Hi,

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

The planet will be 98% as dense as earth but itâ€™s radius will be 2% bigger than earthâ€™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 Iâ€™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 Iâ€™d like to work out orbits and the mean resonance ratio of a large inner moon (4.6368 Ã— 10 to the power of 22 kilograms) and a small outer moon (2.87776 Ã— 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 Iâ€™m trying to figure out the likeliest configuration)?

Later on Iâ€™d like to figure out when the moons will be in conjunction and what the tides on the planet would be like, for now Iâ€™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 itâ€™s just a hobby).

Thanks again

Karl