I am a staunch believer that any near-future permanent colony on the Moon or Mars should be built underground, at least the living spaces, due to the harmful effects of radiation.

What is the gradient (or is there a rule of thumb) for depth beneath the surface that we should build the habitats for living, if the powers-that-be agree with me?

In essence, what depth will shield our colonists and to how much? This is about living space and depth, not about plants, operations, or geographical location of where to colonize first.

  • $\begingroup$ Aren't these basic facts you should know before becoming a 'staunch believer'? $\endgroup$
    – NPSF3000
    Oct 18, 2015 at 3:04
  • $\begingroup$ As a staunch believer, do you have a source on the magnitude and spectrum of solar radiation I can look at to do the calculations necessary for the answer to this question? $\endgroup$
    – Plutoro
    Oct 18, 2015 at 3:20
  • 2
    $\begingroup$ This board is for the building of fictional worlds. While a question like yours might have been asked in a fictional context ("how deep will my characters have to dig?") you appear to be asking in the real world, which is off-topic. $\endgroup$
    – o.m.
    Oct 18, 2015 at 8:56
  • $\begingroup$ I disagree about this being off-topic, because any evidence that this is not being asked in a fictional way is hard to find. I don't see how it's being asked in a real-world context. The phrase "any near-future permanent colony" can apply to fictional colonies, too. $\endgroup$
    – HDE 226868
    Oct 25, 2015 at 14:02
  • $\begingroup$ @NPSF3000 - yes, after my statement I make it clear that I believe strongly that it is due to the harmful effects of radiation. So, for my speculative science question, I request if there are any facts that can assist with understanding how deep we should dig. $\endgroup$
    – Mikey
    Nov 2, 2015 at 7:11

1 Answer 1


According to this article, the most irradiated spots on mars receive about .3 sieverts of radiation per year. The normal background radiation for humans on earth is about .003 sieverts. According to this paper, the thickness of concrete required to reduce the intensity of gamma radiation by a factor of 10 is 372 mm (and this is assuming the highest energy gamma radiation). The surface of Mars is basaltic, but I'm assuming its absorption properties are similar to that of concrete. Thus, to reduce the amount of gamma radiation penetrating the habitat by a factor of 100 (from .3 to .003), you would need 744 mm, or about 29 inches of rock above you.

  • $\begingroup$ So basically, if you dig 2500mm height space into the ground and then pile the removed ground with some concrete type binder on top of the construction, you'll have roughly twice the needed protection? $\endgroup$ Oct 18, 2015 at 4:28
  • $\begingroup$ Even better. That would be 100 times the needed protection. That's how exponents work. $\endgroup$
    – Plutoro
    Oct 18, 2015 at 4:30
  • $\begingroup$ True, forgot that. I was focused on thickness and forgot the actual effect. That should be a simple and cheap way to build. $\endgroup$ Oct 18, 2015 at 4:35
  • $\begingroup$ Sunlight could be piped underground using mirrors. The shafts would have a couple of 90 degree bends in them with more mirrors to reflect the light around the corners. Since the Martian atmosphere is mostly CO2, the mirrors could be front surfaced, possibly aluminum on Mylar. Most of the harmful radiation would just pass through without reflecting and be absorbed by the rock. Enough mirrors could bring in more light than we get on a clear day on Earth. It might even make it necessary to air condition the colony. $\endgroup$ Oct 18, 2015 at 22:22
  • $\begingroup$ Another positive of using reflectors is that you can have redundancy in case a rock penetrates the outer 'window,' you'll still have another window (like a periscope) that a rock won't penetrate since it's z-shaped from the surface. $\endgroup$
    – Mikey
    Oct 19, 2015 at 2:04

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