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Will be possible in the far future to use megascale engineering to create an artificial hollow moon to give Earth a big moon in the sky like in billion years ago? Since it would be hollow, it would not have surface gravity (this is what people say), so tides would not be a problem. Am I right?

If Mars and Venus get terraformed, they could have large moons as well.

But how big this artificial moon would look in order to appear 5 times bigger than our current moon? And how bright would it look if it have the same albedo as the real moon?

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    $\begingroup$ The apparent size depends on the orbital radius, which affects speed. Do you mind it orbiting the Earth, for example, several times a day? Or you want it to have monthly cicles? $\endgroup$
    – Rafael
    Commented Jun 21, 2018 at 15:12
  • $\begingroup$ Stealing one from Jupiter or Saturn is the plan for when we lose the moon so much that the tides start to not work and the oceans die due to lack of currents. Documentary: If We Had No Moon. Why are you doing this if keeping the tides right isn't the objective? $\endgroup$
    – Mazura
    Commented Jun 21, 2018 at 19:31
  • $\begingroup$ Far future? It was done in the 1960s: en.wikipedia.org/wiki/Project_Echo $\endgroup$
    – jamesqf
    Commented Jun 22, 2018 at 5:23

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Yes definitely i mean to be honest this isn't even necessarily a "far future" concept. the moon is basically a sphere so assuming you a using a sphere five times the radius of our moon (or 17 megametres in diameter) then it would have a surface area of 9.5*10^14 square metres which does sound like a lot but using aluminium foil of thickness 0.1 mm is only 2.6 * 10^14 kg of aluminium which isn't too much in the grand scale of things only four-thousand years worth of earths current aluminium production. Yes a lot but with time, automation and economies of scale it can definately be done, and can always be made easier with thinner foil.

it would have some gravity but it would be totally insignificant compared to the actual moon. Getting all that mass up there isn't a problem either to be honest since its just plain metal you could send it up by mass driver or just use a system like an orbital ring to avoid inefficient rockets. The total mass/effort needed could also be cut down by moving the 'moon' to an lower orbit.

As for albedo luckily for you its possible to simply tailor the appearance by anodising different "pixels" into the aluminium (even if these pixels were a metre across you'd need a phenomenally good telescope to notice). the sphere would keep its spherical shape thanks to an extremely small internal pressure provided by some gas (probably hydrogen).

I would be worried about the photon pressure on such a large but light object however its possible that additional mass could be added to compensate or that thrust could be provided via the planets magnetic field and small automated system. Alternatively the moons 'dark side' could have its optical properties adjusted to match.

Why would you do it? No idea whatsoever.

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  • $\begingroup$ FWIW you can roughly estimate the total world production of Al to date as 1x10^9 Ton = 10^12 kg, from here. $\endgroup$
    – Rafael
    Commented Jun 21, 2018 at 15:16
  • $\begingroup$ A bit of care is needed to prevent it from collapsing. You probably need it to spin. Not sure how the spin would affect a hollow object's eccentricity though. $\endgroup$
    – Rafael
    Commented Jun 21, 2018 at 15:21
  • $\begingroup$ My Aluminium production estimates came from here minerals.usgs.gov/minerals/pubs/commodity/aluminum/… again i think that simply adding pressure with the use of internal gas (similar to a balloon) would prevent collapse. though obviously air would not be ideal as its oxygen content could corrode the sphere $\endgroup$
    – Ummdustry
    Commented Jun 21, 2018 at 16:01
  • $\begingroup$ perhaps helium or another noble gas. I don't think you need a lot of it, though. The main challenge is the aluminium, and the whole lot of maintenance to fix holes from small debris kicking it $\endgroup$
    – Rafael
    Commented Jun 21, 2018 at 16:39
  • $\begingroup$ @Rafael I don't think holes would be a problem either, after all you can't see them from the ground and its likely that the large volume of gas accompanied by the very low pressure would mean that a leak would have a very slow flow rate compared to the volume of the gas. $\endgroup$
    – Ummdustry
    Commented Jun 21, 2018 at 17:33
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One at a time:

Will be possible in the far future to use megascale engineering to create an artificial hollow moon to give Earth a big moon in the sky like in billion years ago? Since it would be hollow, it would not have surface gravity (this is what people say), so tides would not be a problem. Am I right?

Wrong! it would still have a gravitational field, and its intensity would depend on the total mass of the hollow moon: it would be weaker if you just had a big big big big balloon in the sky, but it would be there.

If Mars and Venus get terraformed, they could have large moons as well.

If you want large moons to be engineered into orbit, like so many Star Wars' Death Stars you're welcome to do so. In fact, I like the idea for Mars, since it's climatic stability would greatly benefit from a moon that helped keeping the planet's axis in place. But it'd have to be a full moon, not hollow.

Albedo is not about being bigger, it's about reflectivity. Just look at our own Moon. It's not 'apparently bigger' than it is. You can choose mirror foils to turn it in a sort of 'disco ball' orbiting in the sky. It would be reflecting a lot of light and appear brighter than our moon.

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  • $\begingroup$ So why people say that a hollow giant sphere wouldn't have surface gravity?Even if has gravity, if the moon is built using lightweight and low density material it would not have too much mass and therefore it would have a small surface gravity right? $\endgroup$
    – user18428
    Commented Jun 22, 2018 at 14:34

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