> Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics.

Respectable background.

> the only clear evidence for how space affects humans is weighted quite strongly against going. That balance could be changed by doing the work to discover the answers to some of the questions posed in the book... So, maybe do the work beforehand?

This is also something which sounds very reasonable.

> I was shocked to learn that no one really knows how to construct a long-term habitable settlement for either the Moon or Mars.

There were serious, funded, government-sponsored plans for Moon bases over the last half a century. Many kinds of experiments run to answer some specific questions. Definitely not all questions got an answer - but I still think to say "no one knows" is overstatement.

First, we're all living in space. Earth is in space. We're all separated from space by some 100 km - a somewhat arbitrary boundary - of literally thin air. So talking about "living in space" we usually mean "not on Earth". Some of us have lived for months on space stations - and came back in rather livable condition. So we can't say that space kills immediately no matter what we do. Usually we talk about such longer-term dangers as different gravity and too much radiation. Of course air, water, food are also needed immediately, but we've had such problems when we developed e.g. underwater transportation, so we have some solutions.

It's true we don't have good experience with different gravity levels. Space stations are the environment of weightlessness, which is somewhat well studied and found rather unhealthy. But nobody lived for any prolonged time in Moon gravity, even less so Mars gravity, or artificial gravity of any level, which could be achieved in livable modules on centrifuges. It's a sad situation that many scientific demands require expensive space stations to keep microgravity environment, so we can't actually study - scientifically, with experiments - the medical effects of artificial gravity. Maybe we'll have rotation space stations soon, when costs for that hopefully decrease.

From what we thought and planned for artificial gravity, it looks feasible to achieve Earth-like conditions in an orbital station - possibly large, but still achievable - or on the surface of the Moon or Mars. We don't have enough data, but theory and plans are such that to say that "no one really knows how to construct" such a thing needs some clarification. Perhaps https://ssi.org/ may help.

Regarding radiation, we do think we can recreate the effect of the Earth atmosphere in a more permanent settlement. Orbital station has to probably be big and expensive - well, everything in space is expensive - but maybe on the Moon surface the radiation shielding could be more easily achievable. Yes, that doesn't look too similar to how we used to live on Earth - but it could be an achievable solution for time periods longer than those months on the space stations with, as expected, better results regarding radiation exposure. So, again, doesn't look like a scientific problem with an uncertainty if it could be achieved at all.

What we do lack is engineering, which requires more money than we had so far. Things slowly change in that area though, so we might take another attempt in engineering solutions in this and subsequent decades.

Can we confidently say that reality kills space settlement dreams? That's still a good question, but we don't seem to have a good answer now.