Space possible errata

[TheDS]

I was taking a look at the descriptions of world types on pp 75-76, where it discusses world size, types of worlds in that size class, and gives some examples from our own solar system. Something wasn't quite adding up, so I did some looking and found some data on the planets/moons and compared this to the examples given and near as I can tell, there are a few errors in the example worlds.

Frex:
Tiny worlds: Callisto, Europa, Io, Luna, and Mercury.
Small worlds: Titan and Mars.
Standard worlds: Earth, Venus, and Triton.
Large worlds: none.

Anyone see the discrepancy here?

Maybe this image will help: http://www.mattjonesblog.com/img/scale/SolScale1.jpg

1. Triton is definitely not big enough to be in the same class as Earth and Venus. It's smaller than Europa, and if Europa, one of the larger moons in our system, is considered to be tiny, I'm pretty sure Triton is tiny too.

2. If Triton is tiny, that means we have a new class of world, the Tiny Hadean! Or, it could be that Callisto, Europa, Io, Luna, and Mercury are small rather than tiny, and then so can be Triton, and no new classes need be made.

The dividing line between small and tiny is whether the world has the mass to hold free Nitrogen. I didn't do the math to check these examples, and up until now I assumed the authors did.

3. The small rock worlds text indicates that small worlds are likely to have thin atmospheres and no surface water, hence Mars. But the small ice world text acknowledges Titan's thick atmosphere and extensive reserves of fluid oceans.

Since the dividing line between small and standard is whether the world has the mass to retain water vapor, then world size plays no part in whether an atmosphere is retained or not, as Titan demonstrates. Lacking a significant atmosphere is the cause for the loss of Mars' water. Again, the question I'm having to ask is whether the authors did the math to see if Mars was categorized correctly.

4. I see nothing which states any lower bound for tiny worlds or upper bound for large worlds. I would assume the rather nebulous values of minimum hydrostatic equilibrium for the former and whatever level of gravity humans can stand long term (2G? 3G?) for the latter.

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Now certainly I may have misunderstood part or all of what was said, and it seems unlikely that such a fine book would overlook such a small detail, so I'm hoping someone can clear up my misunderstanding.

[Anthony]

World 'size' in space is defined by what gases can be held; thus, a 'large' world at 10 AU (1/100 the illumination, 32% of the surface temperature) is actually smaller than a 'large' world at 1 AU.

[TheDS]

Ah! The sidebar on p86 explains this! Thanks, I didn't realize temperature played such a large role. So used to "size" being more or less a function of physical volume exclusively. Thanks!

[Nemoricus]

Originally Posted by TheDS

4. I see nothing which states any lower bound for tiny worlds or upper bound for large worlds. I would assume the rather nebulous values of minimum hydrostatic equilibrium for the former and whatever level of gravity humans can stand long term (2G? 3G?) for the latter.

The upper bound for large worlds is the ability to retain hydrogen in their atmospheres, at which point the planet would become a gas giant.

There is effectively no lower limit to the size of a tiny world, which means that for all practical purposes hydrostatic equilibrium should be the threshold.

The bounds for the planet categories is given by the table on page 85. Manipulation of the equation given in the sidebar on page 86 shows that these limits correspond to the MMWR.

[Agemegos]

In GURPS Space the terms "tiny", "small", "standard" and "large" aren't don't really refer to size categories (though they are related to size). They are "world types", not sizes. They are defined by the minimum molecular weight of any gas the planet can retain in its atmosphere, which involves temperature and escape velocity, or in even more basic terms temperature, diameter, and mass or surface gravity.

Yes, this is confusing. Also, it results in a subtle trend of planets and moons being smaller in parts of the system that are presently cool than in the warm parts. Realistically, it might be more reasonable to have the cool bodies the same size as the warm ones or even slightly larger, and to have the trend with temperature be towards more atmospheres of light compounds instead of towards smaller diameter.

This bug in the system (it isn't documented, so it isn't a feature) is a result of chapter 5 reusing the type-first procedures of chapter 4 (which are magnificent for their purposes) instead of making up new procedures for core type to be determined first, then mass or diameter, then diameter or mass, then surface gravity, then atmosphere type, then blackbody temperature, then greenhouse factor and albedo, then surface temperature (and then if necessary accounting for runaway glaciation or runaway greenhouse heating).