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If this hypothetical moon were to come to exist between the Earth and Moon, how long would it survive? And could it transition to the moon's orbit, as a subsatellite?

To clairify, this scenario is intended to occur after a second giant-impact event, resulting in a second, smaller moon capable of creating a bi-lunar(?) eclipse, albeit with the new moon presumably colliding with Earth or the Moon after an extended period of time.

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    $\begingroup$ This is going to depend heavily on its velocity at the time it comes into existence. Any specifications in that regard? $\endgroup$
    – manveti
    Commented Nov 8, 2019 at 1:31
  • $\begingroup$ Could it perhaps make use of the Earth-Moon L1 Lagrangian point? $\endgroup$
    – Arkenstein XII
    Commented Nov 8, 2019 at 1:56
  • $\begingroup$ @manveti my thinking was an impact event along the lines of the giant impact hypothesis, but at a slightly smaller scale. I'm afraid I don't currently have specifics beyond that though. $\endgroup$
    – Wax
    Commented Nov 8, 2019 at 4:47
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    $\begingroup$ Wax - you can probably get an impact with Earth or the Moon, have that satellite escape Earth's gravity forever, or arrange for a stable orbit for years if not millennia around the Earth (not sure if you can get it to orbit the moon from that starting point without additional interventions) - this greatly depends on parameters such as velocity, mass and orbit. If you edit your question to describe the desired outcome, or any additional constraints (e.g. this an alternate Solar system where Earth naturally came to have two moons) you are likely to get much better answers. $\endgroup$
    – G0BLiN
    Commented Nov 8, 2019 at 9:00
  • $\begingroup$ @G0BLiN More info has been added, I just hope it's not too vague. $\endgroup$
    – Wax
    Commented Nov 9, 2019 at 11:16

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Halfway between Moon and Earth the body will still be within the Hill sphere of Earth, above the geosynchronous orbit.

Thus, if it has the appropriate velocity, it can orbit Earth.

If this orbit is long term stable or not depends on its parameters: drag at that distance is practically null, but if the orbit dives periodically in the upper layer of the atmosphere the loss of momentum can cause a significant orbital decay.

Influences of other bodies can also destabilize the orbit, but again, I cannot judge its stability without more info.

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  • $\begingroup$ Additional info has been added $\endgroup$
    – Wax
    Commented Nov 9, 2019 at 11:17
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An orbital resonance of 1:2 or 2:3 can make the whole system generally stable

The tides will be even more complex matter than they are now.

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  • $\begingroup$ Just to clarify, would 1:2 mean the inner moon orbits the Earth twice for every single outer moon orbit, or vise versa? $\endgroup$
    – Wax
    Commented Nov 10, 2019 at 1:09
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    $\begingroup$ It could be only the inner moon circling faster and not the other way round. See Kepler laws. Or if you want a rather unstable example, see Mars and it's Phobos and Deimos. Phobos will fall on Mars in a few million years - and probably is a product of a larger satellite falling on Mars earlier. $\endgroup$
    – fraxinus
    Commented Nov 10, 2019 at 7:22

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