Daniel Deudney’s sweeping text provides a net assessment of space activity, asking whether space expansion, which up until now has largely been advocated without critical review, is a positive development for humanity and the Earth or whether it might instead have negative implications. He weighs into the current debate on which approaches to space activity are most likely to realize international security objectives and then goes further to examine the desirability of future large-scale space expansion. Ultimately, he finds that as a whole, space activity to date has made the Earth less secure and that future large-scale space activities, particularly efforts to expand human presence elsewhere in the solar system, would have disastrous consequences for humans and Earth.

These arguments are intriguing, and Deudney’s identification of the need for systematic examination of the potential negative outcomes associated with space activity is laudable. Although his conclusions ultimately go too far in painting the dangers associated with space expansion, the thoughtful and systematic effort he puts into this analysis is a welcome contribution that should spark an important debate.

To support his effort to evaluate space activity as a whole, Deudney places the disparate strands of past and present space scholarship and advocacy into three groupings: military space expansionism, planetary security space expansionism, and habitat space expansionism. The first two focus primarily on activities in near-Earth space. Military space expansionism promotes the development and use of space technologies for national security. Planetary security space expansionism promotes arms control and cooperation and calls for increased attention to satellites that contribute to our ability to address planetary challenges such as climate change. Habitat space expansionism centers on a desire to develop large-scale space infrastructure in near-Earth space, ostensibly to solve problems related to energy, resources, and habitat on Earth. It also promotes human expansion and settlement throughout the solar system to achieve goals of human survival and growth.

To evaluate the relative merit of these three programs, Deudney draws on full geopolitical theory, which examines how geography and technology determine the viability and occurrence of various political arrangements. A key concept in this theory is violence interdependence: the ability of people to inflict violence on one another. When violence interdependence is at very high levels (i.e., populations can cause each other mortal harm), government is necessary. Over time, technological developments in communication, transportation, and destruction have led to increases in violence interdependence and corresponding increases in the size of governed spaces, from villages to nation-states.

Having established his framework, Deudney turns to an evaluation of the three space expansion programs. He first argues that ballistic missiles are properly classified as space technologies. Ballistic missiles play a critical role in delivering nuclear weapons, which he argues has resulted in intense violence interdependence on the planetary level. For this reason, Deudney claims that the net effect of space technology thus far has been to decrease international security. He argues that military space expansionist proposals to develop anti-satellite weapon technology would be similarly harmful, because of this technology’s ability to target military communications and early-warning satellites.

Deudney much favors planetary security space expansionism, with its emphasis on arms control, which he sees as a method to adequately contain the dangers created by ballistic missiles and nuclear weapons. Approaching this issue from the perspective of geopolitical theory is interesting and novel. However, Deudney presents the debate as a dichotomous choice between unbridled weapons development and mutually agreed-on arms control. The discussion would have been more illuminating if he had expanded on how his ideas interact with the complex literature on technical and strategic approaches to deterrence, de-escalation, and other methods to defeat threats to space assets. He also largely sidesteps the issue of how to achieve space arms control, a challenge that is central to the success of his proposal.

Perhaps of most interest because it is most novel is Deudney’s critical review of habitat space expansionist proposals. According to this program, space settlements would begin with small, Earth-dependent populations and eventually become large and self-sustaining. Deudney draws on history to show that large colonies tend to become politically independent—often through violence—particularly when operating at large distances and when populations experience significant differences in circumstances, criteria that those living in space colonies would certainly meet.

The vast distances between likely settlement locations in the solar system would tend to decrease violence interdependence. However, Deudney argues that space populations will necessarily develop the technology to move asteroids in order to enable resource mining and that this technology could easily be weaponized, giving any space colony a potentially catastrophic weapon. The feasibility of this technology and its eventual ubiquity are more controversial than Deudney suggests, but if it were to be developed, it would greatly increase violence interdependence. According to Deudney’s evaluation, the overarching result would be a dangerous, anarchic solar system prone to high levels of violence.

Although Deudney paints a vivid picture of one potential future, it seems reasonable that space expansion will be significantly less dramatic. European explorers who colonized the Americas did so in part because of the numerous resources available there. The land, particularly in temperate climates, was generally hospitable to development. As noted earlier, there is no strong economic case for settling space: it is much easier and less expensive to recycle or reduce our consumption of resources on Earth than to attempt to gather new resources from asteroids, and it is much easier to build homes where there is arable land and a breathable atmosphere.

Space settlements are apt to look more like Antarctic bases or small desert villages than like the conquest of the Americas. Even if these settlements grow, they are likely to be at a disadvantage to those on Earth because of the harsh conditions in which they exist. Deudney, for example, argues that an enormous human habitat in Earth orbit could become an eighth continent that holds the planetary high ground with the ability to bomb any location on Earth, giving it the potential to “hold the rest of terrestrial humanity in a position of permanent subservience” (p. 326). However, he insufficiently acknowledges the reciprocal ability for such a structure to be targeted from anywhere on Earth, as well as the significant and inherent vulnerability of a space habitat operating in the vacuum of space and requiring complex closed-loop life support systems.

In this book, Deudney rightly points out that many advocates of space expansion take an uncritical approach to space activity and that the rationalizations provided—economic growth, addressing Earth’s environmental or resource challenges, even national security—often do not fully justify the effort. As former NASA administrator Mike Griffin stated in 2007, these are acceptable reasons appropriate for policy or academic debates, but they are not the real reasons for undertaking human space activity. In many cases, space advocates are driven by an innate desire to pursue these goals. Introducing a dose of rigorous criticism to challenge these ambitions and evaluate their long-term effects is reasonable.

Ultimately, Deudney goes too far in his dark view of space expansion, at one point stating that “major movement into the heavens promises a descent into hell” (p. 355). However, his efforts to critically review space expansionist plans, particularly their political implications, is important and useful. Early in his book, Deudney argues that space expansionists view humanity as traveling along a linear path across time in which technological advances and spatial expansion improve the human condition. Full geopolitics, by contrast, argues that the results of technological advances and spatial expansion, whether desirable or disastrous, depend on whether they are matched by appropriate configurations of restraining institutions. Rather than ending all human space activity or continuing to blindly push forward into space, Deudney’s work has opened the door for creative debate and development of a middle path that expressly considers the political element of space expansion to design a smarter path forward.