Atmosphere

After determining world size, the next step in the Traveller world generation process is to roll for atmosphere. Normally, this is done with a roll of 2d6-7. Add the world’s size number to this roll.

In addition, I am using the aforementioned “hard science” optional rules. These are:

  • If Size is 0–2, Atmosphere is set to 0. The world is too small to retain an atmosphere.

  • If Size is 3–4 and Atmosphere is 0–2, set Atmosphere to 0.

  • If Size is 3–4 and Atmosphere is 3–5, set Atmosphere to 1.

  • If Size is 3–4 and Atmopshere is 6+, set Atmosphere to A.

Minor Issues

One minor issue I am having is that the atmosphere types are fairly ill defined. What is a “corrosive” atmosphere, for example? Or an “insidious” one? The old Classic Traveller Special Supplement 2 has more information, but it is not part of Mongoose Traveller. However, one important bit of information that we should take from SS2 is that Hydrogen atmospheres are considered “insidious”.

Venus, incidentally, is considered to be “corrosive” under Traveller rules, which illustrates just how broad the atmospheric types are.

The Atmospheric table

Atmospheric types are:

Digit Atmosphere Pressure (bar) Survival Gear Examples
0 None 0 Vacc Suit Asteroids, Moon
1 Trace 0.0001 – 0.09 Vacc Suit Mars
2 Very thin, Tainted 0.1 – 0.42 Respirator+Filter
3 Very thin 0.1 – 0.42 Respirator
4 Thin, tainted 0.43 – 0.7 Filter
5 Thin 0.43 – 0.7
6 Standard 0.71 – 1.49 Earth
7 Standard, tainted 0.71 – 1.49
8 Dense 1.5 – 2.49
9 Dense, tainted 1.5 – 2.49
10 (A) Exotic Varies Air supply Titan?
11 (B) Corrosive Varies Vacc suit Venus
12 (C) Insidious Varies Vacc suit
13 (D) Dense, High 2.5+
14 (E) Thin, Low 0.5 or less
15 (F) Unusual Varies Varies

The last three types warrant quick explanations.

Dense, High: A breathable atmosphere that is too dense for humans to breathe. Colonies might exist on tall peaks.

Thin, Low: This world’s atmosphere is concentrated in deep canyons or other depressions; it is too think to be breathable everywhere else.

Unusual: Catch-all for the strange. The Traveller rules give one notable example, a rocky core surrounded by hundreds of kilometers of water.

Changes

With world sizes up to 15, we’ve also effectively extended the atmospheric table and made some types of atmospheres more likely. The question is – does the system need an adjustment to accommodate our super-Earths?

Surprisingly, I think the percentage for existing atmospheric types are actually OK (top header is size):

10

11

12

13

14

15

Thin (5)

2,78%

Standard (6)

5,56%

2,78%

Std, Taint (7)

8,33%

5,56%

2,78%

Dense (8)

11,11%

8,33%

5,56%

2,78%

Dense, Taint (9)

13,89%

11,11%

8,33%

5,56%

2,78%

Exotic (10)

16,67%

13,89%

11,11%

8,33%

5,56%

2,78%

Corrosive (11)

13,89%

16,67%

13,89%

11,11%

8,33%

5,56%

Insidious (12)

11,11%

13,89%

16,67%

13,89%

11,11%

8,33%

Dense, High (13)

8,33%

11,11%

13,89%

16,67%

13,89%

11,11%

Thin, Low (14)

5,56%

8,33%

11,11%

13,89%

16,67%

13,89%

Unusual (15)

2,78%

5,56%

8,33%

11,11%

13,89%

16,67%

So a Size 15 world has a one-in six chance of having an “unusual” atmosphere. That’s okay, considering a) only 0.46% of our worlds will fall into this size category and b) it seems to match the exoplanets we have found; quite a few of them are really strange compared to the well-behaved planets in our solar system.

How do we catch higher rolls, though? The maximum result possible is 20. I am thinking that an easy fix is probably the best: Results of 16-17 count as “Exotic”, 18-20 count as “Insidious”.

Probabilities are:

10

11

12

13

14

15

16 (Exotic)

2,78%

5,56%

8,33%

11,11%

13,89%

17 (Exotic)

2,78%

5,56%

8,33%

11,11%

18 (Insidious)

2,78%

5,56%

8,33%

19 (Insidious)

2,78%

5,56%

20 (Insidious)

2,78%

Granted, it’s a kludge, but it looks fairly reasonable to me; presumably one could make the middle entry (roll result of 18) Corrosive instead, Either way, I think this will work – it just needs to get playtested.

2 thoughts on “Atmosphere”


    1. Oooh, thank you – those are great links!

      I guess if you want to use published UWPs without changes you only have one of two solutions; accept (and try to explain) the silliness, or change the interpretation. I personally think that “broken” UWPs ought to be errataed – and of course if you create new worlds you can just avoid being too silly in the first place.

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