Al Globus and I have just released a pre-print of a new paper on radiation shielding for space colonies. It contains some important new insights about where the first space colonies are likely to be built, and what sort of shielding requirements they will face. Here’s the full abstract:
We examine the radiation shielding requirements for protecting the inhabitants of space settlements located in orbit. In particular, we recommend a threshold of 20 mSv/year based on the most relevant existing standards. Space settlement studies in the 1970s assumed that lunar regolith with a mass equivalent to Earth’s atmosphere above high altitude cities, roughly 5 tons per square meter, would be sufficient to meet a 5 mSv/year threshold at the Earth-Moon L5 point, their recommended settlement location. Using OLTARIS, NASA’s online radiation computational tool, we found this to be far too little for their 5 mSv/year threshold. Even at our 20 mSv/year threshold about 10 tons/m2 of lunar regolith is required. Fortunately, radiation shielding mass requirements can be radically reduced by using better materials and/or by placing settlements in low Earth orbit (LEO) rather than above the Van Allen Belts. Specifically, 67 tons of water or polyethylene radiation shielding per square meter of hull is sufficient in free space and settlements in a circular 500-600 km equatorial Earth orbit may require no shielding at all to meet the 20 mSv/year threshold. This has strong implications for the best paths towards space settlement as the first settlements may not need extraterrestrial mining and processing. For settlements in LEO, transportation to and from Earth is (relatively) easy, implying a smaller step between large space hotels or low-g retirement homes and the first settlements. It is important to note that there are significant uncertainties in our understanding of the effects of low-level continuous high-energy particle radiation on human tissue that, when resolved, may invalidate these findings.
It all boils down to a couple of key findings:
- Earlier space settlement design studies, focused on colonies at L4 or L5, grossly underestimated the amount of shielding needed; but:
- Habitats in a low Earth orbit over the equator need little to no extra radiation shielding, as they already get all they need from the magnetosphere and the Earth itself.
Since radiation shielding, outside of LEO, is the bulk of the mass of the entire space colony, this makes a really huge difference in the feasibility of the whole concept. It means that the first space colonies almost certainly won’t be built in L4 or L5; they’ll be built in LEO, right over the equator. The materials, then, will probably be launched from Earth as well; a few hundred Falcon Heavy launches over the course of a couple of decades would suffice for a small colony (though at that flight rate, other launch options would no doubt spring up as well). We can also predict that these launches will be made from some launch site on the equator, since that increases the payload (or decreases the cost) of each launch, and will be quite worth the trouble when that many launches are needed.
If you’ve been playing High Frontier, you probably knew most of this already. But it’s pretty big news in the space community, which has been laboring under misconceptions about radiation shielding since the 1970s.
Read the full paper here, and share your thoughts about it below!