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It seems that energy production in space is a limiting factor. We can use photovoltaics, but they're of minimal value past Mars, and many interesting places (like the moon) have very long nights. The only other obvious power source for spacecraft and installations seems to be nuclear, either fission or RTG. But we are generally hesitant to put large quantities of radioactive material on a rocket full of flammable materials, since they do occasionally explode.

It seems that capture of a uranium-rich asteroid would solve this problem. Obviously we would then have a hundred new technological problems to solve, but in principle this would seem to be a good and necessary first step to a lot of interesting targets.

Has this been considered in depth? If not, is that because there's some obvious flaw I'm missing? Would this not actually solve any problems? Or is it just a matter of trivial practicalities like "What uranium-rich asteroid?" and "nobody's ever built a space-based uranium refinery."

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Asteroids fall into one of four categories:

  • C-type, consisting mostly of carbon
  • S-type, consisting mostly of silicate minerals but they can also contain metals
  • M-type, these are metallic, mostly composed of iron & nickel
  • D-type, these are dark asteroids composed of water ice, carbon monoxide and rock minerals

The proportion of each type of asteroid is:

Asteroid Type   % of Asteroids
 C-type               75
 S-type               17
 M-type               <7
 D-type               <1

If you are targeting a uranium rich asteroid you would be targeting the S and M types, which is about one in four.

Looking for one that contains uranium makes the task more difficult. Getting one that is "rich" in uranium is even more difficult. Also, do uranium rich asteroids exist? The costs in mining asteroids, let alone very specific asteroids will be extremely expensive.

The issues associated with asteroid mining, in general, have been considered and it comes down to economics. Is it cheaper to find a uranium rich asteroid, take it, process it and get what you want from it so that it can be used as an energy source, than it is to use an alternate energy source?

Technically and scientifically, most things are possible, but economics will dictate what will happen when it comes to using celestial resources.

The answer provided to Mining Helium-3 on the moon and sending it to Earth? addresses other issues with mining celestial resources.

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    $\begingroup$ You may actually want to focus on S-type asteroids. Uranium is lithophilic, so would tend to be in rocky matter. $\endgroup$ – Oscar Lanzi Apr 16 at 23:54
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The best source I could find places the quantities of uranium at 10-25 parts per billion, and thorium at 30-120 parts per billion, while the lowest concentration that's been mined on Earth was 50,000 parts per billion. Photovoltaics should be acceptable at least to 3.2 AU where the outer main belt of asteroids lies, giving roughly 133 W/m^2. That can be enhanced through the use of lightweight metal mirrors. That doesn't mean all is lost, though; water-rich asteroids should still be a good source of deuterium, so if we can ever develop D/D fusion reactors, it may then be possible to build high-performance spacecraft capable of pushing large payloads in reasonable timeframes.

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Having a lot of uranium in space clearly opens new possibilities, but before these options become relevant there are some practical issues: you have to find and capture the asteroid, then mine and process the Uranium before it can be utilized. All these steps are very difficult for this use-case.

Uranium quite difficult to process and is very energy dense so it's comparatively cheap to ship to space. So I'd argue it would be a lesser interesting asteroid, compared to one with say a lot of water.

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