This is my first post. I apologise for any infractions I may have committed, I will try to be better in my next posts. To clarify, this question is not about if Earth had 'two moons'. This is instead referring to a theoretical submoon entity, with proportions near identical to that of Deimos. This submoon, theoretically, orbits around Luna in a fairly standard orbit outside of its Roche Limit, with little elliptical curve. This submoon would be covered with highly reflective silver particles. This submoon will be called Selene. If this moon were to appear, with the extremely unlikely chances of it surviving entry, capture, and survival against the Earth occurring, what would the implications be? Would the tidal forces just rip it apart? Would it act as a satellite to the moon?

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    $\begingroup$ Hello @Rory02, welcome to Worldbuilding. Please note that asking more than one question is literally a reason to vote to close a question. Further, asking an open-ended question - like asking for "implications" - is prohibited in the Help Center. Please take the time to read through our tour and the following two Help Center pages: help center and help center. Finally, inviting us to answer quickly is an unreasonable expectation. Out of curiosity, what's your rush to build a fictional world? $\endgroup$
    – JBH
    Commented Mar 23 at 2:07
  • $\begingroup$ Oh, I apologise. It is simply 'please reply at your earliest convenience'. However, I did not want this to sound like an Email. As for the multiple questions, I understand now, and I will keep it to just one straightforward question. Regarding the open-ended question, I am vaguely confused about that particular aspect. Could you please clarify what qualifies as 'open-ended'? $\endgroup$
    – Rory 02
    Commented Mar 23 at 3:45
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    $\begingroup$ More generally about submoons: worldbuilding.stackexchange.com/a/249535/16949 $\endgroup$
    – jcaron
    Commented Mar 23 at 13:22
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    $\begingroup$ Stack Exchange is designed to focus on one specific and reasonably objective problem with a definitive solution. It's not a place to ask questions that have a long (or never-ending) list of things as an answer. Think of it this way, Stack Exchange expects you to ask a question that will have one (just one) best answer. So when you ask for "implications," you're asking for a whole list of things. How will you choose a best answer? You can't. The help center prohibits questions where all answers have equal value, which is what asking for "implications" does. There's no single best answer. $\endgroup$
    – JBH
    Commented Mar 23 at 17:34
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    $\begingroup$ Very well then, thank you very much, @JBH. I will attempt to avoid this in the future. $\endgroup$
    – Rory 02
    Commented Mar 23 at 17:53

1 Answer 1


On Earth: no detectable effects. Deimos's mass is about 50 million times less than the Moon's, so its influence on Earth's tides is also about 50 million times less, regardless of whether it is a bit closer or a bit farther away.

On Deimos:

Moon-Deimos Roche limit is (with moon M, Deimos D, mass m, radius R)

$d = R_D(2m_M/m_D)^\frac{1}{3} = 2900 km$

That's less than a moon radius above the surface of the moon, and only about 1% of an Earth-moon distance. It's safe from being ripped apart or captured by the Earth, even if we double that distance, and we can give it an almost perfectly circular orbit around the Moon. It'll eventually go eccentric or get smashed apart by passing meteorites, but it should be fine for a few million years.

  • $\begingroup$ Wikipedia says that Lunar orbits above 690km are unstable due to perturbations from Earth, so I'm skeptical about this. $\endgroup$
    – N. Virgo
    Commented Mar 23 at 16:14
  • $\begingroup$ @N.Virgo for what it's worth before answering I fed it into a simulator (with nothing in the universe but Earth-mass, Moon-mass, and Deimos-mass) and ran it for five thousand years and the orbit was still round. I suspect that the Wikipedia reference material may be referring only to orbits oriented away from the Moon's equator to allow for measurement of the poles. Following the reference got me 'Stable circular lunar orbits do exist below an inclination of 39.6º, says Ely, but they spend so much time near the equator that "they are terrible orbits for covering the poles."' $\endgroup$
    – g s
    Commented Mar 23 at 16:38
  • $\begingroup$ Also note that the 700km altitude is d=2400km. The moon's radius is about 1700km. $\endgroup$
    – g s
    Commented Mar 23 at 16:45

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