How to design a planet with two moons

[kwinsw]

Hi,

I was wondering if anyone could help me. I’m trying to design a planet with two moons for a story I’m writing.

The planet will be 98% as dense as earth but it’s radius will be 2% bigger than earth’s, giving it about 99.996% earth gravity (if my calculations are correct, which is by no means guaranteed).

I want the orbits of the two moons to be stable because they are in mean resonance to each other. So far I’ve been using, amongst other things, the research of Canup and Stewart, which you can see here.

http://www.xtec.es/recursos/astronom/moon/canupe.htm

Following their information I’d like to work out orbits and the mean resonance ratio of a large inner moon (4.6368 × 10 to the power of 22 kilograms) and a small outer moon (2.87776 × 10 to the power of 22 kilograms).

So now for the question; can anyone help me work out what the semimajor axes of these moons need to be for them to enter a stable relationship with each other and their primary (I know terrestrial multiple moon systems are unlikely, but I’m trying to figure out the likeliest configuration)?

Later on I’d like to figure out when the moons will be in conjunction and what the tides on the planet would be like, for now I’d be very happy to figure out semimajor axes and orbital eccentricity.

Any help would really be much appreciated. If I ever get published I will, of course, acknowledge anyone who helps (but for the moment it’s just a hobby).


Thanks again

Karl

[Silly Elois]

maybe its a little off topic, but anyone know where i can get an acura car manual, specifically for a legend?

[SoulThief]

Have you tried searching google for a program to simulate teh conditions that you are looking for. I did a simple search on "planet simulation orbit" and the first site I got was http://www.arachnoid.com/gravitation/

I did not bother to look beyond that, but I am sure that with enough effort an appropriate simulation program could be found to help you test the validity of your settings.

ST

[hegemon]

Poul Anderson always did the math in designing his star systems. I remember a fanzine article he wrote on the subject, but can't remember which fanzine, and the orbital mechanics are way beyond my ability. I can handle a two body problem, but three body problems are notoriously hard, though I suspect the special case you ask about is doable.

[kwinsw]

Have you tried searching google for a program to simulate teh conditions that you are looking for. I did a simple search on "planet simulation orbit" and the first site I got was http://www.arachnoid.com/gravitation/

I did not bother to look beyond that, but I am sure that with enough effort an appropriate simulation program could be found to help you test the validity of your settings.

ST

Thanks SoulThief. I have looked online. But the problem isn't working out the orbits, there are lots of guides for that, it's working out what the orbits would have to be in order to remain stable. I'll take a look at that site, though - maybe it will provide some information I haven't found elsewhere.

Thanks again

[kwinsw]

Poul Anderson always did the math in designing his star systems. I remember a fanzine article he wrote on the subject, but can't remember which fanzine, and the orbital mechanics are way beyond my ability. I can handle a two body problem, but three body problems are notoriously hard, though I suspect the special case you ask about is doable.

That sounds like a really cool article. I'll do a search for it, thanks.

[kwinsw]

maybe its a little off topic, but anyone know where i can get an acura car manual, specifically for a legend?

There plenty of PDFs you can download for a fee, but I guess you've seen them.

Have you tried bittorrent? I did a quick search on some torrent search sites, there were no legend manuals to download but there were manuals for some other models. If you look around on some bittorrent and file sharing networks you might get lucky.

And now that Opera 9 includes a bittorrent client you don't even have to install any dodgy software to download torrents.

www.opera.com

[hegemon]

One obvious case to consider is where the second moon is at a trojan point relative to the planet and the first moon. That should be easy to compute and stable. Only -- I can't remember if the object at the trojan point has to be much smaller than the other two objects. Anyway, something to think about.

[Ehkzu]

re: looking for a specific car manual

Look for a user's forum. They'll know. I belong to one for VW Eurovan owners, on Yahoo Groups, for example. See if there's an Acura forum there first.

[Ehkzu]

re: two moon problem

I'm a little dubious about the likelihood of an Earth-size-pretty-much planet having a big moon and a small one. In any solar system you're going to get gravitational "jostling" from the central star (not to mention a multiple star system, as a majority are), along with the other planets--especially big ones, of course. Jupiter has had a dramatic effect on our system, for example.

That might not affect moons orbiting a big fat planet, but in this circumstance I think it's likely such perturbations will make the moons collide or one of them get tossed out of orbit. There's probably a reason why none of the rocky planets in our solar system have multiple moons. (I don't count Mars's--they're just captured asteroids).

However, I'[m not an astrophysicist so anything I suggest would need to be confirmed by someone with formal training.

Just remember it's not all calculus out there. Chaos math often rules, ultimately.

[admin]

Keep in mind that we don't need something good enough for a paper in Science, just reasonable speculation for an sf story.

A lot of science fiction writers just mention that the planet has two moons (local color) and leave it at that. But, we'll assume you want to sell to Analog or Cosmos or, just for your own satisfaction, want to get it right. In that case, I'd read up on the way resonence works for the moons of Jupiter and Saturn, and mimic that.