The Weinersmiths researched what the near future of space settlement might be like, inspired by statements from advocates of extraterrestrial colonization like Elon Musk who claimed that Mars colonies would be established in the early 21st century.

It also argues that the political reasons for the original Space Race, which took place side-by-side with decolonization when the US and USSR competed to impress newly formed countries, are no longer relevant.

The authors conclude that space settlement is unlikely to improve humanity's survival unless economic self-sufficiency could be achieved, which would require millions of settlers or incredibly advanced hypothetical robotics.

Return trips would have strictly limited launch windows due to the need for gravity assists, suggesting that even the fastest Mars missions would be lengthier than prior space stays.

The book discusses the environmental hazards of extraterrestrial settlements and states that, even in the case of a doomsday event, Earth would be more habitable than other environments in the Solar System.

It notes that the Moon is also extremely difficult to settle, as its surface is covered in regolith and its environment lacks elements essential to human life like carbon and phosphorus.

Some of the hypothetical scenarios discussed involve survival homicide and cannibalism, such as a situation in which food and air must be preserved by killing off some crew members or settlers.

Existing precedents for survival homicides, such as US v. Holmes and R v Dudley and Stephens, are discussed alongside the likelihood that space settlements would operate under the laws of whichever nation's citizens were sent.

[11] The authors argue that the governance style of space settlements, particularly small ones, would probably shift toward authoritarianism, given the strict control of machinery and mandatory recycling required for survival.

They also suggest that settlements would require a substantial population, allowing for redundancy if people quit or die or machines break, since replacements could not be quickly sent from Earth.

[16] Kim Kovacs of BookBrowse thought that A City on Mars was deeper than the average pop science book, with its humor helping let laypeople follow along with the cutting edge of space technology.

[3] Chris Lee of Ars Technica praised the book for raising the risks of not taking seriously the political structure of a future space settlement, writing "do you really want to create a group of hungry, disgruntled miners that are also able to sling very large rocks at the Earth?