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So the consequences on Earth are somewhat easier to see, as it is mostly just about having cheaper energy. The most obvious downside back on Earth is that climate change doesn't have an easy solution, but we can probably fix it with a mix of renewables and energy storage instead(and it might even be cheaper regardless of whether fusion is possible). The problem with climate change is more economics than it is physics. One possibility is that this could also serve as a means for additional space development using space based solar. While the economics don't currently work well, this could change in the future if energy demands increase without better alternatives.

Anyway, the main interesting problem is about what this does to the possibilities for space colonization. Even going to Mars long term becomes much harder when the best energy sources are nuclear fission or solar. Anything past Jupiter is likely off the table because the energy would get too expensive. The JUNO probe is the first to use solar that far out, and it was much less efficient as a result.

What options might be possible as other alternatives to fusion power for space colonies? Would it still be plausible to have extrasolar colonization at all without fusion power?

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    $\begingroup$ If fusion does not work, your sun will be a bit dark. Are you asking what would happen if we fail to economically tame fusion for commercial use? $\endgroup$
    – PcMan
    Jan 2, 2021 at 7:08
  • $\begingroup$ Look at the space exploration as we know it. Artificial fusion power does not work. How does the space exploration look like? $\endgroup$
    – Suma
    Jan 2, 2021 at 18:17
  • $\begingroup$ Please explain what you mean by " The JUNO probe is the first to use solar that far out, and it was much less efficient as a result.". In what way was the Juno probe "less efficient"? The only way one can say that is that the solar panels weighed more. 340kg producing 480w(decreasing 1% per year), vs for example Galileo's 2 RTG's weighing 210kg producing 490w(decreasing 1.2% per year). But weighed against the whole probe's mass of 1600kg, that's not unreasonable at all, and much cheaper to build. $\endgroup$
    – PcMan
    Jan 2, 2021 at 19:18

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For inside the solar system, fusion is unnecessary. For outside the solar system, fusion might be insufficient (if you want to get there in a lifetime).

Inside the solar system you can easily get where you need to go using fission power, or instead of powering your craft directly with solar, you fuel up with hydrogen extracted using solar through electrolysis. Past the asteroid belt there's plenty of ice. Using electrolysis is going to be a good idea because you're going to need oxygen anyway. There's also very little constraint on the size of your engine or fuel tank when going from space station to space station, because you aren't fighting against gravity.

Basically a good way to do it would be to set off a few nukes behind your ship, then when you get half way, flip your ship around and detonate some more in front of your ship to slow down.

https://en.m.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)

As for the actual power supply for day to day use on a colony, plenty of moons and dwarf planets have things you can burn and geological activity you can harness. You've got literal geysers of hydrocarbons on some bodies.

https://en.m.wikipedia.org/wiki/Cryovolcano

https://space.stackexchange.com/questions/2328/is-solar-system-active-cryovolcanism-a-potential-viable-power-source-for-future

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    $\begingroup$ "plenty of moons and dwarf planets have things you can burn": but no oxygen to burn them with unless you electrolyze some water, which itself gives you all the hydrogen the oxygen you get can burn. Those hydrocarbons are going to be raw material, not an energy source. $\endgroup$
    – Christopher James Huff
    Jan 2, 2021 at 14:25
  • $\begingroup$ At least SpaceX is looking at using the Sabatier process to produce methane on Mars. Elon Musk has even mentioned producing methane on Mars for trips back to Earth, although that raises questions.One benefit is that you can use that process even in the absence of ice. It's currently used on the ISS for life support. Conceivably, if you're set up to use methane anyway, it might be worth it to use some in-situ. Fuel energy density when considering oxidizers may also be a factor. $\endgroup$
    – Morgan
    Jan 2, 2021 at 15:52
  • $\begingroup$ space.stackexchange.com/questions/27245/… $\endgroup$
    – Morgan
    Jan 2, 2021 at 15:54
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Solar power falls off with distance from the sun, but there's no such difficulty with fission power...in fact, it becomes somewhat more efficient since solar heating of radiators is reduced. There's no problem with using nuclear out past Mars. And even if/when we do have usable fusion power, it's likely that fission will perform better at relatively small scales. There are kilowatt-scale fission reactors currently in development that weigh only a couple hundred kilograms, while the DEMO fusion power station concept is in the 2-4 gigawatt range and would be a massive facility. Fusion power reactors are likely to have a quite large minimum size, and may well be much heavier for the power they produce than fission reactors.

The limiting factor with fission is availability of fissile materials. Mars is likely to have concentrated ores, moons with saltwater subsurface oceans may have usable quantities dissolved in the water, but extracting the needed fuels from asteroids is going to be energy intensive and require processing a lot of material. The solar system as a whole is not particularly short on fissile materials (or fertile materials that can be bred into usable fission fuels).

Solar also isn't useless in the outer system. Spacecraft are limited by the need to carry their power supply, but colonies could make practical use of solar power at much greater distances by using concentrating reflectors. For long term energy independence, a colony may find it worth investing the material in very large solar arrays even if they produce a fraction of what they would near Earth.

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