Freelance Traveller

The Physical World

by Richard Honeycutt

This article originally appeared in the January/February 2024 issue.

Traveller was unique as an RPG from the beginning as it presented a wide array of tools for building believable science fiction backgrounds. From animal generation, which produced seemingly valid food chains, to procedures for generating entire worlds, Traveller gave all the appearances of producing hard science settings to play in. Book 6: Scouts and World Builder’s Handbook continued to promote the idea that Traveller was, and still is, a “hard” science RPG. Under closer scrutiny, its clear that the scientific accuracy is a clever illusion. Given the time when Traveller was created, this  was not an issue as the main point of the game was to be fast and fun to play. Many aspects were abstracted and simplified. In those days, most players only had dice, pens and paper. Calculators were out of reach of many and home computers were virtually unheard of. All of the procedures placed their emphasis on simplicity.  Speed and ease of use trumped accuracy.

Nowhere in the rules does this show itself more glaringly than the UWP procedures for world building.

As a result, one can create airless, waterless rockballs that support populations in the tens of billions, tiny worlds with thick breathable atmospheres next to perfect garden worlds than are inexplicably untouched by settlers. Worlds like these can be interesting as anomalies, but the current UWP procedure creates such planets far too often to explain believably.

I find that this hinders the suspension of disbelief for me and I have worked to create a variant that suits me better. While I would be among the first to deny that my variant is true to real world planetology, I feel that it produces worlds that follow known science more closely than the old procedure. It shows a better correlation between world size, atmospheric pressure, hydrographic percentage and average temperature than present methods.

I do not use ‘taint’ in my methods. As the only significant in-game effect of taint being a determiner for characters to wear filter/respirator masks when going outside, I feel that ‘taint’ is little more than background fluff that should be added by the worldbuilder to support the background. I prefer to see the atmosphere number as being a direct indicator of surface pressure and not of composition. Atmospheric composition is beyond the scope of this simple writing. Traveller generally treats atmosphere as being a “standard oxygen/nitrogen mix” anyway. One place where ‘taint’ can have a direct impact is by adding to the greenhouse factor of a world. However, that can be simulated easily after the worldbuilding process is complete by multiplying the temperatures by 1 plus the small amount that the greenhouse is increased. To increase the greenhouse factor by .08 (CO₂ released by volcanic action, for example), multiply the calculated temperature by 1.08.

The UWP process described in MegaTraveller and DGP’s World Builder’s Handbook specifically designates atmospheres as being nitrogen/oxygen mixes with possible taints for all atmospheres except vacuum, exotic, corrosive, and insidious atmosphere types. It also specifically lists the hydrographics percentage as being liquid water except for exotic, corrosive, and insidious atmospheres. The extended system generation also expects the main world to be placed in the habitable zone.

The procedure presented here is more involved, but despite requiring a few simple calculations that can be done on a basic student’s calculator, it is not that difficult. It is also an eternal work-in-progress. However, the parts here are workable for worlds in the habitable zone, a limitation shared with the original UWP process upon which my variant is loosely based.

I’ve tried to avoid tables and special-case DMs so that it should be very easy to use in a spreadsheet.

Step 1

Determine system presence, star type and size.  Use your preferred method here.

The luminosity of the star(s) and the main world orbital distance are needed to calculate the ‘effective luminosity’ for the world.

L_eff = L/d2

where

L_eff is the Effective Luminosity
L is the actual luminosity of the star. Both L and Leff are measured in terms of Sol (i.e., L_Sol = 1.0)
d is the distance of the world from the star in AU

To be in the habitable zone, the effective luminosity should be between 0.5 and 1.5

Alternatively, you can just pick an effective luminosity within that range, closer to 1.5 for warmer and closer to 0.5 for cooler, and determine the orbital distance after picking your star, then placing the world there. Earth’s effective luminosity L_eff = 1.0.

From this, calculate the world’s temperature multiplier. The temperature multiplier is related to the world’s average surface temp which can be determined in later steps.

TM = L_eff^0.25
or
TM = ⁴√L_eff

where
TM is the Temperature Multiplier
L_eff is the Effective Luminosity calculated previously.

This number will be used in determining the atmosphere and hydrosphere UWP values

Example: A G6V star has luminosity L = 0.792 and the world orbits at d = 0.95 AU for an effective luminosity L_eff = (0.792/0.952) = 0.878

The temperature multiplier TM = (0.8780.25) = 0.968

For comparison, for Earth, L_eff = 1.0 and TM = 1.0.

Step 2

For the basic size S of the world, roll 2d6-2 for diameter in thousands of miles (that is, if you roll 8, the world’s diameter is approximately 8,000 miles, or 12,900 km). If you further refine the value, keep it as a decimal number. For example, if you decide the world is 7,846 miles in diameter, then the value used in later steps where the world’s size is called for would be 7.846

Decide on the density D of the world in terms of Earth’s average density (that is, treat Earth’s density as D_Earth = 1.0, and set your world’s density as a multiplier to Earth’s density). This can be determined randomly as in World Builder’s Handbook or just pick a likely value. For worlds in the habitable zone, this can be left as 1.0, but depending on their actual composition, a solid planet (i.e., not a gas giant) can have a density D of 0.5 or greater.

Now calculate the surface gravity g. This value will be used in later steps.

g = D×S÷8

where

g is the surface gravity, in terms of Earth (g_Earth = 1.0)
D is the world’s density in terms of Earth (D_Earth = 1.0)
S is the world’s size (diameter), in thousands of miles

For example, a world with a diameter of 7,846 miles and a density of 1.0 has a surface gravity of 0.98 (1.0×7.846÷8).

(For those who are interested, D_Earth = 5.519 grams per cubic centimeter, and g_Earth = 9.81 meters per second per second.)

Step 3

For the amount of atmosphere the world has accumulated, roll A* = 2d6-7+size. During system formation, larger bodies will accrete more material than smaller bodies. Use this, gravity and temperature to find the UWP value. Keep decimal places for more accuracy. This value will then be used to find surface pressure.

A = A*×g÷TM
P = A²÷49

where

A* is the accumulated material generated by the dice
g is the surface gravity previously calculated
TM is the temperature multiplier previously calculated
A is the final Atmosphere value for the UWP. It should be rounded to the nearest integer after calculating the surface pressure P.
P is the surface pressure of the world, in terms of Earth (that is, P_Earth = 1.0)

I chose to use gravity and temperature in this manner because gravity prevents the loss of the atmosphere to space, therefore, stronger gravity means there is more gases retained. Also, surface pressure is proportional to gravity. High temperatures tend to allow gases to escape the atmosphere more easily, so the amount of gas in the atmosphere is inversely proportional to temperature and the temperature multiplier relates to the world’s blackbody temperature.

The determination of surface pressure simply gives a curve that matches (somewhat) the pressures given in World Builder’s Handbook for each UWP step. A UWP value of ‘7' is equal to 1 atm of pressure.

Example: our world accreted A* = 5.846 (rolled 5- 7 + 7.846). This gives an ‘atmosphere’ value A of 5.92 (5.846×.98÷.968), which in turn gives a surface pressure P = 5.92²/49 ≈ 0.72

Step 4

For the amount of water the world has accumulated, roll H* = 2d6-7+size. During system formation, larger bodies will accrete more material than smaller bodies. Use this, the atmospheric pressure P and temperature multiplier TM (both calculated previously) to find the UWP value. Keep decimal places for more accuracy.

H = H*×P^0.5÷TM
or
H = H*×√P÷TM

where

H* is the accumulated material generated by the dice
TM is the temperature multiplier previously calculated
H is the final Atmosphere value for the UWP. It should be rounded to the nearest integer after calculating the surface pressure P.
P is the surface pressure of the world (previously calculated), in terms of Earth (that is, P_Earth = 1.0)

I chose to use atmospheric pressure and temperature in this manner because pressure prevents the evaporation of water into the atmosphere. Therefore, greater pressure means there is less water evaporated and more remaining in the oceans. High temperatures tend to allow water vapor to evaporate into the atmosphere more easily, so the amount of water remaining in the hydrosphere is inversely proportional to temperature and the temperature multiplier relates to the world’s blackbody temperature.

Example: our world accreted H* = 8.846 (rolled 8 - 7 + 7.846). This gives a ‘Hydrosphere’ value of ~7.70    H = (8.846×√0.72÷0.968) ≈ 7.75, so water covers ~78% of the surface.

Step 5

Knowing the atmosphere value and hydrographic % can allow us to approximate the world’s albedo α and greenhouse γ values. With this information, we can estimate the average surface temperature T.

T = 279×(1-α)^0.25×TM×γ
or
T = 279×⁴√(1-α)×TM×γ

where

T is the average surface temperature in kelvins
TM is the temperature multiplier (previously calculated)
α is the world’s albedo (calculated below)
γ is the world’s greenhouse factor (calculated below)

A world’s hydrographic percentage can be used to estimate a world’s albedo.

CC ≈ H×0.7
L = 1.0-H
α_cloud ~ 0.6
α_land ~ 0.15
α_water ~ 0.02
α = (CC×α_cloud)+((1.0-CC)×((L×α_land)+(H×α_water))

where

CC is the cloud coverage as a percentage of the world’s surface
L is the percentage of land coverage of the world’s surface
H is the world’s hydrographic percentage (calculated previously)
α_x is the albedo of x (i.e., α_cloud is the albedo of cloud cover, etc.). An unsubscripted α is the albedo of the world as a whole.

Our sample world has a cloud coverage CC=0.78×0.7= or 0.546, land coverage L=0.22 and a sea coverage H=0.78, giving an albedo of

α=(0.546×0.6)+((1.0-0.546)×((0.22×0.15)+(0.78×0.02))
α=0.3276+(0.454×(0.033+0.0156))
α=0.3276+(0.454×0.0486)
α=0.3276+0.0221
α=0.3497

Atmospheric pressure P and hydrographics percentage H can be used to estimate the base greenhouse factor γ:

γ ≈ 1.0 + (P^0.5×0.05) + (H×0.1)
or
γ ≈ 1.0 + (√P×0.05) + (H×0.1)

where

γ is the base greenhouse factor for the world
P is the world’s surface pressure (calculated previously)
H is the world’s hydrographic percentage (calculated previously)

Our sample world has a greenhouse factor of

γ=1.0+(√0.72×0.05)+(0.78×0.1)
γ=1.0+(0.8485×0.05)+0.078
γ=1.0+0.0424+0.078
γ≈1.12

Our sample world’s average surface temperature T is

T=279×4√(1-0.3497)×0.968×1.12

T=279×0.898×0.968×1.12
T=272

The world’s UWP (or at least the physical stats) will be x-868xxx-x, slightly smaller than Earth, with a slightly thinner atmosphere, and somewhat cooler.

Some Additional Examples

Let’s try this method on a couple of random systems.

Example 1

Let’s place a K4V star with a world in the habitable zone; we can just choose an effective luminosity of 1.15 which gives TM=4√1.15=1.0355.

I roll 2d6-2 for a size of ‘9‘, and assume the same density as Earth (1.0). This gives g=1.0×9÷8=1.125.

I roll 2d6-7+’9‘ for atmosphere accumulated of ‘8‘. Atmosphere then becomes A=8×1.125÷1.0355= 8.69, and pressure is then P=8.692÷49=1.77.

I roll 2d6-7+’9‘ for hydro accumulated of ‘7‘. Hydrosphere then becomes H=7×√1.77÷1.0355=8.99, or 90%.

Cloud cover CC=0.9×0.7=0.63, so the albedo

α=(0.63×0.6)+(0.37×((0.1×0.15)+(0.9×0.02))
α=0.378+(0.37×(0.015+0.018))
α=0.378+(0.37×0.033)
α=0.378+012
α=0.39

and the greenhouse factor

γ=1.0+(√1.77×0.05)+(0.9×0.1)
γ=1.0+(0.4207×0.05)+(0.09)
γ=1.0+0.021+0.09
γ=1.11

giving an average surface temperature of

T=279×4√(1.0-0.39)×1.0355×1.11
T=279×4√0.61×1.0355×1.11
T=279×0.8837×1.0355×1.11
T=283.4K

and a physical UWP of x-989xxx-x

This world is slightly cooler than Earth (288K), because the high hydrographic percentage (and therefore greater cloud cover and albedo) offsets the higher insolation and heat retention of the atmosphere.

Example 2

Let’s place a F9V star with a world in the habitable zone; we can just choose an effective luminosity of 0.75 which gives a TM=4√0.75=0.93

I roll 2d6-2 for a size of ‘5‘, and assume the same density as Earth (1.0). This gives g=1.0×5÷8=0.625.

I roll 2d6-7+’5‘ for atmosphere accumulated of ‘7‘. Atmosphere then becomes A=7×0.625÷0.93=4.7, and pressure P=4.72÷49=0.45.

I roll 2d6-7+’5‘ for hydro accumulated of ‘5‘. Hydrographics then becomes H=5×√0.45÷0.93=3.6, or 36%

Cloud cover CC=0.36×0.7=0.252, so the albedo

α=(0.252×0.6)+((1.0-0.252)×((0.64×0.15)+(0.36×0.02)))
α=0.1512+(0.748×(0.096+0.0072))
α=0.1512+(0.748×0.1032)
α=0.1512+0.0772
α=0.2284

and the greenhouse factor

γ=1.0+(√0.45×0.05)+(0.36×0.1)
γ=1.0+(0.6708×0.05)+0.036
γ=1.0+0.0335+0.036
γ=1.07

giving an average surface temperature of

T= 279×4√(1-0.2284)×0.93×1.07
T= 279×4√0.7716×0.93×1.07
T= 279×0.9372×0.93×1.07
T=260

and a physical UWP of x-554xxx-x

a small cool world with scattered cold seas, likely frozen over at higher latitudes and possibly well into the middle latitudes.

Notes

1. Validity

This procedure rapidly breaks down when used outside of the habitable zone and can give ‘odd’ results if used in the inner, middle or outer zones. This is because of the importance of liquid water in determining the composition of a world’s atmosphere.

2. CO2, Liquid Water, and the ‘Cold Trap’

Liquid water is important for removing CO2 from the atmosphere as part of the carbonate-silicate cycle where the CO2 is locked into the crust. If the CO2 that is locked in the crust of Earth were released, Earth would have a CO2 atmosphere with a pressure in excess of 60 atm. Because water is kept close to the surface by the ‘cold trap’ where temperatures force water vapor to condense in the troposphere, relatively little water is lost by UV disassociation which allows for the hydrogen to escape the atmosphere. The height of the ‘cold trap’ can be estimated by using the adiabatic lapse rate of the atmosphere.

Venus’ ‘cold trap’ is at a very high altitude which allowed for the hydrogen to be lost after the water is broken down by UV rays. The oxygen then recombines with other elements. With the water being lost, the carbonate-silicate cycle was broken and the CO2 remained in the atmosphere. Runaway greenhouse pushed the cold trap higher.

On Mars, in the middle zone, the carbonate-silicate cycle was broken by the fact that water is frozen into the crust. Liquid water does not exist to remove the CO2 from the atmosphere, thus CO2 is the main component of the atmosphere of Mars.

3. Cooler Stars

Type ‘M’ stars might not have a habitable zone, and for those that do, the world will be tidally locked. A habitable zone around a type ‘M’ star will be extremely close to the star and being tidally locked will mean that the magnetosphere will be very weak if it exists at all; the atmosphere will be eroded away by the solar wind… and then there are the flares.

Red dwarfs will have huge convection zones which will give them very active photospheres with strong magnetic fields. This will cause them to flare more often and more violently than the Sun.

For type ‘K’ stars, any world in the habitable will most likely be tidally locked.

 

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