Aliens show up in a generation ship. They don't particularly care about living on a planet specifically, and they don't have any interest in conquering Earth for our land or our water or our biodiversity or anything like that. After all, they've been living in space for generations and they would be perfectly happy to just hang out in the asteroid belt and build more space colonies.

Except... they need uranium and thorium, which aren't particularly common elements, but are concentrated by geological processes.

So: what are the best places to go mining for those things? Are they inevitably going to end up interacting with us on Earth, or can we just be annoyed at them from afar as they settle in around Mars, Venus, or Mercury?

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    $\begingroup$ the asteroid belt by a huge margin, you don't need to mine halfway through a planet to get to it. $\endgroup$
    – John
    Commented Aug 25, 2022 at 19:02
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    $\begingroup$ @John You don't need to mine halfway through a planet to get to it on Earth, either; geology conveniently concentrates it well above background abundance in ores in the crust. If there are asteroids with uranium concentrations at least higher than you can find in Earthling granite, though, that would be good to know. $\endgroup$ Commented Aug 25, 2022 at 19:11
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    $\begingroup$ I know I'm being a pest, but it might be easier to ask NASA this question because from a worldbuilding perspective, does the answer to this question matter? How many people reading your story would (or even could) know that you're right or wrong? And if you write a good story, how many of those people would care? And if NASA or the ESA (etc.) haven't cataloged these sources, who's to know the answer? In your imaginary world (see help center) put the sources where you need them. $\endgroup$
    – JBH
    Commented Aug 26, 2022 at 0:09
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    $\begingroup$ NASA has a remarkable customer service department dedicated to engaging with the public - it's a fundamental part of NASAs mission (and in their best interest). The last time I contacted NASA (I was 13) I asked them to review my design for a rocket engine. They sent me the coolest packet containing (then...) recent research into ion drives and how propulsion technology was being used to further NASA's mission. I still have that packet 40+ years later. $\endgroup$
    – JBH
    Commented Aug 26, 2022 at 2:41
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    $\begingroup$ Done the aliens have fuision power generation? If don't,how did the reach the solar system? or do they have specialized uses for fission energy? $\endgroup$ Commented Aug 26, 2022 at 6:14

5 Answers 5


They probably will come to earth, after some detours

Let's first start off by narrowing our search window. We know heavy elements tend to concentrate in inner orbits, so we can exclude the gas giants(and Kuiper belt) on that alone.

So how do the rocky planets stack up?

From what we can tell¹, Earth far outweighs the rest of the solar system in terms of uranium/thorium concentrations, with an average of uranium 2.8 PPM (parts per million), while uranium ore deposits we currently extract have over 1000 PPM of uranium[[1]]. Thorium is much more common, in the neighborhood of 56 parts per million. However, it's insoluble, so it doesn't concentrate as well.

Venus could have similar concentrations[S], but uhh... good luck getting it. Unless the aliens screw up massively and anger us humans, mining on earth will be much more efficient than on Venus. Y'know, pressure and acid rain.

The moon has a region that may reach 2.1 PPM, but it's only a small portion, and again, not exactly confident in these measurements. Barely worth it. Next.

Mars is estimated to have around 1 and 5 PPM of uranium and thorium, respectively, but we aren't exactly confident in this measurement.

Mercury doesn't seem to have much; the MESSENGER probe only detected around 100 parts per billion(.1 PPM). Thorium was slightly more common at around 200 parts billion, which is peanuts.

Lastly, the asteroid belt. It might be 8 PPB, or lower. We don't really know, and as JBH said, that's a shame. NASA should really be studying the composition of the asteroid belt harder.

So yes, Earth has the highest concentrations of fissile material-but is it the most efficient option?

  • Again, Venus is right out the window. It's simply too hostile to be efficient.
  • Mercury is a joke when it comes to contents
  • The same applies to the asteroid belt
  • Mars has decent concentrations, and lower gravity (1/3 G)
  • The Moon is close to a large deposit(earth), and has much lower gravity (1/8 G)

So we have two major alternatives: Mars and the Moon

Mars would likely be a major operation. It has a decent enough concentration of uranium, and it's much easier to move into orbit than what's here on earth. There's also the added benefit of not having to spend a lot of resources dealing with humans(either destroying them, or trading with them).

Now, if their demand is great enough, Earth is the next target. It's got infrastructure already in place for the extraction and refinery of uranium, so that's a lot less in startup costs, and there's probably more of it here, too.

And the moon? That would be extracted on the way to Earth, as a forward operating base of sorts. Gravity is likely weak enough to make extraction profitable.

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    $\begingroup$ I'll add sources and clean up formatting later, I'm tired right now and it gets the point across "well enough". Also I feel that the way I worded the answer is skirting the line close to "not answering the question" as the question is "Where to get it?" Oh well. $\endgroup$ Commented Mar 31, 2023 at 6:20
  • $\begingroup$ Earth is also very hostile to anybody who is not human (we got used to it), since its atmosphere is highly corrosive and most of the stuff readily burns in it after only mild heating. $\endgroup$
    – alamar
    Commented Apr 5, 2023 at 10:19
  • $\begingroup$ Why not Io? Lots of potential there. $\endgroup$
    – jdunlop
    Commented Apr 5, 2023 at 18:27
  • $\begingroup$ @jdunlop Leaving aside the hideous radiation environment, and all the volcanos, it's so far down Jupiter's gravity well that it takes more dV to leave than Earth does (and a lot more to land, since you can aerobrake in Earth's atmosphere). $\endgroup$
    – Cadence
    Commented Apr 7, 2023 at 4:53
  • $\begingroup$ ....I meant to include something about that in my answer @Cadence, back in the first draft. I originally said asteroid belt because of the huge energy req to get out of Earth's gravity well. $\endgroup$ Commented Apr 8, 2023 at 3:29

It depends on the engine technology of the aliens.

If their engines are relatively weak, such that gravity wells and atmospheres are worth considering, Mars and the asteroid belt are their best bets. Escaping gravity wells can be a huge cost, especially when we're talking about moving something really heavy that you need in relatively large amounts, like fuel.

If their engines are strong and gravity wells and atmospheres are negligible issues, mining is instead the main cost. Asteroids again will likely be decent choices, but geologic processes may concentrate fissile fuels in relatively rich deposits on planets. Or they could just trade with the Earthlings who already dig up mountains of the stuff, and who would pay top dollar even for alien trash.


Asteroid Belt

Asteroids are the ideal place to mine practically any metal because they do not form by the rule of averages like planets do. Planets form by colliding a bunch of different asteroids and comets made of different stuff until you get a big ball of everything all mashed and mixed together, and only through tectonic and whether patterns do you get any sort of concentrations of ores... but asteroids are WAY better than this because most asteroids will be formed primarily from the remains of 1 stellar event instead of many.

Because different stellar events, produce different elements, it means that most asteroids will not contain any uranium/thorium to speak of. The thing is that most of the non-hydrogen/helium mass in the solar system comes from dying stars and white dwarf supernovas; so, here on earth we have to separate out massive amounts of oxygen, silicon, carbon, iron, etc. But, an asteroid formed from a very rare neutron star collision, will have very little amounts of these elements, and hundreds if not thousands of times as much uranium/thorium as you will find anywhere on Earth.

So, instead of having to melt thousands of tons of ore over and over again to try to get a few kg of pure uranium on Earth, the aliens could scan thousands of asteroids for radioactive signatures, and mine only those formed from neutron star remnants.

Source: https://svs.gsfc.nasa.gov/13873


I am not sure what form of difinitive answer you are after, as their is currently little information on the mineral composition of the planets other than earth. The best that can be offered is conjecture.

If it helps, there is evidence that Mars at one time had experienced spontaneous natural nuclear explosins.

Previously, it had been hypothesized that Mars had been the location of large natural nuclear reactors[1,2], as are known to have occurred on Earth [3] This hypothesis was prompted by evidence of a large nuclear energy release in Mars past, and was considered the simplest hypothesis to explain the available data. Evidence of large scale nuclear activity on Mars comes from a variety of sources. It has been a long standing paradox that uranium, thorium and potassium, appear hyper-abundant on Mars surface when compared to Mars meteorites, which are believed to sample subsurface rocks. [4] This suggested a thin debris layer on the surface of Mars, enriched in Th, K and U , and dispersed by some impact or explosion. Thorium and radioactive potassium appear concentrated in the northern Mare Acidalium in the region of approximately 50 W 55N with a smaller concentration in Utopia Planum centered at approximately 90E and 55N with an additional small concentration at the approximate antipode of the Acidalium hot spot ( see Figure 1and 2). This pattern suggested a massive explosion, such as the explosive disassembly of a large natural nuclear reactor, producing a global debris pattern , with a shock wave wrapping around the planet and colliding with itself at the approximate antipode.

Just how much uranium do your aliens require? The evidence seems to indicate that there should be ample supplies on Mars.

  • $\begingroup$ I think the researcher who wrote that article may be questionable. This snippet sounds like its taking a turn into Ancient Aliens territory near the end, and this story seems to have been picked up by Fox News and no one else. I mean this is Worldbuilding, so build what you want, but just for the record... $\endgroup$
    – Atog
    Commented Apr 7, 2023 at 3:54
  • $\begingroup$ Anyway, this is good, let's build on J. E. Brandenburg's world, where some past alien power tested an atmospheric thermonuclear weapon on Mars millennia ago... Maybe one of the best tricks for starfaring race is to look for parts of a solar system where a prior starfaring race left some useful junk behind. Maybe there are rumors of a lost stash of U235 like we have pirate treasure on Earth. $\endgroup$
    – Atog
    Commented Apr 7, 2023 at 4:00

If they are technologically advanced enough probably star-lifting. Our star has the majority of matter in solar system. It is just a matter of filtering that matter to get the elements you need. And it is quite fortunately that you get practically limitless amount of energy in situ to fuel that process. That also put their colonies closer to the star which only make sense. Why would you settle in the asteroid field/Mars instead near the sun, where you get all the energy you would need?

Well, that is assuming aliens arrived either in large numbers, or they plan to raise their numbers. If they are small in number (1 billion of aliens IS a small number) they could just trade with Earth.

But the real question would be why do they need fission fuel? Using solar power is way more efficient, even for ships. You can use star-powered lasers to accelerate colony ships leaving the solar system - so there is little need for fission/fussion. Only reason for usage of fission I can think of is that they didn't control their star (so, they are refugees). But that explain the usage before comming here, where they could easily take control of the sun.


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