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I'm planning to design a world with a moon whose orbit is VERY slow (takes more than a century to complete a full circle). Said moon's orbit would take them close enough for their atmosphere to flow into the planet's. This would be significant mostly due to the moon having a much different composition than ours.

Would this be possible and if so, how big could said moon be? Would it be possible for the overflow to only go one way, maybe due to the planet's larger gravity pool?

Brythan
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Berry M.
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    Hi Berry, and welcome to Worldbuilding SE. I'm pretty sure we've had some questions already on this subject. Try searching for Roche limit. Also, please [edit] your question and tell us more about the planet and moon involved; particularly their masses. I have a very hard time seeing how you could design a system where the moon needs over a hundred (Earth) years to orbit the planet; it would likely need to be too far away. In our solar system, orbiting the Sun, you'd need to be somewhere between Uranus (84 Earth years) and Neptune (165 Earth years) to get such an orbital period! – user Jun 08 '16 at 15:26
  • Note that if your moon was a comet instead, a hundred year period between approaches would make more sense. You may want to consider what are the important points of your story. I.e. what do you want to have occurring. Then we could better fill in the details. Is this an Earth-like planet? Or something different? Humans on it? Or something alien? – Brythan Jun 08 '16 at 16:54
  • This is a plot point in Crash Of Moons... a very cheesy 50s sci-fi movie. – Schwern Jun 08 '16 at 17:10
  • Thank you all! I love this section of Stack Exchange. Such enthousiasm! – Berry M. Jun 08 '16 at 19:34

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The short answer is no, for various reasons. The first is that atmospheres just aren't very thick, so the two bodies have to very close to each other in order for the atmospheres to interact. This will cause one or both bodies to start to disintegrate (see the Roche Limit). I don't think you realize just how thin atmospheres are. The Earth's atmosphere is 99% below 30 km, and the Earth's radius is about 6400 km.

Also, consider a satellite in NEO (Near Earth Orbit). It has about a 90 minute period. That's about 17,000 mph. If this is a near pass by an object in a larger orbit, the speed must be much greater in order to let the object get farther away from the earth. But then the orbit can't be "sharp" enough at close approach to do a (roughly) 180 degree turn - that only happens for something like LEO. Although an orbit can get closer than the 90 miles or so (early satellite orbits) the gravity pull on the orbit simply doesn't increase much.

Also, if two bodies passed close enough to mix atmospheres, the atmosphere gases would be the least of your problems. At the speed they're going, you would get really massive devastation from the shock waves - 17,000 miles plus over a HUGE area.

WhatRoughBeast
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