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Sometime in the far future, the Earth gets destroyed. Before the Earth is destroyed, humans formed a small colony on the Moon which is capable of self-sustaining itself for the next decade or so. What would be the most logical, physics-obeying situation that would happen to the civilisation on the Moon?

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    $\begingroup$ Hello and welcome to Worldbuilding! I hope that you will find your experience on this site both interesting and useful. You may want to take the tour and read the help center in order to become better acquainted with the rules and expectations of this site. (1) The title asks what would happend to the Moon, whereas the text of the question asks what would happen to the civilization on the Moon. Very different questions. (2) What would happen to the Moon depends on how the Earth gets destroyed, and the question does not say. It does not even say what it means by Earth getting destroyed. $\endgroup$
    – AlexP
    Mar 23, 2023 at 8:53
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    $\begingroup$ Mainly I think we need to know what you mean by "destroyed". A nuclear holocaust is a lot different than the earth being vaporized by aliens or impacted by a 500 mile wide asteroid moving at half the speed of light. $\endgroup$
    – JamieB
    Mar 23, 2023 at 13:30
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    $\begingroup$ One would think that, if Earth were randomly vaporized, that the Moon would simply drift off into the Sun's orbit. I find it unlikely that humans will survive as they would require considerable technology to survive and it seems unlikely that they'll have the industrial base to support the life support equipment necessary; e.g., performing maintenance, etc. $\endgroup$
    – dreamforge
    Mar 24, 2023 at 7:28
  • $\begingroup$ @dreamforge is right. The Moon's orbital velocity is about three orders of magnitude shy of being able to escape the Sun, and that's presuming that it's flung in the correct direction. The Moon would become a particularly large asteroid. $\endgroup$ Mar 24, 2023 at 15:56

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It depends on how the earth is destroyed

Consider three scenarios.

  1. The surface of the earth is destroyed. Nanites, nuclear bombs, plagues, something kills everything on the surface of the earth. The moon and the earth will just keep orbiting the sun, and it doesn't care what happened to humans.

  2. The earth is exploded equally in all directions. The kinetic energy of the moon is about 10000 times smaller than the binding energy of the earth so the moon is probably massively damaged and de-orbited.

  3. The earth is de-orbited and flung into the sun. The earth has even more kinetic energy than binding energy, so this is also catastrophic for the moon.

As such, the only safe destruction is 1. The earth's surface is damaged and all Earthbound humanity destroyed, but it remains a big rock that's going around the sun. If you are fine with everyone on the moon dying, 2 or 3 is fine.

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  • $\begingroup$ An interesting flavour of 1 is if some kind of experiment gone wrong or magic space weapon turns the Earth into a black hole. There'd just be a tiny, weird distortion in the sky if you knew where to look, though time and time again it'd pass in front of the Sun, which I imagine would be a noteworthy sight. $\endgroup$
    – biziclop
    Mar 23, 2023 at 11:21
  • $\begingroup$ They asked for logical and physics obeying, so I didn't suggest anything that needed magical space weapons. $\endgroup$
    – Nepene Nep
    Mar 23, 2023 at 12:57
  • $\begingroup$ The Moon is not mechanically attached to the Earth by a rigid rod; It's the Earth's gravity that keeps it in orbit. As such, the Moon will almost certainly remain in a stable solar orbit if you deorbit the Earth, unless your deceleration of Earth to the deorbiting is gentle enough to drag the moon along instead of letting it escape, or the Earth passes close enough to tear it to shreds, or smacks into it. $\endgroup$
    – notovny
    Mar 23, 2023 at 13:31
  • $\begingroup$ Building on #2: if the mass of the Earth is pushed away from the center (I remember the discussions of "spherical horses" in my physics classes), then the moon will either be sucked into the expanding mass of the Earth (the same mass is there, but it's expansion means it's getting closer to the moon). Or, if it moves fast enough, it pushes the moon away, but eventually absorbs it. There's a very small chance of a Space: 1999 kind of moon-spinning-off-into-space scenario, but that seems the least likely event. I can't find a way of saving the lunar occupants in case #2. $\endgroup$
    – JBH
    Mar 23, 2023 at 16:45
  • $\begingroup$ IN # 2, the debris of Earth would Eventually expand into a radius equal to the distance of the Moon and keep on expanding past the moon and thus the surface of the Moon would be scoured by the expanding debris field and the relative orbital velocity of the Moon. $\endgroup$ Mar 23, 2023 at 19:01

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