A City on Mars by Kelly and Zach Weinersmith

A City on Mars asks in its subtitle “Can We Settle Space, Should We Settle Space, and Have We Really Thought This Through?” When I read part of the book to decide how to vote on its place in the 2024 Hugo Awards category of Best Related Work I thought that the answers were “probably not, probably not, and definitely not” but allowed for the possibility that after reading the whole thing they might be even simpler: no, no, and no. Now that I have taken time to read the full argument, I think the answers are “maybe but not for a long time, only under certain circumstances and again not for a long time, and oh hell no.” That’s not really the story the Weinersmiths wanted to tell when they started their project, but it’s the conclusion they came to after a long and fair look at many aspects of space settlement.

The text is breezy and conversational, punctuated by well-chosen illustrations by Zach, but the subject and conclusions are plenty serious. The American government, along with two of the world’s richest men, are investing significant resources into promoting and achieving some form of space settlement in the nearish future. Elon Musk’s promises, and especially his timetables, on that front have turned out to be vaporware, but his company continues to push in that direction. His attitude toward law, as revealed in his actions in early 2025, shows the importance of a topic that the Weinersmiths devote considerable attention: the laws and agreements that regulate human activities in space, how those came to be, how they work in both theory and practice, and how they will shape attempts at settling celestial bodies beyond the Earth. Those are topics that tend to get short shrift in much writing about space settlement. That’s unfortunate because attempting to place a new settlement on the Moon or on Mars would be an immense social effort, and those don’t happen outside of legal and political frameworks. The Weinersmiths tackle legal and social questions last because those are the most difficult; although they don’t directly say that God gave physicists the easy problems, it’s true.

But make no mistake, the physical problems of establishing permanent human habitats on the Moon or on Mars, let alone one that could exist without regular shipments of goods and people from Earth, are immense. Even the most basic trip to Mars and back will be unprecedented for the crew. The longest of the Apollo missions was roughly twelve and a half days. Typical plans for Mars missions consider a duration of 400 to 450 days, nearly 30 times as long. The main difference from long-term stays on the International Space Station (ISS) — apart from the impossibility of rescue should something go terribly wrong, Andy Weir notwithstanding — is that the crew will be outside the Earth’s protective magnetosphere the whole time. The Weinersmiths give a details and clear explanation of radiation hazards that can be expected on such a mission, and it boils down to an extremely difficult problem. The sun produces a steady stream of radiation, and astronauts will be exposed to higher doses and different types than what they experience on Earth, types that the human body has not evolved to deal with. (For radiation experience on Earth, the Weinersmiths note, “The thin shell of dead skin around you provides a natural shield, and your internal machinery is reasonably good at repairing or destroying radiation-damaged cells.” (p. 52)) Solar radiation can be blocked by sufficient shielding. The problem with that — in addition to the challenges of launching the extra mass of shielding into space — is that solar radiation is not the only kind out there in space. There is also “galactic cosmic radiation,” mostly protons or helium atoms, but occasionally atomic bits of heavier elements such as iron. When this kind of radiation hits the shielding designed to protect against solar radiation, it can cause “cascades of secondary and biologically dangerous particles, sometimes known as a ‘nuclear shower.'” (p. 57) In fact, they note, “if your radiation shield is super thick and is made from aluminum, you can end up being exposed to even more radiation than you would have if you had no shielding at all.” (p. 57)

And that’s just getting there and back again. Neither Mars nor the Moon has a magnetosphere, so building some form of radiation protection will be important for as long as people want to try to live there. That most likely means living underground. The Weinersmiths also get into the unknowns involved with living in gravity that is substantially less than Earth normal. They detail what is known about life in microgravity thanks to experience with the ISS; they emphasize that for practical purposes nothing is known about long-term effects in the Moon’s one-sixth gravity, and nothing at all about what Martian gravity might be like for permanent human residents. Space medicine in general is limited in its applicability to the relatively normal people who would built a permanent settlement because astronauts from the beginning of space travel to the present day are a highly selective elite. Speaking of normal people, nothing is known about human reproduction in space or anywhere off the Earth, for obvious reasons. For equally obvious reasons, getting pregnant, having babies, and rearing children will be essential to any permanent space settlement. Can fetuses develop normally in low gravity? Can children grow and thrive in the kind of environment likely to be present in a space settlement? Nobody knows.

The list of physical and physiological challenges and unknowns goes on and on. The Weinersmiths present the current state of knowledge and discussion clearly, fairly, and interestingly. Mars and the Moon are dry; A City on Mars is not.

Then there are the economic, social and political challenges. What is valuable enough on the Moon, Mars or elsewhere in accessible space to spend gazillions of simoleons to get there and stay there? Nothing, so far. The Weinersmiths point to the 1970s as the high point of when space settlement might have been seen to be economically sensible. From the first satellite launches through the Saturn V, the cost per kilogram of sending mass into space had fallen exponentially. At the same time, Earth’s population was rising exponentially, and reasonable people could see space as an escape valve. But the famines that were forecast to cost hundreds of millions of lives in that decade did not come to pass. The cost of orbiting matter leveled off. As the education of women around the world improved and access to birth control became more widespread, population growth also leveled off. At present, well over half of the people in the world live in countries whose birth rates are at or below replacement. There just isn’t an economic case for space settlement.

The Weinersmiths take settlement enthusiasts most thoroughly to task when it comes to law and politics. For example, the Weidersmiths cite a Starlink end user agreement that disclaims any Earth-based government’s authority over activities on Mars. Pure bunkum.

This clause is like many statements about outer space settlement: it was promoted by a powerful advocate, widely shared and commented upon, and profoundly misleading. Earth-based governments do have authority over Mars activities—Mars is regulated by long-standing treaties and is an international commons. Admittedly, the treaties are weird and vague, but they do exist and can’t be de-existed via a Terms of Service agreement. (pp. 1–2)

Parts IV and V of A City on Mars discuss space law, how it got that way, and what the future might hold. Their main point is that even without a supra-national authority, international law is a real thing, a combination of treaties and agreements plus actual practice that has become customary. Laws and customs governing the sea are a reasonable analogue to space. The Weinersmiths also describe how Earth’s nation-states declared Antarctica an international commons in the late 1950s and how they have managed that status, peacefully, in the nearly three-quarters of a century since then. Some of the results are weird — territorial claims to slices of Antarctica are neither renounced nor actively pursued — but by and large the framework has held, and international cooperation in Antarctica has itself become a norm acquiring the force of custom. The Weinersmiths also put the kibosh on notions of lunar or Martian declarations of independence, showing how dependent settlements would continue to be on Earth, and how militarily Earth would have the upper hand.

Settling nearer parts of space — eventually — will be a hard technical problem. It will be an even harder human problem. It will take a lot of work from a lot of people over a long period time to have any chance of succeeding at all, and more for it to have a chance of advancing human flourishing. The Weinersmiths have some recommendations on how to get there from here, because their research into the nitty gritty has not dimmed their fascination with space and other worlds. But pipe dreams of doing it within a decade or so may have the perverse effect of setting real progress back by many more decades. “Going to the stars,” they write, “will not make us wise. We have to become wise if we want to go to the stars.” (p. 388)

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I gave A City on Mars my top vote in the 2024 Hugo Awards for the category of Best Related Work, which it went on to win. Reading the full book confirmed my initial impression that it deserved to win.