Worldbuilding - satellite size limits?

[dalthor]

So, messing around with world building, and got to thinking...and that should scare you.

Since I'm building a T5 universe, I posted it here.

Anyway, I generated a size-8 mainworld (MW) in the habitable zone. It had two satellites, and BOTH ROLLS came up as BigWorlds which were LARGER than the MW. This got me wondering about logical satellite sizes.

As I understand physics, the closer a satellite gets to the size of the world it orbits, the more likely you will get a barycenter rather than an actual orbit, and neither world is a true satellite. I'd rather not have that situation during standard world generation, fun and/or realistic as it would be.

[[ Admittedly, there will be times I want to have that barycentric relationship, but it will be a manual process. ]]

We'll assume WORLD means the main body/planet/GG, and SATELLITE means the object that will orbit that WORLD. That SATELLITE could conceivably be a radworld or other world type, and still be a satellite, especially around a GG.

What is a logical limit for satellite size - i.e. a body that orbits another body?

I was thinking about several factors, and ignoring mass:
- The size of the WORLD
- The orbital position around that WORLD
- I can't see a BigWorld as a satellite, except maybe around a size Y GG

My thoughts led me to this:

o Replace the BigWorld entry in the Satellite tables with Planetoid
o If that satellite size is less than 0, it is removed (usually applies to Worldlets, size = 1D - 3) -- no change to existing info
o If that satellite size is exactly 0, it is a standard ring (planetoid belt??) -- again, not a change
o Max SATELLITE size is the LESSER of WORLD size/4 or the orbit around that WORLD.

Does this work? I want to keep it somewhat simple, and yet plausible.

Oh, and might it be wise to have a forum for system and/or world building? That seems to be a decent idea, but probably isn't viable due to differences in versions.

Sorry about the grinding sounds and smoke, but y'all know how it is.

 

[aramis]

Terra-Luna barycenter is still inside earth 4,671 km of 6,378 km radius.

The distance is 384472 km or so, so that's almost 1% of the distance.

Wikipedia has the relevant formulae in the page Barycenter

 

[Fovean]

In my MgT campaign I've been using (Mainworld Size)-2 as maximum Satellite size. But this is an interesting system, I'll have to revisit this idea....

 

[The Pakkrat]

Who's Moon IS this?

You might try:

The Mainworld is now the satellite of the BigWorld, StormWorld, InnerWorld that was just rolled much larger than the Mainworld.

This could make your system special in that now your Mainworld remains in the Habitable Zone (HZ-1, HZ, HZ+1), only now - instead of a Gas Giant - it revolves around a larger planet in that orbit.

For example: I rolled a White Dwarf in HZ-1, the hot edge of the Green Belt. Distributing Gas Giants, the largest Giant got put in the first satellite orbit of the White Dwarf in planetary orbit 4. Both were revolving around a F5 V primary star, (immaterial). Then the Gas Giant produced a satellite of its own, a BigWorld. That's a lot of activity in planetary orbit 4! A satellite of a Gas Giant revolving about a White Dwarf revolving about a F5 V star. o:

Else, I like your fourth, safer option of setting a maximum Size of the Mainworld's satellite.

But go with what makes sense. It keeps the grognards happier.

From Cartography high above Roethoeegaeaegz, this is the Pakkrat for Net-7 News.

 

[jcrocker]

If it turns out the moon is bigger, no problem - it's got fluid oceans or nitric acid atmosphere or some other really unappetizing physical features that mean people will work there if there's a really good reason to, but they live on the other body.

You could still leave it as a moon. Say there was a big collision eons ago, the 'main world' ended up with most of the dense core, the moon is bigger but less dense so it still orbits the smaller more massive partner. Does that seem less than likely to us? Yes, but twenty years ago, I doubt the real-world Trappist system would have been deemed believable if they wrote it in a movie.

You could even have that as the adventure seed - sure, the Second Survey said that the moon had a lower density, but your patron came into some sensor data that casts doubt on that. And why is that detailed sensor scans are prohibited in that system, anyway? The tabloid media people are saying it's the Ancients again, but they do that anyway to boost circulation. But what if they're right this time?

 

[Timerover51]

So, messing around with world building, and got to thinking...and that should scare you.

Since I'm building a T5 universe, I posted it here.

Anyway, I generated a size-8 mainworld (MW) in the habitable zone. It had two satellites, and BOTH ROLLS came up as BigWorlds which were LARGER than the MW. This got me wondering about logical satellite sizes.

As I understand physics, the closer a satellite gets to the size of the world it orbits, the more likely you will get a barycenter rather than an actual orbit, and neither world is a true satellite. I'd rather not have that situation during standard world generation, fun and/or realistic as it would be.

[[ Admittedly, there will be times I want to have that barycentric relationship, but it will be a manual process. ]]

We'll assume WORLD means the main body/planet/GG, and SATELLITE means the object that will orbit that WORLD. That SATELLITE could conceivably be a radworld or other world type, and still be a satellite, especially around a GG.

What is a logical limit for satellite size - i.e. a body that orbits another body?

I was thinking about several factors, and ignoring mass:
- The size of the WORLD
- The orbital position around that WORLD
- I can't see a BigWorld as a satellite, except maybe around a size Y GG

My thoughts led me to this:

o Replace the BigWorld entry in the Satellite tables with Planetoid
o If that satellite size is less than 0, it is removed (usually applies to Worldlets, size = 1D - 3) -- no change to existing info
o If that satellite size is exactly 0, it is a standard ring (planetoid belt??) -- again, not a change
o Max SATELLITE size is the LESSER of WORLD size/4 or the orbit around that WORLD.

Does this work? I want to keep it somewhat simple, and yet plausible.

Oh, and might it be wise to have a forum for system and/or world building? That seems to be a decent idea, but probably isn't viable due to differences in versions.

Sorry about the grinding sounds and smoke, but y'all know how it is.

Am I correct in thinking that all of your world-building is done using dice?

 

[dalthor]

Right now, this is tied to an extended system generator that I am writing.

It uses the T5 rules to generate the complete system for a given hex. When I was walking thru the process manually, I ran in to the issue; checking test output, I saw the same type of thing.

T5.09 page 413 has table S for number of satellites for worlds; it appears you the then roll on the two tables for HOSPITABLE and OUTER satellites.

Those tables can generate results including worldlets, inferno, bigworld, and others. When you then generate the size, you can get a satellite that is larger than the mainworld.

You can change the MW to be a satellite of that larger planet, but that really isn't what you want to hard-code into an application, and hand-waiving it (ancients or whatever) really gets old.

Personally, I'm thinking about several possibilities, in order of preference:

1 - limit the satellite size; I prefer limit of 1/4 size of MW

2 - keep the results, and swap the MW to be the satellite

3 - tho highly unlikely, but possible, the two are close enough to the same size that they orbit each other. I don't know that this could happen in the RW, since you'd expect only one "normal" sized world in an orbit, and the rest to be true satellites of that "normal" world. As I understand normal system accretion, this really shouldn't happen with planetary bodies.

4 - Ignore the satellite altogether

I might use number 3 ONCE as a plot device; it might be natural; it might be a captured world, or it may have been engineered.

I've been thinking about adding a size table of my own, similar to this

Satellite size (1d6)
1 1d10 meters
2 1d10 * 100 meters
3 1d10 * 1000 meters
4-6 size is MW size/4

Anyway, thoughts appreciated.

 

[Daddicus]

Two (or more) objects will orbit about the center of gravity. In our solar system, Earth is the only planet where the barycenter is at a significant distance from the center of the main.

However, that's only a true statement because Pluto is no longer a planet. Pluto/Charon's barycenter is actually outside of Pluto's diameter.

It seems that in the spirit of the rules, one should either cap the size of the satellite at the size of the main, or else swap the roles. In either case, this is a good thing to add to the errata threads.

 

[dalthor]

Two (or more) objects will orbit about the center of gravity. In our solar system, Earth is the only planet where the barycenter is at a significant distance from the center of the main.

However, that's only a true statement because Pluto is no longer a planet. Pluto/Charon's barycenter is actually outside of Pluto's diameter.

It seems that in the spirit of the rules, one should either cap the size of the satellite at the size of the main, or else swap the roles. In either case, this is a good thing to add to the errata threads.

I seem to recall a real-world size limit regarding non-stellar orbital bodies, I just cannot find it for reference. One body would use the majority of the mass in that orbit, and one or more smaller bodies would tend to be the satellites -- a function of tidal effects once that [theoretical?] size limit is reached.

I'm not too worried about where the barycenter is located, more concerned about logical size limits for those orbiting bodies.

I will add something to the errata thread.

 

[dalthor]

Also, the key word in this whole discussion is SATELLITE.

I suppose a lot is riding on the definition of that term.

I use the common astronomical definition wherein a satellite is a natural body that revolves around a planet. Essentially, a moon.

For my purposes, anything else is a case NOT considered in this discussion.

Furthermore, I will limit this definition to "orbital dynamics such the that barycenter of those objects falls inside the diameter of a single body."

This would mean that Pluto-Charon, for example, would be excluded, since the barycenter of the two lies outside the diameter of the larger body. Those two would be classified as a double planet rather than a planet and moon (satellite) for my purposes.

Mayhap this will help clarify what I seek to define.

 

[lgriffin]

Sorry to necro this thread, but having just read it, I wanted to remember there was a limitation on size. In both CT (Book 6) and MT, a satellite's size was the world it was orbiting's size - 1D. If the world was a small GG 2D6-6; a large GG 2d6-4. A zero exactly was a ring and less than zero was a small (~200km) planet(old).

So, there is precedent for limiting the size of a planet's satellites.

Lee