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If we moved Enceladus (not Europa) into Earth's orbit (ignoring the plausibility of it), and gave it a breathable atmosphere, what effects would it have on the Earth? How would it affect us?

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    $\begingroup$ Not part off an answer, but it's not massive enough to hold on to a breathable atmos. at this distance from the sun (temperature). $\endgroup$ Commented Feb 6, 2020 at 18:09
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    $\begingroup$ @BLT-Bub this argument arises often, but seems to miss the point that atmosphere escape applies over geological and astronomical timescales. It isn't really relevant if the atmosphere disappears in a million years, or even in ten thousand years, especially if the means for generating said atmosphere were still available to keep it topped up. $\endgroup$ Commented Feb 6, 2020 at 19:31
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    $\begingroup$ @StarfishPrime ... or indeed a planet-enclosing forcefield or gas-impermeable sphere. Just saying... Enceladus is small enough that it'd otherwise lose the atmos quicker than that. $\endgroup$ Commented Feb 6, 2020 at 19:45
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    $\begingroup$ Oh, sure, but if the OP was okay with grossly temporary solutions, why didn't they say so? $\endgroup$
    – Zeiss Ikon
    Commented Feb 6, 2020 at 19:53

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Presuming you'd park Enceladus well away from Luna's orbit, you'd get either much stronger and more complex tides (if closer) or slightly increased and more complex tides (if beyond).

However, I'm not sure you could park another large moon inside our existing Moon's orbit without disrupting the system with three-body effects, so you'd probably have to park it further out -- which would be a problem in terms of keeping Enceladus bound to Earth, because the Moon is much further out than most other moons in the system, relative to the sizes of their primaries. Try to put a second moon in even a 4:3 resonance beyond Luna (that'd be a 40-day orbit, roughly) and you've have a problem with the "new" moon wandering away into a halo or horseshoe orbit of the Sun, rather than being a satellite "of the Earth" as you seem to intend.

Most likely, because of the very large size of our Moon relative to the Earth, there isn't a way to bring in a second body of similar size/mass to Luna and not disrupt Luna's orbit in some way -- the two bodies are too close to the same mass to use Lagrange libration points, and even our distant Moon is too close (and too non-uniform) to park them in orbit of each other without perturbation by the Earth, Sun, and Jupiter causing them to drift out of that arrangement.

I was reminded in comments, however, that Enceladus is actually significantly lighter than Luna -- more than 600x lighter, in fact, which is well below the threshold to be metastable in the L4 or L5 Lagrange points. So the simple solution appears to be to park Enceladus 60 degrees ahead or or behind the Moon in the same orbit (the additional mass added to the system, in this case, isn't enough to noticeably change the Moon's period).

Doing this will insert a second, weaker high tide each day, either four hours ahead of the main one or four hours behind it. There are some locations (such as the Bay of Fundy, where flow restrictions make the tides much more spectacular than is common) where this will cause significant complications, but in most locations, the overall effect will be small.

Of course, there will be a second moon in the sky, with phase running about five days ahead or behind Luna's -- it will be smaller, but might be as bright or even a good bit brighter (since the artifically maintained atmosphere mildly implies surface water and clouds, which will have a far higher albedo than the asphalt pavement darkness of Luna's surface). This would have some knock-on effects on nocturnal life -- night hunters would have more light, longer, most nights, for instance. The second light in the sky might also confuse some night fliers for a while, because they'd have to navigate by one beacon or the other -- it might take thousands of years to select out the moths and night flies that can't deal with the change.

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  • $\begingroup$ Given the staggering difficulties involved in moving something as big as Enceladus out of a deep gravity well and then gently inject it into a terrestrial orbit, anyone capable of doing such a thing could just chuck it into an Earth-Moon Lagrange point and use the same transport mechanism it was moved with for stationkeeping. $\endgroup$ Commented Feb 6, 2020 at 19:33
  • $\begingroup$ Sure -- if they want to stick around for millions of years to adjust the orbits every few months or years, to keep from coming back from a sublight trip to M31 to find Enceladus or Luna missing and Earth with a new crust and no atmosphere... $\endgroup$
    – Zeiss Ikon
    Commented Feb 6, 2020 at 19:50
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    $\begingroup$ "The gift that keeps on giving -- provided you're willing and able to pay for it for the rest of the Sun's lifetime." Ethical demigods owe it to the normals to use solutions that stay solved. $\endgroup$
    – Zeiss Ikon
    Commented Feb 6, 2020 at 19:54
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    $\begingroup$ Actually, Enceladus is a quite a bit smaller and lighter than the Moon. Moon's 7.35E22kg, vs Enceladus at 1.08E22 kh. Mass limit for stability on L4 or L5 is a shade under 25:1, and Moon to Enceladus is about 680:1 $\endgroup$
    – notovny
    Commented Feb 6, 2020 at 21:16
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    $\begingroup$ @notovny that's a very good point (especially once I'd realised you meant E20 for Enceladus, not E22!) $\endgroup$ Commented Feb 6, 2020 at 21:40
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The Orbit Matters

If Enceladus is parked opposite Luna, at a matching orbit, then these changes will happen at minimum : Tides will occur at the same timing (remember, both close and far sides of the Earth flex out) , and they will be stronger . The night sky will be brighter on average, and dark nights will be rare. Enceladus has a lighter density and much lighter mass, so its orbit will change at a different rate than Luna's, but that's a millions-of-years change.

If Enceladus is parked around and close to Luna so that the orbital plane passes through the Earth, then the tides on Earth will be stronger in general, with a new oscillation from the distance to Enceladus.

If Enceladus is parked around and close to Luna so that the orbital plane is flat to Earth, then Earth's tides will be stronger in general, and generally the same as before.

As mentioned in a previous answer, most Earth-centric orbits will result in calamity, with Enceladus flung away or falling into Earth. Enceladus has to be in resonance with Luna's orbit around Earth (L1-L5), or Enceladus has to be a significant distance away.

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