Print Story Ask HuSi: Orbital Mechanics
By Gedvondur (Thu Mar 03, 2011 at 12:58:15 AM EST) RPG, Nerds, Ask Husi (all tags)
I’ve got some questions about how planets and moons work and I need to know if a possible RPG creation of mine is an impossibility.

Full explanation inside:


I run a science fiction table-top RPG based in the Traveller universe called The Marshall Cooperative. In this, my little group of intrepid space-farers go to planets that might be ready to begin the process of (re)joining the 4th Imperium after decades of isolation following the AI Virus incident.

The Scenario

I am thinking about basing the next run of my game on a world I call Callisto. I want Callisto to have two sides, a “day” side and a “night” side. Callisto can be a moon or a planet.

Here’s my quandary:

1. I need the “day” side to not be a broiled desert.
2. I need the night side to not be a freezing hellhole.
3. I need to understand how weather could possibly work.
4. I want this to be at least decent pseudo-science.

I know I’m killing a million cat-girls by even bringing up the idea of a reasonable orbital mechanic here, but I don’t just want to brush the idea aside. Jarring departures from reality take players out of the game if they are not based on at least pseudo-science.

I imagine the night side to be a place much like Zanga Marsh in World of Warcraft. Towering mushrooms, tons of fungus, strange things that live in the dark and enough bio-luminescence to give it that creepy blue feeling. Another example of how I view it would be Alan Dean Foster’s description of Longtunnel in the book Flinx in Flux (Pip and Flinx) series.

The day side will be more normal, with some broiling deserts but jungle zones near the poles.

I keep thinking that there should be some other moons/orbital bodies that shed some reflected illumination on the night side, and/or moderating the light getting to the day side but I fear that is WAY too complicated.

So tell me fellow HuSites: Is there an orbital mechanic that can be described to work in any of these examples? Or is this stretching things too far?

EDIT Friday March 3, 10:12AM CDT

Wow, lots of good input, guys.  I'm SUPER busy today, but I will get back to all of you soon.
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Ask HuSi: Orbital Mechanics | 27 comments (27 topical, 0 hidden) | Trackback
Not too difficult, really by notafurry (4.00 / 1) #1 Thu Mar 03, 2011 at 01:47:00 AM EST
Large moon tidally locked to a big gas giant. Distant hot primary.

Day side - warm, lit by gas giant, but not roasting. You need to assume massive magnetic field / radiation belt.

Night side - cool, warmed by primary and lit a little by the star and by the constant aurora created by the gas giant's radiation flux.

It doesn't quite work in real life, I think, but it's close enough for pseudo-science.

Assuming an atmosphere by Scrymarch (2.00 / 0) #3 Thu Mar 03, 2011 at 02:02:16 AM EST
... to distribute the heat from day to night side?

Iambic Web Certified

[ Parent ]
Current theory... by Vulch (2.00 / 0) #5 Thu Mar 03, 2011 at 04:20:08 AM EST
It will be a very windy place. Hot air will rise over the Hot Pole, effectively a very big permanent thermal, and be replaced by cooler air flowing in at ground level. The hot air will spread out at high level and eventually land up over the Cold Pole, sinking as it cools so there's a downdraft there. The heat transferred by the atmospheric currents looks to be enough to mediate the extremes at the two temperature poles, but you still wouldn't want to live at either one.

For a bit of night light for some of the year at least make the planet orbit around a red dwarf, which would probably be the case anyway for a tidal lock, and put that in distant orbit around something much brighter.

[ Parent ]
Options by TheophileEscargot (4.00 / 1) #2 Thu Mar 03, 2011 at 01:56:40 AM EST
Could the planet be in the L1 Lagrange point between a large bright star and a small red dwarf or brown dwarf that mostly radiates in the infrared? The infra-red would heat the dark side, but might be not much use for seeing with, if there's a lot of water vapour or CO2 in the atmosphere scattering and absorbing it.

Or (or as well) you could have a lot of radioactive heating of the planet from the inside. Would probably mean a lot of active volcanoes.

Or you could have a planet with a large close moon that drags huge tides around, transferring heat from light side to dark side. Problem is that might also cause the planet to spin slowly.

Or, a small, dense, solid planetoid with hollow channels though the middle, wobbling slightly due to gravitational influences, with convection currents passing straight through the middle and heating up the dark side.
It is unlikely that the good of a snail should reside in its shell: so is it likely that the good of a man should?

A tidally-locked satellite rotates by ammoniacal (2.00 / 0) #4 Thu Mar 03, 2011 at 02:51:34 AM EST
to slowly to form a geodynamic core. assuming the core is even molten. Therefore, there will be no satisfactory magnetosphere to protect the surface from hellish stellar radiation. Don't forget that the body needs to be large-ish if you want it to retain a useful atmosphere.

Also, this site is a Red Zone. Stand to and prepare to be boarded.

"To this day that was the most bullshit caesar salad I have every experienced..." - triggerfinger

Notable exception: by ana (4.00 / 2) #7 Thu Mar 03, 2011 at 07:20:45 AM EST
Io. The problem there is it's in an orbital resonance with the other Galilean moons, and the orbit is maintained in a slightly eccentric form. This causes tidal flexing of the object (also the tides raised by the passing of Europa and the others), which keeps Io geologically (iologically?) active.

There's a substantial amount of research on planetary atmosphere models just now, what with all the exoplanets popping out of the ether lately. I wonder if there's a plausibly readable book or review article on the subject. Lisa Kaltenegger has done some talking to the press; dunno if she's written anything accessible.

"And this ... is a piece of Synergy." --Kellnerin

[ Parent ]
I defer to «the» Rocket Scientist. by ammoniacal (2.00 / 0) #9 Thu Mar 03, 2011 at 07:46:29 AM EST

"To this day that was the most bullshit caesar salad I have every experienced..." - triggerfinger

[ Parent ]
Brain Aldiss by Phage (2.00 / 0) #6 Thu Mar 03, 2011 at 06:32:46 AM EST
Answered this one.

an idea by gzt (2.00 / 0) #8 Thu Mar 03, 2011 at 07:26:20 AM EST
simple: axis of rotation perpendicular to ecliptic, that axis is in tidal lock to the sun. ie, polar circle reaches to equator, north pole always points to sun. to explain this, perhaps something hit the planet. unfortunately, a moon would just bog this up. if on the plane of orbit, that would throw off pointing at the sun, if on the plane of the rotation, that's not all that helpful for illumination and it would be weird.

to explain it, some object could've hit it and thrown it out of kilter. venus rotates backwards, so why not?

with a thick atmosphere and a lot of water, this might work. but probably not.

Uh... by ana (2.00 / 0) #13 Thu Mar 03, 2011 at 09:31:27 AM EST
Uranus, for example, has its axis of rotation tilted 97 degrees to the ecliptic, i.e. nearly in the plane of the orbit. So during its year, the sun passes close to overhead at both poles, but for a large part of the year (spring/fall, if you will), the daily rotation of the planet makes the sun rise & set most everywhere on the planet, like we're accustomed to here.

I think I've heard it said that some of the fluffy hot Jupiter style planets may be in such a configuration (instead of being tide-locked).

To *keep* the rotation axis pointed at the sun would require a huge torque.

"And this ... is a piece of Synergy." --Kellnerin

[ Parent ]
There's no need to get personal [n/t] by Herring (2.00 / 0) #15 Thu Mar 03, 2011 at 09:42:52 AM EST

christ, we're all old now - StackyMcRacky
[ Parent ]
you're right about the torque. by gzt (2.00 / 0) #20 Thu Mar 03, 2011 at 10:18:02 AM EST
not tenable. so i don't think this is doable, given that you want a planet that rotates at a reasonable rate but also has a light side and a dark side.

[ Parent ]
Couple ideas from existing SciFi by wiredog (2.00 / 0) #10 Thu Mar 03, 2011 at 09:20:36 AM EST
Not exactly what you're looking for, but:
Uller Uprising.
The axis of Uller is in the same plane as the orbit, so that at a certain time of the year the north pole is pointed directly at the sun, while at the opposite end of the orbit it points directly away. The result is highly exaggerated seasons. At the poles the temperature runs from 120°C to a low of -80°C. At the equator it remains not far from 10°C all year round. Strong winds blow during the summer and winter, from the hot to the cold pole; few winds during the spring and fall.[xii] The appearance of the poles varies during the year from baked deserts to glaciers covered with solid CO2. Free water exists in the equatorial regions all year round.

Four Day Planet

Fenris is the second planet of a G4 star, six hundred and fifty light-years to the Galactic southwest of the Sol System. Everything else equal, it should have been pretty much Terra type; closer to a cooler primary and getting about the same amount of radiation. At least, that's what the book says. I was born on Fenris, and have never been off it in the seventeen years since.

Everything else, however, is not equal. The Fenris year is a trifle shorter than the Terran year we use for Atomic Era dating, eight thousand and a few odd Galactic Standard hours. In that time, Fenris makes almost exactly four axial rotations. This means that on one side the sun is continuously in the sky for a thousand hours, pouring down unceasing heat, while the other side is in shadow. You sleep eight hours, and when you get up and go outside—in an insulated vehicle, or an extreme-environment suit—you find that the shadows have moved only an inch or so, and it's that much hotter. Finally, the sun crawls down to the horizon and hangs there for a few days—periods of twenty-four G.S. hours—and then slides slowly out of sight. Then, for about a hundred hours, there is a beautiful unfading sunset, and it's really pleasant outdoors. Then it gets darker and colder until, just before sunrise, it gets almost cold enough to freeze CO2. Then the sun comes up, and we begin all over again.

Earth First!
(We can strip mine the rest later.)

Four day planet by Herring (4.00 / 2) #16 Thu Mar 03, 2011 at 09:43:44 AM EST
Is that four simultaneous days in one 24 hour rotation?

christ, we're all old now - StackyMcRacky
[ Parent ]
No, four rotations/year. by wiredog (2.00 / 0) #21 Thu Mar 03, 2011 at 11:04:34 AM EST

Earth First!
(We can strip mine the rest later.)

[ Parent ]
Almost like some sort of cubic arrangement? by mrgoat (4.00 / 2) #27 Thu Mar 03, 2011 at 06:09:59 PM EST

--top hat--
[ Parent ]
"Towering mushrooms, lots of fungus" by ni (2.00 / 0) #11 Thu Mar 03, 2011 at 09:26:34 AM EST
What do you propose they're eating? Sunlight is the primary energy source for life on earth, and the only one driving the production of the biomass fungus consumes.

Orbital mechanics are the least of your scientific concerns: you have life happening without energy input.

I doubt geothermal heat would do the trick. Unless you have a storm that brings several million tonnes of biomass across the light/dark divide every night, I don't think large fungus (or much else) works.

"These days it seems like sometimes dreams of Italian hyper-gonadism are all a man's got to keep him going." -- CRwM

Suggestion: by ni (2.00 / 0) #12 Thu Mar 03, 2011 at 09:30:26 AM EST
Lots of very radioactive rocks strewn around, and very, very weird life (no primary decomposers!)

"These days it seems like sometimes dreams of Italian hyper-gonadism are all a man's got to keep him going." -- CRwM
[ Parent ]
I was going to suggest absurdly long hyphae by ambrosen (2.00 / 0) #14 Thu Mar 03, 2011 at 09:37:34 AM EST
Reaching from the day zone. But I've no idea why the fungi would choose to fruit in the dark zone. Unless their fruiting bodies couldn't survive the heat.

[ Parent ]
Maybe with really, really strong wind? by ni (2.00 / 0) #17 Thu Mar 03, 2011 at 09:49:01 AM EST
Otherwise you'd have the produced spores germinating in the dark zone, where they'd promptly die for lack of energy, and the species wouldn't last very long.

"These days it seems like sometimes dreams of Italian hyper-gonadism are all a man's got to keep him going." -- CRwM
[ Parent ]
That, or the spores cannibalise their parents. by ambrosen (2.00 / 0) #18 Thu Mar 03, 2011 at 09:58:15 AM EST
Until they can get food from the productive side. The spores may be light enough to aerosolize, though.

[ Parent ]
I wonder... by ana (2.00 / 0) #19 Thu Mar 03, 2011 at 10:11:14 AM EST
There's something in the mechanics of the earth-moon system (probably perturbations from the sun, but i'm not sure) that makes the moon swing from side to side a bit, showing us a tad over 50% of its surface.

The twilight zone of a planet tide-locked to the sun could be subject to occasional bouts of daylight (and night); maybe your fungi could do something with that. And I'm afraid I'm not sure how periodic the moon's nutations (did I get the right word?) are.

"And this ... is a piece of Synergy." --Kellnerin

[ Parent ]
Wrong word. by ana (2.00 / 0) #22 Thu Mar 03, 2011 at 03:14:33 PM EST
It appears I meant Libration, which allows about 59% of the moon's surface to be seen from the earth. Basically the problem is that the moon rotates at a constant rate (very nearly), but because the orbit is slightly elliptical (driven and maintained by the sun's gravity), it doesn't orbit at a constant rate. The average rate exactly matches the rotation rate (so there's no secular trend to turn around backwards), but instantaneously, it's kind of like a pendulum swinging in time with the orbit.

"And this ... is a piece of Synergy." --Kellnerin

[ Parent ]
I'm prone to showing more of by ni (4.00 / 3) #23 Thu Mar 03, 2011 at 03:25:06 PM EST
my surface due to libation, too.

"These days it seems like sometimes dreams of Italian hyper-gonadism are all a man's got to keep him going." -- CRwM
[ Parent ]
The wiki article by ana (4.00 / 2) #24 Thu Mar 03, 2011 at 03:41:55 PM EST
specifically says "not to be confused with... libation..."

"And this ... is a piece of Synergy." --Kellnerin

[ Parent ]
Dammit, ni by ambrosen (4.00 / 1) #25 Thu Mar 03, 2011 at 04:06:28 PM EST
you stole my joke before I'd scrolled down to it.

[ Parent ]
Strong winds should preclude by ammoniacal (2.00 / 0) #26 Thu Mar 03, 2011 at 04:42:03 PM EST
the Yes album cover "towering mushrooms" scenario Ged mentioned, from a structural viewpoint.

"To this day that was the most bullshit caesar salad I have every experienced..." - triggerfinger

[ Parent ]
Ask HuSi: Orbital Mechanics | 27 comments (27 topical, 0 hidden) | Trackback