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I'm working on a story that has several worlds but I focus only two of them now. World A is our present, world B is a fantasy-ish one which is very similar to ours but evolution has been different so there are more intelligent species not just humans and some other plantation as well. Magic in some forms are present.

On that world the Moon is partially destroyed and got closer to the Earth because of the changed magnetic fields and/or changed gravity. It happens in a 100 or 1000 years cycle. World B should slowly be destroyed by this.

I'm thinking a past-war leftover (like a big ship or something) floating around the solar system which mass is bigger so it change the gravity as it comes closer to the planet. Also it knock a small piece from the Moon every time.

How and would it be even possible? Does the fact the the Moon is changing has another affect on the planet?

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  • $\begingroup$ So you want that moon having been destroyed partially by some impact event, and then coming too close to that world in a 100 or 1000 year cycle because of an altered orbit? And that altered orbit could be because of some other object that comes so close to that moon that it destroys it further a bit? But the whole thing should still follow a stable periodical pattern? $\endgroup$ – Khris Oct 20 '17 at 6:50
  • $\begingroup$ Yes it should be more or less stable between the cycles. Of course there could be effects like earthquakes or changed weather but the planet should be still livable. $\endgroup$ – user43947 Oct 20 '17 at 7:16
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I'm thinking a past-war leftover (like a big ship or something) floating around the solar system which mass is bigger so it change the gravity as it comes closer to the planet. Also it knock a small piece from the Moon every time.

You are basically asking to have the moon closer than the Roche limit of the other body

the Roche limit or Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction

This makes the body way bigger than the moon itself (else it would be destroyed by the moon, not vice versa), and thus less likely to be a war leftover.

Moreover, if the body is so big, your concern should not be the moon being chopped away, but probably the planet itself being heavily disturbed by this body.

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  • $\begingroup$ So how about that the moon got closer to the Roche limit causing it to fall apart slowly so it's mass doesn't stop the other body anymore? $\endgroup$ – user43947 Oct 20 '17 at 7:18
  • $\begingroup$ The problem is that the other body should affect the moon in such a strong way but not the planet. I can think of a moon in an extremely elliptic orbit that gets affected by the other body only in its apogee, but that would mean that the orbit would get disturbed greatly so that the moon's period would change, it could even get completely thrown out of the orbit and leave that planet. $\endgroup$ – Khris Oct 20 '17 at 7:35
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I don’t think that regular collisions between the moon and another object causing pieces to be knocked off the moon are even remotely plausible. That said if you are planning to use magic in some form in the story then that may be your answer. The problem will be that going far enough into magic to make this possible is likely to leave you with consistency problems. For example people may ask why, if bits of the moon can be knocked off by magic couldn’t this magic be used to good effect in all other elements of your story?

If the moon were to have large pieces removed from it somehow it would reduce the tides on the earth. It would also effect the orbital period of the moon every time a piece was removed.

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  • $\begingroup$ Sure I can use magic as an explanation. Maybe there is an old magic/curse and the last person who could stop it is long gone. There is a religious cult/leadership who sacrifice humans to stop this anyway. Although I did not plan to introduce magic as detailed as that. But it's a good idea! $\endgroup$ – user43947 Oct 20 '17 at 10:17

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