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Imagine one day the moon suddenly transformed into a colossal elephant's foot and we need to get to the moon to obtain sample... Could it be done using modern day technology to mine the uranium for science? I don't need man on the moon just a good sizable sample in the most economic reusable rocket will do. The moon now is consist of 50% U235, 49% radium and 1% other.

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  • $\begingroup$ You should clarify if you want a sample or a mining operation, and if the moon becomes composed of 50/50 uraninum-radium or if it is an elephants foot (uranium/concrete) $\endgroup$ – Innovine Dec 12 '19 at 6:18
  • $\begingroup$ @Innovine: I don't think mining is possible but I could be wrong but the temperature should take care of anything we throw in. $\endgroup$ – user6760 Dec 12 '19 at 6:20
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    $\begingroup$ Forgive my ignorance, but wouldn't it be critical mass and just explode? $\endgroup$ – mwarren Dec 12 '19 at 8:47
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    $\begingroup$ @mwarren is exactly right! 1 kg of Uranium-235 that fissions releases 17 kiloton of energy. If half the Moon's mass becomes U-235, it would release 624 quadrillion gigatons of energy in a giant explosio. And this does not count the radium. $\endgroup$ – Klaus Æ. Mogensen Dec 12 '19 at 10:39
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    $\begingroup$ That large a mass of natural uranium would still be supercritical, based on the natural (small) percentage of U-235, not to mention continual breeding of Pu-239 from spontaneous fission neutrons. And what would alpha particles from the radium produce? Don't recall, exactly, but it won't be stable. Even 50% natural uranium would explode with enough energy to melt the facing side of the Earth and blow off the atmosphere. $\endgroup$ – Zeiss Ikon Dec 12 '19 at 13:20
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The soviets successfully performed three automated lunar sample return missions in the early 70's. https://en.m.wikipedia.org/wiki/Luna_16 A sample return mission is not too difficult, just very expensive. This is assuming the moon becomes natural uranium.

The Chernobyl elephants foot is radioactive and dangerous because of fission byproducts, not because of the uranium content. Uranium, especially naturally occurring uranium, is relatively harmless. It needs to be pureified and then run in a nuclear fission reactor to break parts of it down into very dangerous, radioactive, fission byproducts.

Radium decays quickly, and is produced as a trace element in the decay chain of uranium, so there is no real explanation for half of the moon to become radium, whether natural or as a result of critical nuclear reactor operation. I am unsure of the bahavior of vast amounts of radium, since it does not occur. The principle danger wïh radium is the gas released, which would probably form ao extremely toxic atmosphere given the quantities you mention. Even if it all outgassed into space, the rate of production would likely leave a faint radon atmosphere. If you insist on having half the moon be made of radium, you probably want to check this with a physicist to see if an atmosphere would exist.

If the moon suddenly transformed into a giant elephants foot (a mix of uranium and concrete, only trace amounds of radium) there would be some difficulty in retrieving samples, but not impossible. Modern electronics can be hardened against those levels of radiation (see the fukushima robotic exploration of the reactor vessels), but a successful sample return mission under these circumstances would be extremely challenging with todays technology.

(note this answer was written before the question was edited to say the moon is 50% u-235. In that case, it just simply and immediately explodes)

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If the Moon were to be instantly transformed as you describe there would be a huge nuclear explosion. A Moon sized chunk of U235 even diluted as described would be fissile and the detonation of the outer layers in a damp squib explosion would compress the inner layers. The shockwave would propagate down into the Moon creating enough explosive force to destroy the Moon in it’s entirely and probably the Earth as well. The Radium would also serve to heat the explosion by virtue of its intense radioactivity.

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  • $\begingroup$ U-235 does not occur in large quantities in nature. The much more common U-238 (accounting for 99% of natural uranium) is not fissile. $\endgroup$ – Innovine Dec 12 '19 at 12:16
  • $\begingroup$ Even if it didn't explode, a critical Moon would be an enormous source of energetic neutrons. That alone would heat it up and probably melt it, complicating any mining operations. I'll let someone else do the sums, but I'd be surprised if any life on Earth survived the neutron irradiation, except maybe in the oceans. $\endgroup$ – Pastychomper thanks Monica Dec 12 '19 at 13:26
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    $\begingroup$ @Innovine Actually, it does. We see almost all U-238 because the U-235 has decayed. Fresh uranium from a supernova or the like will be in the ballpark of equal quantities of each. $\endgroup$ – Loren Pechtel Dec 13 '19 at 2:03
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The solar system will be vaporized, and the entire galaxy likely sterilized.

U-235 is fissile, meaning that it will emit a heck of a lot of energy if 50kg or so is put in one place. If half the mass of the moon is suddenly transformed into U-235, this is around 3.6x10^22 kg, and able to start a supercritical nuclear reaction (explosion). The fissile energy of uranium is approximately 82 TJ per kilo, times the moons mass is 2.4x10^36 J. To put that into perspective, this is an energy output on the order of a billion billion suns, briefly dwarfing the entire galactic energy output.

There will be nothing left of the Earth. There might not even be anything left of the rest of the solar system. This event will probably sterilize the rest of the galaxy.

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    $\begingroup$ Seems right. One nitpick though: No fusion, since all the moon is already fissile material. Fusing fissiles is not possible/endothermic. It could only fuse if the insdes were made from light elements (like a boosted nuke, that has D-T gas in the middle) $\endgroup$ – b.Lorenz Dec 12 '19 at 13:18
  • $\begingroup$ I am not sure about this. The energy towards the core will be unlike anything experienced in our galactic neighbourhood. I think its possible extremely exotic elements will be fused together. Whether this is endothermic or not seems unanswerable. I've removed the paragraph on fusion since it's a realm of physics we haven't explored. $\endgroup$ – Innovine Dec 12 '19 at 13:31
  • $\begingroup$ This seems like wise policy. But supernovas operate at comparable energies, and still though they produce heavy elemenets, do not fuse them... But anyway, the fission bang is big enough in itself. $\endgroup$ – b.Lorenz Dec 12 '19 at 13:49
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    $\begingroup$ Also the galaxy sterilization seems unlikely. Supernovas can output more energy (up to 10^42 Joules, it seems: en.wikipedia.org/wiki/Supernova#Energy_output), happen regularly, and yet we are here. And though the massive amount of fission products will be a threat, they will take a hundred-thousand year to cross the Milky Way (even if accelerated to near c), and would mostly decay by then $\endgroup$ – b.Lorenz Dec 12 '19 at 13:56
  • $\begingroup$ The nature of the explosion also depends on what "suddenly" means. If the transformation to U235 was chemical-reaction fast, then the result would be pretty nasty, but not nearly a total use of the U235 as fuel. The shock wave from the nuclear process would out-pace the chemical process, and disperse the material in front. Large parts of the moon would get blown away before they could convert. Still nothing left of Earth though. Might even still take out the rest of the solar system. $\endgroup$ – puppetsock Dec 12 '19 at 14:46

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