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They would occasionally get so close to each other that I'm imagining their orbits as either going chaotic or drifting towards a situation in which they are orbiting each other as well as the planet - but I don't think it works with two moons so similar in size and mass.

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history-monk.livejournal.com

They are quite close to a 6:7 resonance but I'm not sure how stable that would be.

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No idea - with the two destroyed moons that are mentioned it doesn't seem likely that it would stay stable for long.

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It would appear that he forgot to apply the square/cube law to his mass calculations.

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Or meant percentage of the Moon's mass.

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Still wouldn't work. The moons are about 1/3 the diameter of our moon, which gives them about 1/27 of the mass.

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Crud - you're right, of course.

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or nearly - the moon is 1738 KM wide, Krypton's moons are given as 1095 and 1017, so about 15% of the mass of the moon. Which works out at about .002 Earth masses.

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The moon has a *radius* of 1738 km. Its diameter is 3476 km.

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DOHHHHHHH!!!!!

REALLY wasn't paying enough attention last night - I've got guests coming at the weekend and spent most of the evening cleaning.

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IANAA, but, as has been pointed out, the two moons are about the same distance from Krypton as our moon is from us. (In fact Koron is only 100 km closer to Krypton than the moon is to us.)

Two moons being that close together probably won't work. For them to have stable orbits, you would probably want them to have orbital periods with a 1:2 resonance: the outer moon makes one orbit in the time it takes for the inner moon to make two. For that to happen their orbital radii would have to have a ratio of 1.59 (cube root of 4 if you want more precision.)

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I'd be happier if there was a big difference, or a Bode's Law ratio between those orbits (and the orbits of the two destroyed moons).

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Bode's Law actually comes pretty close to putting planets into 1:2 orbital ratios. (It predicts a spacing ratio of 1.6 between Earth and Mars, for example, while the real ratio is 1.5.)

The 4 Galilean moons of Jupiter have orbital periods that all fall pretty closely into 1:2 orbital ratios, as you move out:

Io: 1.77 days
Europa: 3.55 days
Ganymede: 7.15 days
Callisto: 16.69 days

If you leave the outer moon, Koron, at 384,000 km, putting the inner moon, Mithen, at 242,000 km would give them a 1:2 resonance.

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Yes - I was agreeing with you and expressing it badly - I meant that the orbit described doesn't work.

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OK. If we assume that all four original moons fell in orbits that obeyed Bode's Law, and the two inner moons are the ones that have gone missing/been destroyed, the two left would have a nice 1:2 ratio, and be Bode's Law compliant.

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These were my figures and I freely admit my math occasionally sucks big time.

The masses were SUPPOSED to be figured with the Earth's Moon as 1.

However, if somebody wants to give me figures that would work better, I have no problem with that either.

I had actually figured the two destroyed moons to be very very small and fast.

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Small and close in works well for that, but they'd barely be visible except as faint moving dots - that silly picture really doesn't work!

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And PLEASE don't get me started on the "science" of Another Earth, at least the bits we see in the trailer, or The Return of the Time Lords...

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Wouldn't dare. I only allow a limited number of physics violations per universe - and the size of habitable planets isn't one of them - today.

But if we assume that the pic is (maybe) Kryptonian landscape art rather than a photo-depiction, and knowing that Earth artists invariably exaggerate the size of the Moon by massive amounts - then there's no reason not to do something like estimate the difference in arc between the two moons and work some figures from that.

Just eyeballing it it looks like one is about 5 times wider (in arc) than the other. Since it's probably not reasonable for one to be 5 times further away from the planet, then one is much smaller - but it doesn't necessarily have to be the closer one that's smaller.

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I worked through these trying to get the visible arcs to make some sense:

	M(M)	diameter(km)	mass (kg)	distance (km)	orbital period (days)	Arc in min.
Koron	0.502	2840.55	3.69E+22	345869.1	21.33	27.43
Mithen	0.017	915.97	1.24E+21	538018.6	41.38	5.69

This makes the two moons very different in size and gives visible arcs in line with the picture (although much smaller than shown but I figure the artist exaggerated - a lot.)

I hope this works better.

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That feels a lot more reasonable - many thanks for going to so much trouble.

I really hate to say how minor the plot point will be, if it happens at all; just Kara missing something because she's not very familiar with tidal cycles - especially since she's from an essentially artificial environment.

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