The two primary methods I use to generate star systems for Traveller games are GURPS Traveller: First In and Traveller5. First In, as might be expected, takes a fairly granular approach and is pretty formula heavy. Not being an astrophysicist myself, I can only assume the additional complexity provides additional accuracy. In contrast, Traveller5 opts for simplicity in exchange for a much less granular product. However, it seems the Traveller5 designers tried to ensure that their results would roughly correspond with a more detailed system like First In. For example, Traveller5 assumes that worlds in orbit 0 or 1 are tide-locked to their star. And that simplification holds up fairly well.
In addition to being more granular, the First In design sequence is also somewhat more realistic than the sequences used by other versions of Traveller. It’s an open question how hard the science of Traveller is supposed to be. Purists might argue that any setting with FTL drives can’t be “hard science fiction,” and certainly many of Traveller’s most direct literary influences are pulp stories, where the emphasis was as much on adventure as science.
But that said, Traveller does retain a certain level of grittiness. And there‘s some evidence that the Third Imperium setting is a little bit harder than the “raw” Traveller ruleset, which can be plenty gonzo if a referee wanted to run it like that. By incorporating just a few elements from First In to Traveller5, we can bring a little more realism to system generation without adding too much complexity.
Planetary Size and Atmosphere
Here’s a good example of where the Third Imperium setting diverges from raw Traveller rules. In Traveller5 a mainworld’s atmosphere is generated by Size plus Flux. But this can easily result in worlds much too small to retain the atmosphere generated. A size 3 world, for example, could have a dense oxygen-nitrogen atmosphere of 8, even though the world simply has too little mass to retain oxygen molecules.
During the T5SS review of Third Imperium sector data, Donald McKinney employed a very simple hack to ensure that the atmosphere code was always paired with a plausible planetary size: If Atmosphere is 1, A, B, or C, adjust Size to 3+. If Atmosphere is 2 or 3, adjust Size to 4+. And if Atmosphere is 4-9, adjust Size to 5+. This simple adjustment goes a long way toward making Traveller worlds that line up with planetary science.
Orbital Zones
Traveller5 does not consider inner and outer orbital zones—the limit points beyond which planets are unlikely to form or be captured. While Traveller5 implies that worlds could be placed all the way out to orbit 17, First In assumes that most main sequence stars rarely have planets beyond orbit 8 or 9.
Rather than grind out a bunch of formulas to determine orbital zones, I use the following reference chart when fleshing out a star system. I keep a version of the following Orbital Zones table in an Excel file and just select the column that most closely matches the spectral type of the primary star. Then I cut and paste this column into a new worksheet that then forms a template for populating the system. Since giants, subgiants, and other stars off the main sequence are fairly rare, this chart works for 99% of the systems I work with.
| Orbital Zones by Spectral Class | Distance (AU) | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| A0 | A5 | F0 | F5 | G0 | G5 | K0 | K5 | M0 | M5 | M8 | Min | Avg | Max | |
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0.15 | 0.20 | 0.30 | ||
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0.30 | 0.40 | 0.55 | |
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 0.55 | 0.70 | 0.85 | |
| 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 0.85 | 1.00 | 1.30 | |
| 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 1.30 | 1.60 | 2.20 | |
| 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 2.20 | 2.80 | 4.00 | |
| 6 | 6 | 6 | 6 | 6 | 6 | 6 | 6 | 6 | 6 | 4.00 | 5.20 | 7.60 | ||
| 7 | 7 | 7 | 7 | 7 | 7 | 7 | 7 | 7 | 7 | 7.60 | 10.00 | 15.00 | ||
| 8 | 8 | 8 | 8 | 8 | 8 | 8 | 8 | 8 | 15.00 | 20.00 | 30.00 | |||
| 9 | 9 | 9 | 9 | 9 | 9 | 9 | 30.00 | 40.00 | 58.00 | |||||
| 10 | 10 | 10 | 58.00 | 77.00 | 115.00 | |||||||||
Light gray cells indicate orbits beyond the inner and outer orbital zones. No worlds should be placed there. Light green cells are the Habitable Zone (HZ) orbits, light orange cells are HZ-1 orbits (with hot or tropical climates), blue cells are the HZ+1 orbits (cold or tundra climates), and light blue cells are HZ+2 orbits (frozen climates). The dark blue line indicates the snow line, the point beyond which the largest gas giants are likely to form during planetary formation. Note that this chart reflects GURPS Space 4e data, which slightly tweaked the First In formulas.
One consequence of the chart is the Traveller5 “number of worlds” is occasionally much larger than the available orbits. An M8 V star, for example, only has 6 open orbits—but could potentially have as many as 20 worlds (the mainworld + 3 belts + 4 gas giants + 12 other worlds) under Traveller5. In such cases, one would probably need to count gas giant satellites in order to ever get to 20 worlds.
It’s worth noting that the T5SS review removed most primary stars with less mass than M3 V: another example of the Third Imperium setting using somewhat different rules than Traveller5.
Mainworld Type
Traveller5 uses a simple Flux roll to determine whether the mainworld is a planet or satellite, without consideration to orbit or mainworld size. In First In, tide-locked worlds do not have satellites, period. The maximum size of a satellite is 7, and the satellite needs to orbit a sufficiently large planet. Once you know the mainworld size and orbit, I use a simple method to determine whether the world is a planet or satellite.
- Orbits 0 or 1, always planet
- Orbit 2+, size 1 or 2, always satellite
- Orbit 2+, Size 34567, Roll 2D + Size. If > 9, the mainworld is a planet, otherwise a satellite
- Orbit 2+, Size 8+, always planet
Forbidden Zones
In First In, “If the star is a member of a multiple star system, planets will not form at certain distances. The presence of a companion star makes a stable planetary orbit impossible in these zones.” The following chart establishes forbidden zones by Traveller5 orbit.
| Companion Distance | Orbit | Forbidden Orbits |
|---|---|---|
| Companion | <0 | 0 |
| ( Close ) | 0 | 0,1 |
| 1 | 0,1,2,3 | |
| 2 | 0,1,2,3,4 | |
| 3 | 0,1,2,3,4,5 | |
| 4 | 1,2,3,4,5 | |
| 5 | 2,3,4,5,6 | |
| { Near } | 6 | 4,5,6,7 |
| 7 | 5,6,7,8 | |
| 8 | 6,7,8,9 | |
| 9 | 7,8,9,10 | |
| 10 | 8,9,10,11 | |
| 11 | 9,10,11,12 | |
| [ Far ] | 12 | 10,11,12,13 |
| 13 | 11,12,13,14 | |
| 14 | 12,13,14,15 | |
| 15 | 13,14,15,16 | |
| 16 | 14,15,16,17 | |
| 17 | 15,16,17 |
By looking at the data like this, it becomes clear how hard it is to have a close companion star and a habitable world.
Example: The Wair System
So, if we put all of these tweaks together and apply them to Traveller5 system generation, we get a bit more realism with not too much more fuss. Since I primarily play in the Third Imperium setting, I generally begin with some basic details of the system. The Traveller Map gives us the stars, the number of worlds, and the number of gas giants and planetary belts. We just need to arrange these details into a coherent pattern.
Let’s use, as an example, Wair: a high population world in Alpha quadrant of Magyar sector, bordering Dark Nebula. Here are the Traveller Map data:
| Hex | Name | UWP | Remarks | {Ix} | (Ex) | [Cx] | N | B | Z | PBG | W | A | Stellar |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0116 | Wair | C89A98C-C | Hi In Wa Pz | { 3 } | (C8E+5) | [CC8F] | - | - | A | 701 | 11 | SoCf | G2 V M6 V |
Pulling the G0 V column from the Orbital Zones table, we pick up the Traveller5 worldgen sequence and roll for the mainworld placement and get HZ+1, which is orbit 4. With its high hydrographic percentage (A), Wair is a good candidate for being a Hoth-type ice world. Because of the world’s size (8), it is a planet.
| Orbit | [Primary] | G2 V |
|---|---|---|
| 0 | ||
| 1 | ||
| 2 | ||
| 3 | ||
| 4 | Wair | C89A98C-C |
| 5 | ||
| 6 | ||
| 7 | ||
| 8 | ||
| 9 | ||
| 10 |
With the mainworld placed, we now locate the M6 V companion. We can use the Traveller5 placement rules for companion stars, but the Forbidden Zones chart establishes some restrictions: since the mainworld is in orbit 4, that precludes the companion from occupying orbits 2, 3, 4, 5, and 6. With some fiddling, we determine that the second star is a near companion in orbit 8. This makes orbits 6 and 7 forbidden zones. We can cut and paste from the Orbital Zone table to establish the entire system template.
| Orbit | [Primary] | G2 V |
|---|---|---|
| 0 | ||
| 1 | ||
| 2 | ||
| 3 | ||
| 4 | Wair | C89A98C-C |
| 5 | ||
| 6 | ||
| 7 | ||
| 8 | [Companion] | M6 V |
| 0 | ||
| 1 | ||
| 2 | ||
| 3 | ||
| 4 |
Now the entire system is ready to be populated using the rest of the Traveller5 Other Worlds sequence. We start by placing the one gas giant, and then move on to placing the 9 remaining worlds.
| Orbit | Braw | G2 V | ||
|---|---|---|---|---|
| 0 | Braw-1 | Y400356-7 | Mi Tz | |
| 1 | Nevis | Y8C7000-0 | Tz | |
| 2 | Gale | FAB9679-8 | Mi Ho | |
| 3 | Dour | YB20000-0 | ||
| 4 | Wair | C89A98C-C | Hi In Wa Pz Co | |
| 5 | Bidean | Size P SGG | ||
| 6 | ||||
| 7 | ||||
| 8 | Merrit | M6 V | ||
| 0 | Trune | YDB4000-0 | Tz | |
| 1 | Opan | YC10000-0 | Co Tz | |
| 2 | Tey | Y8B5000-0 | Fr | |
| 3 | Min | F3108BC-7 | ||
| 4 | Hirta | Y300433-7 | Mi |
By using the charts, the system details were generated fairly quickly. The Wair system appears to be a resource-rich, highly efficient economic center that only lacks a nice shirt-sleeve world. The distant secondary world of Min, orbiting the M6 V companion Merrit, seems interesting: a small, airless, pre-high population world ruled by a charismatic dictator. Political exiles, perhaps? Maybe an Aslan community?
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