For story reasons, I need my mages to convert a moon on a collision course with their planet into a ring around the planet. The only thing they can do is hold the moon still. They can't do so indefinitely, and they can't change or reverse its momentum, so the only remaining option is to hold the moon within the Roche Limit and let the planet's gravity tear it apart, thus forming a ring. Disregard for now any potential impacts the formation of a ring would cause; the mages have protective measures in place for that.

Question: What would be the effects (on the surface of the planet) of a stationary moon within the Roche Limit for however long it takes for the planet's gravity to ensure a ring will form?


  • The moon is the exact composition of Mimas, but it is 95 times smaller, having a mass of ~ $3.15 \cdot 10^{17}$ kg.
  • The mages stop the moon slowly; you shouldn't need to worry about abrupt halts pulverizing everything.
  • The moon is locked over the position of the mages. So for instance if the planet was Earth, and the moon was over South Africa, it would remain directly over South Africa until it was sufficiently torn apart.
  • Don't worry about any kinetic energy converting into heat in the moon as it slows down. The mages are slowly removing the kinetic energy altogether.
  • Assume the planet is Earth.
  • I am unsure at this point if the planet already has a moon or not. Leave it out of the equation.
  • The mages hold the moon within the Roche Limit until the formation of the ring is guaranteed. The ring should preferably form anywhere within 1000 years. Any longer than that won't work. The quicker it forms the better.
  • I'm only concerned with the effects of the stationary moon as it breaks apart. Large chunks descending to the surface are permissible.
  • 2
    $\begingroup$ It's probably easier to slow the moon until it slots into the desired orbit. Holding it actually still is a bit complex. For instance, what is it stationary with relation to? The planet? The star? The mages? Does it orbit? Is it held in a geosynchronous (or sol-synchronous) position? $\endgroup$
    – Cadence
    Commented Jul 16, 2018 at 21:40
  • $\begingroup$ @Cadence I've made an edit to address that relativity issue, thanks for pointing it out. It would be easier to slot it into orbit around the planet, but I need a ring, not a moon. It's very important to the story. $\endgroup$ Commented Jul 16, 2018 at 21:43
  • $\begingroup$ Maybe I'm misunderstanding, but you seem to be worried over nothing. Being in orbit isn't what makes a moon a moon - rings are in orbit too. (Or, the particles that make them up are.) Any given quantity of "stuff" in orbit over a planet will eventually split into a ring if it's within its Roche limit. $\endgroup$
    – Cadence
    Commented Jul 16, 2018 at 23:37
  • $\begingroup$ So if the mages slot it into orbit, it will break apart rather than just collide with the surface? $\endgroup$ Commented Jul 16, 2018 at 23:55
  • 2
    $\begingroup$ Yes. If it's in orbit, by definition it won't collide with the surface. Even if the tidal forces tear it apart, those parts will stay in or near the larger body's orbit. (A body that breaks up for another reason - like an explosion - might scatter debris further out.) $\endgroup$
    – Cadence
    Commented Jul 17, 2018 at 0:03

1 Answer 1


Big misunderstanding here:

Lets look at what "stop" really means.

You have to define a context for stop. 1) Relative to the wizard, 2) relative to the planet, 3) relative to the star, 4) relative to the galaxy.

3) and 4) are clearly useless for you--both result in it flying off at great speed. 1) and 2) are problematic because both require the expenditure of great energy to keep the moon from simply falling down. All the chunks that come off simply fall onto the planet, you don't get a ring.

The only way you are going to get anything like what you want is if the wizards simply slowed it a bit so it passed behind the planet rather than hitting it. The problem here is that while conceivably this would result in a capture it would not result in anything like a circular orbit--your ring is very lopsided indeed and I'm not aware of anything that could circularize it.

Thus, if you have a ring it's because the wizards deliberately slowed it more than they needed to in order to circularize and make a ring.


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