate at all times (such as the Babylon 5 station). These space stations can be quite exotic, being a meeting (and trading) place for the local race and aliens from all over the explored galaxy.

Planet Locations

First, use the rules on pg 92-93 of Book 3: Scouts to determine the numbers and positions of your planets in a system. Once you have done that, work out the distances of the orbits from the systems sun – to do that, use the (extremely arbitrary) system here:

Inner Zone planets all have orbits roughly 0.25 AU apart (so the first planet would be 0.25 AU from the sun, the second 0.5 and so on).

Habitable Zone planets all have orbits roughly 0.5 AU away from the Inner Zone planets and each other (if there aren’t any Inner Zone planets in your system, assume that the first planet orbits 0.5 AU from the sun and work out from there).

Outer Zone planets start 1d3 AU from the Habitable Zone planets, and then double each orbital distance after that.

Example: having worked out a system on the Scouts system, we end up with 9 planetary orbits: 2 Inner Zone planets, 2 Habitable Zone planets and 5 Outer Zone planets:

Orbit  1      2      3      4      5      6      7      8      9     (Orbit Number)
*      I      I      H      H      O      O      O      O      O     (Zone: I:Inner, H:Habitable, O:Outer)
0    0.25    0.5    1.0    1.5    2.5     5     10     20     40     (AU from Star)

For simplification, I am rounding the Astronomical Unit (AU) measurement to 150 million km.

Determine Jump Gate Location

Once you have worked out the planets and their orbits it’s time to find out where the jump gate actually is and how far away it is. Roll 1d6 on the Focal Planet table to find out which planet the gate is oriented on:

Focal Planet
Roll Type of Planet
1 Inner Zone
2 Non-Mainworld Habitable Zone
3 Mainworld
4 Mainworld
5 Non-Mainworld Habitable Zone
6 or more Outer Zone
DM +1 if major system; DM +2 if racial Home system (not cumulative)

Having done this, roll another d6 to find out in what position the gate is in:

Gate Location
Roll Lagrange Point
1 Roll 1d6: 1-3, L1& 4-6, L2
2-3 L4
4-5 L5
6 Roll 1d6: 1-3, Choose; 4-5: L3; 6: Choose, and place a second gate on a different focal planet.

From this, you can work out the distances from the gate to the planet that it is oriented on:

Gate Distance
Lag. Pt. Distance
L1, L2 Orbit Number/2 1 million km
L3 2 Orbital Radius in millions of km
L4, L5 Orbital Radius in millions of km

The distinction between L1/L2 or L4/L5 is only important if the GM wishes to use that info to flesh out additional details.

For our example system, L1/L2 are 1.5 million km, L3 is 300 million km, and L4/L5 are 150 million km from the world.