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The civilization in question here has colonized the solar system. They have established settlements on most solid planets. Of course, a space-faring people generates plenty of trash, but they have a few ideas.

Is it possible for this trash to be sent into space, and formed into an artificial moon? Could humans potentially live on it?

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    $\begingroup$ Similar to Gas Giants as waste disposal sites. The answers and comments would seem immediately relevant to your question. $\endgroup$ – StephenG Dec 6 '18 at 9:03
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    $\begingroup$ This is supposedly type 1 civilization and they consume energy directly from the star, at this level they should not be producing lots of unusable/unrecyclable materials but anyhow it would likely be a safe haven for all politicians... $\endgroup$ – user6760 Dec 6 '18 at 9:08
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    $\begingroup$ Obligatory Futurama reference: theinfosphere.org/A_Big_Piece_of_Garbage $\endgroup$ – RonJohn Dec 6 '18 at 10:32
  • $\begingroup$ Given enough time this would strip your planet bare... $\endgroup$ – Pixel Dec 6 '18 at 15:12
  • $\begingroup$ Reminds me of this: youtube.com/watch?v=bL5TRaI9u0k see background, wonder if they are doing just that. $\endgroup$ – mxmissile Dec 6 '18 at 15:21
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The main obstacle to what you propose is cost.

Just look at what happens today in every city of this world: transportation is cheap, yet we barely move our garbage few km out the door and throw it in some landfill. Garbage has no value, and whatever one does with it has to be dirty cheap. No long distance transportation, no fancy things.

Same would hold for your civilization: why using fuel, ships and launch windows just to discard garbage? If your people are traveling space, it means more of the planet is available for landfill and garbage storage, where it won't risk impacting some transiting space ship.

Even worse, building up a moon requires additional effort to set up and control its orbit. Just dump it on the planet.

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  • $\begingroup$ "Garbage has no value" right, but what about the space it occupies? It might still make economical sense in a world with extreme population density. Not sure these worlds really make sense themselves, but they're a common trope in science fiction anyway. $\endgroup$ – Aaron Dec 6 '18 at 15:17
  • $\begingroup$ I thought I saw it was economical for some companies to mine for precious resources and materials in old garbage dumps in europe. $\endgroup$ – whn Dec 6 '18 at 15:21
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    $\begingroup$ @Aaron: Actually, I would argue the value of garbage increases with the scarcity of space => if space is so scarce you cannot dump garbage, how do you get raw materials in the first place? By recycling garbage, of course. And in this case, it's counter-productive to throw it at the moon. $\endgroup$ – Matthieu M. Dec 6 '18 at 15:39
  • $\begingroup$ @MatthieuM. I expect garbage would be recycled as much as possible but that there'd still be some unrecoverable waste you'd need to deal with. If that's indeed the case, maybe you could export your waste to places where storing it / trying to squeeze the last sliver of value out of it would make more economical sense. Or maybe there would be some ways to dump it into space that would have a low-ish operational cost (maybe a space elevator?). In any case I agree it's hard to find a sensible reason to aggregate it into a moon. $\endgroup$ – Aaron Dec 6 '18 at 16:17
  • $\begingroup$ We will put garbage on a platform and send it down river. If we had anti-gravity space garbage platforms that could lift it for free into space, there wouldn't be a cost. $\endgroup$ – Bill K Dec 6 '18 at 17:37
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Using this as a reference for amount of rubbish generated, averages to about 281.5 kg/person per year.

Our moon has a mass of 7.342×10^22 kg, so doing the math, the current population of earth would need 34.63 billion years in order to produce the amount of rubbish needed to create a moon with the mass of our own. See here for a few more details. Also keep in mind that our moon (or other planetary objects with similar mass/size) won't be able to hold an atmosphere, so having humans living on the Trash Planet (ignoring other issues) would mean it would need to be much larger.

Since your civilization is a space faring one, with populations on a number of different planets, it's logical to assume there's quite a few more of them. Taking that into account, we could calculate the total population needed in order to produce the amount of rubbish needed in a more feasible amount of time. So if say, you wanted to build it in the span of 200 years, you'd need a population of 1.304×10^18. That's about 170 million Earths worth of people. Considering this all takes place in the future, you would assume waste would be somewhat reduce (hopefully recycling and such). You could of course have the future people go in the complete opposite direction (what with rubbish being needed for the Trash Planet construction) and have them produce more rubbish, but even doubling the amount of rubbish per person and doubling the construction time to 400 still means you need roughly 44 million Earths worth of people.

All that aside, making an artificial world also seems somewhat tough, at least according to this thread. While you're not really looking to make a whole planet, making it habitable in any form (even with Moon bases etc) seems unfeasible given the timespan needed to get it to settle down.

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    $\begingroup$ Presumably putting that amount of rubbish into orbit would have a pretty substantial impact on the source planet too... How much of it would be from surface layers, say (very little rubbish comes from the planetary core!)? $\endgroup$ – Matthew Dec 6 '18 at 13:30
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    $\begingroup$ @Matthew It would mean a 1.2% loss of mass for Earth. That is actually quite substantial. $\endgroup$ – DRF Dec 6 '18 at 14:03
  • $\begingroup$ Basically the entire crust - that would definitely impact life on the planet $\endgroup$ – Matthew Dec 6 '18 at 14:26
  • $\begingroup$ Of course, a spacefaring civilization would utilize planets it is not living on. We are quite far from being a mature spacefaring civilization, and yet we have proposals tool dissasemble mercury... $\endgroup$ – b.Lorenz Dec 6 '18 at 15:03
  • $\begingroup$ Our moon is exceptionally big compared to the size of our planet, we are practically a twin-planet. What about "a" moon with the relative size of one of the moons of Jupiter or Saturn? $\endgroup$ – Bill K Dec 6 '18 at 17:39

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