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I mentioned in a previous answer the problem with building magnetic shielding would require basically reassembling a entire planet to give it a van Allen belt.

Imaging space travel works, which I don't, wouldn't it be easier to just build a super space station? Assembling a $10^{20}$kg planet which might be able to produce $10^{15}$ W over $10^{9}$ years, versus packing a space station with uranium which would require about $10^{10}$ kg for the same result. Given radiation problems isn't it cheaper to just build a giant space station than terraform?

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  • $\begingroup$ why would you add a magnetosphere. a magnetosphere would be one of the basic things you look for in perspective planet. most of the planets in the solar system have one after all. $\endgroup$
    – John
    Dec 8, 2020 at 5:30

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I am a huge fan of building space stations. You can build one more, repeat. That's the way to spread humanity.

If your station can sustain humans for 1e9 years, it is also the way if you want to move to other stars eventually.

If your station needs 1e10 kg of mass, you can build 1e10 of them for the same cost as you would have for one 1e20 kg planet. That's matter-efficient.

I know you're just talking about the magnetic shielding, but once you have it you can do a lot of other things with it, too.

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If you're talking about giving a planet magnetic shielding against radiation, would you really need to reassemble the planet? One proposal for terraforming Mars involves putting a powerful magnet in the planet's L1 point, where it would block radiation coming from the Sun (the main source of dangerous radiation).

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Terraforming is extremely costly in terms of energy, takes a long time, is inherently destructive toward whatever was there in the first place, and after you've done that, you're stuck with a planet, which has a gravity well and breathable atmosphere, both of which increase the cost of leaving said planet.

Or, you could build a few space stations, maybe buried in hollowed-out asteroids, and have as much space much cheaper, quicker, more customizable, and leaving the station is so cheap you could potentially build a short-range personal vehicle right now for under $100,000US [1]. The only downside is that you need to ship in lots and lots of resources to build and maintain your habitat, whereas with a planet you could source a lot of that locally (see also: Musk Vs Besos on Mars Vs O'neill Cylinders).

[1] The cheap low-tech intraorbital vehicle you could throw together in a garage with some sheet-metal would not have very good radiation shielding. It'd also not be very fast, but if your neighbors are in the same or very close orbit, the energy it'd take to travel to and fro would be quite low, compared to interplanetary travel. The exact pricetag is less relevant than the fact that you can get away with cheap low-tech intraorbital vehicles if you can make them air-tight.

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    $\begingroup$ As much space is arguable, a planets surface is massive, you would need to build a LOT of space stations to come close. $\endgroup$
    – John
    Dec 8, 2020 at 5:37
  • $\begingroup$ @John Still easier than terraforming. And if you mined out the planet, those resources would provide far more habitable space as stations than as a planet. $\endgroup$
    – DKNguyen
    Dec 8, 2020 at 18:46
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    $\begingroup$ @DKNguyen mining out the planet would require lifting the mined resources out of the gravity well. The energy to do so would probably approach terraforming scales. $\endgroup$
    – GOATNine
    Dec 8, 2020 at 19:40
  • $\begingroup$ @GOATNine way more than terraforming. All the petroleum on earth could not lift even a single percent of its mass off world. $\endgroup$
    – John
    Dec 8, 2020 at 20:42
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    $\begingroup$ @DKNguyen of course the question is about quality of life as much as it is raw survival. humans can survive off wheat and beans but few are willing too. There was a time when meat and bathing were considered luxuries. Your living accommodations have to be desirable if you want people to use it. Also the nice thing about terraforming is you don't have to build every square foot of living space, you use living things to do a lot of the work. $\endgroup$
    – John
    Dec 8, 2020 at 21:28
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Terraforming is a waste of time and energy-O'neill Cylinders are the way to go. If you send out just a couple self-replicating robots with the blueprints for one installed, you can have many times earth's living space in less than 35 replication cycles.

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Both terraforming and building mega-space stations have advantages and disadvantages and while I agree that building mega-stations is the way to go, there are some valid arguments for terraforming too:

  • Terraforming advantages
    • Becomes very cheap at long time scales. Terraforming a planet in a couple decades or even centuries is expensive. Doing so if you have a couple thousand or ten thousand years? It gets much cheaper. For example, terraformers could just nudge some large asteroids and other space-bodies on collision courses, seed the planet with extremophile life, and then just come back thousands of years later.
    • In-situ resource utilization makes construction simpler than building in space. Lifting mass out of gravity wells is expensive. If you can, for example, set up self-replicating machines on the surface which mine and melt ice, you don't need to deal with moving large amounts of materials to space.
    • Higher density and quantity of rare elements. This is similar to the last point but it's simply a fact that mass collects in planets. Yes, there are your occasional solid-gold asteroids but mining, especially on an un-terraformed world, is much easier than collecting space rocks or lifting material out of a gravity well to build a station.
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