How large can a moon be?
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BlackBat242
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Well, in CT, LBB-6 Scouts, page 33 says "Planetary size -1D. For large gas giant, 2D-4. For small gas giant, 2D-6. If size* 0, use R. If size* less than 0, use S.
S=small world (200 km)
R=ring
0=planetoid belt/asteroid
*I assume they meant "if resulting size ="
Thus, a normal planet's moon can be no larger than 4,000 miles (6,400 km) in diameter, a small gas giant's 6,000 m (9,600 km), and a large gas giant can have a moon up to 8,000 m (12,800 km) in diameter.
Note that Earth is size 8 (7,972 m/12,756 km), and Luna is size 2 (2,172 m/3,475 km).
Jupiter's largest moon is Ganymede at size 3 (3,312 m/5,300 km).
Saturn's largest moon is Titan at size 3 (3,437 m/5,500 km).
S=small world (200 km)
R=ring
0=planetoid belt/asteroid
*I assume they meant "if resulting size ="
Thus, a normal planet's moon can be no larger than 4,000 miles (6,400 km) in diameter, a small gas giant's 6,000 m (9,600 km), and a large gas giant can have a moon up to 8,000 m (12,800 km) in diameter.
Note that Earth is size 8 (7,972 m/12,756 km), and Luna is size 2 (2,172 m/3,475 km).
Jupiter's largest moon is Ganymede at size 3 (3,312 m/5,300 km).
Saturn's largest moon is Titan at size 3 (3,437 m/5,500 km).
Duh! Forgot to look at LBB6!
How about this one. I am re-reading one of the "Flinx" books by Allen Dean Foster. It talks about the planets Moth and Flame in almost the same orbit so that Moth trails behind Flame. Could this be an example of a captured planet at orbit x.9 coupled with orbit y or y and y.1 ?
Edit: Make that Alan Dean Foster. I did not have the book with me at the time I wrote this and was not going to try to spell it from memory. The Tar-Aim Krang is the name of this book, but the main character is in several books.
How about this one. I am re-reading one of the "Flinx" books by Allen Dean Foster. It talks about the planets Moth and Flame in almost the same orbit so that Moth trails behind Flame. Could this be an example of a captured planet at orbit x.9 coupled with orbit y or y and y.1 ?
Edit: Make that Alan Dean Foster. I did not have the book with me at the time I wrote this and was not going to try to spell it from memory. The Tar-Aim Krang is the name of this book, but the main character is in several books.
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do some googling and see if the planets have been detailed in some fashion either officially or unofficially by fans.
BlackBat242
SOC-14 1K
From the article you linked:
"For extrasolar systems, Canup and Ward suggest, the largest satellites of a Jupiter-mass planet would be Moon-to-Mars sized, so that Jovian-sized exoplanets would not be expected to host satellites as large as Earth. This is relevant to the potential habitability of satellites in extrasolar systems."
Mars = 6,795 km (4,247 miles).
Interesting.
Actually, I was wondering about how to do this using Traveller rules rather than how it was explained in Foster's books. He gives a lot more detail than I included in my question. Moth is a ringed planet that somehow had 2 sections of the rings removed (no explanation given) to make it look like a moth. It trails Flame in the orbit so the system looks like a moth chasing after a flame.
Sorry, both those propositions sound like nonsense. Sections removed from the rings would close up in a matter of days from the inner part of the rings rotating faster than the outer parts. And mutual perturbations between two planets in the same orbit any closer than 60 degrees apart would speed Moth up into a higher orbit where is would drop back and slow Flame into a lower orbit where it would rush forwards. They would separate in a matter of months, and years later Flame would close in on Moth from behind and they would exchange orbits again. The cycle is not closed, however, and soon in geological time either they would collide or they would both be perturbed into entirely different orbits.
The suggestion is not science fiction, it is space fairytale.
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Indeed. By all means put sci-fi eye candy in your TU or other game if you want it there. It's not as though contragrav, reactionless thrusters, Jump Drives, nuclear dampeners, black globe generators, meson guns, or psionics are hard SF either. Traveller, or any interstellar setting for that matter, has to go soft somewhere, or it would be pretty dull if it didn't.
Just don't ask the sciency guys to rationalise your whims for you and get cross with them when they say they can't. It isn't their fault if you are asking for the impossible.
One of the ideas that I've incorporated IMTU is that the mass my be distributed through several satellites, or there may be one more massive satellite and some relatively insignificant moonlets.
I'm a sciency-guy too.
I just don't apply that science as rigorously to a roleplaying game that's got the ability to emulate (to some cobbled up extent) The Foundation Series, Dune, AND Star Wars.
I'm all for dramatic license, hence Transporters and Warp in Trek, Folding Space in Dune, Jump Space in Traveller.
We all know none of these are real, and I think are in agreement, that they never will be.
I just don't apply that science as rigorously to a roleplaying game that's got the ability to emulate (to some cobbled up extent) The Foundation Series, Dune, AND Star Wars.
I'm all for dramatic license, hence Transporters and Warp in Trek, Folding Space in Dune, Jump Space in Traveller.
We all know none of these are real, and I think are in agreement, that they never will be.
Given the headline, I thought this was going to be dated 04/01 or in the Onion.
Out IRL, there's probably no hard limit.
A few considerations:
1. A "moon" must be notably smaller than the combined primary body it orbits; the center of mass of the primary-satellite system must reside, at least for part of the moon's orbit, within the body of the primary. Otherwise, it's not a "moon", but a "binary companion".
2. The moon must not be so large that its own gravity initiates fusion; it must be sub-stellar mass. Otherwise, it's a star. (As would its larger-mass primary be.)
3. It must orbit the primary outside the Roche limit.
4. It must actually orbit its primary. Compare Cruithne which is not Terra's "second moon".
Other than these considerations, anything goes -- especially within the model behind LBB6. (Remember, as per LBB6, Regina/Regina/Spinward Marches is a moon of the GG Assiniboia.)
It's unlikely that a small GG could orbit a large GG (since they'd siphon gas from each other and eventually migrate to the middle), but anything rocky should be workable, at least for a few hundred million years... note that such a megamoon would certainly be "face-locked" to its primary, similar to the way Luna is with Terra; otherwise tidal forces would break it down in (cosmically) short order...
Some might disagree.
There are two figures... one is that for non-caputured bodies, which I've heard 1% primary's mass being the 'generous' upper limit and 10% the 'you have got to be kidding' line.
For captured or collision formed moons, I believe the limit is about 75-90%, at which point it is a double planet (since the baricenter is most likely going to be outside the larger.
For captured or collision formed moons, I believe the limit is about 75-90%, at which point it is a double planet (since the baricenter is most likely going to be outside the larger.
Icosahedron
SOC-14 1K
Thanks for that link to the Roche limit, Boomslang. A while back, I had an adventure idea based on the premise of a region of 'negative gravity' on the surface of a moon, inside which the primary planet 'lifted' things from the moon's surface. I reckoned it might be possible, but I couldn't figure it out. (cos I wasn't using the equation for tidal force, doh!) But I got it now, and the adventure can go ahead. I've just got to figure the diameter of the area, and I should be able to do that.
Cheers.
Cheers.
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