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Dark Skies
The recent book Dark Skies tries to make the case that space settlement could be disastrous to humanity.

Not so dark skies

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In the book Dark Skies, Daniel Deudney examines space settlement[1] in detail and comes to the conclusion that it is so likely to exterminate humanity or have other serious consequences that it should not be undertaken at all, or at least not for several centuries, giving time to improve homo sapiens’ habits. Deudney comes to his surprising conclusion by applying geopolitics, a part of political science that studies “the practice of states controlling and competing for territory,”[2] among other things, to space settlement, which Deudney describes as “habitat expansionism.” Deudney uses a version of geopolitical theory to generate 12 propositions and then applies them to predict the future, coming to the conclusion that space settlement is an existential threat to humanity and should be viewed in the same category as nuclear war. Dark Skies is a difficult read but it is also a detailed and extensive critique of space settlement that deserves a thoughtful response.

General critique

This article examines Deudney’s six major threats to Earth and humanity and compares each with and without space settlements, something oddly left out of the book. This paper finds major problems with most of the six, especially the suitability of asteroids as weapons, which is poor. This article also examines five likely, relatively near-term, threats to Earth and finds that without space settlements any of these threats could exterminate humanity. With space settlements most of the threats would be reduced from existential to catastrophic. In all cases space settlements could help Earth recover.

Deudney uses a version of geopolitical theory to generate 12 propositions and then applies them to predict the future, coming to the conclusion that space settlement is an existential threat to humanity and should be viewed in the same category as nuclear war.

It should be noted that Dark Skies is for the most part a well thought out, carefully reasoned, knowledgeable critique of space settlement. In particular, it correctly points out that space settlement advocates have not spent much time and energy examining the potential downsides of space settlement, a fault that should be remedied. However, the core conclusion, that space settlement is a serious threat to humanity that must be strangled in the crib, is simply wrong. At the simplest level, the list of survivable threats to a space settling civilization is much longer than that for a society that stays exclusively on Earth, particularly an Earth with thousands of nuclear warheads.

There are some serious general problems that play a role in generating Deudney’s conclusions. These include:

  1. For the six threats identified, Dark Skies does not compare a future with space settlement versus a future without space settlement. When this comparison is done it becomes clear that the “no space settlement case” is far more dangerous to humanity’s long term survival than is the space settlement case.
  2. Deliberately-engineered asteroid attacks play an important role in four of the threats, but as weapons asteroids are far inferior to nuclear bombs, making weaponized asteroids somewhat superfluous.[3]
  3. The difficulty of weaponizing asteroids is substantially understated.
  4. Nothing is quantified, ever. So much so that this author thought geopolitics might not quantify, but an examination of Introduction to Geopolitics by Colin Flint does reveal tables of numbers for various purposes.
  5. Geopolitical analysis has always been on a more or less constant sphere (Earth). However, the spatial relationships among free space settlements and between those and planetary settlements are not on a sphere, or even close. For example, the physical relationship between free space settlements constantly varies due to orbital mechanics. Note that France and Germany always share a common border, but the distance between Mars and Earth varies from 54.6 to 401 million kilometers. There is little or no data or experience regarding the geopolitical effects of these unique spatial relationships on a space settlement society and Dark Skies makes no attempt to show how geopolitical theory must be modified (or not) to take this fundamental change into account.

Early in Dark Skies, Deudney states that militarization has dominated space development to date and therefore space development is responsible for the existential threat of nuclear war. He sees nuclear-tipped ICBMs as space weapons because they pass through space on the way to their targets. However, in Space Weapons Earth Wars and elsewhere, only weapons based exclusively in space are considered “space” weapons.

More to the point, the US space program has been conducted without ICBMs after the first launchers, and civilian space development has made little contribution to ICBM programs. Painting space settlement with the ICBM brush is a little like attributing tank warfare to the automotive industry. In any case, abandoning space settlement is unlikely to reduce the ICBM threat to Earth and certainly means that nuclear war on Earth is an existential threat to humanity as opposed to “just” a severe blow to our home planet.

All of these threats have horrendous consequences but a vigorous and capable space settlement based society in Earth’s vicinity can reduce some threats.

Finally, space development is dominated not by national space programs, either civilian or military, but rather by commerce. Communication satellites, especially video direct to consumers on the ground, and Earth observation systems generate far more revenue than is spent by national governments on space activities. For example, according to, the total space economy was $383.5 billion in 2017. NASA’s 2017 budget was around $19.6 billion.

Threats to Earth regardless of space settlements

Table 1 is a partial list of potential existential threats to humanity when there are no space settlements. In all cases an advanced space settlement society could provide refuge and, in most cases, could repopulate Earth with at least those species in use or archived in space settlements.

Items on this list were chosen because they

  1. Could manifest soon, in many cases within a few years or decades.
  2. Threaten an Earth-bound society with extermination.
  3. Are relatively uncontroversial, as most knowledgeable observers would agree the threat is real.

Table 1: Existential Threats. The second column is the time the threat is likely to strike.

Threat Time to initiation
Major asteroid hit Anytime, likely thousands or even millions of years for large objects. Decades or centuries for smaller, less dangerous ones.
Supervolcano eruptions Anytime
Nuclear war Anytime, likely years or decades
Pandemic, usually not existential Anytime, likely years or decades
Technology run amok, e.g., the Grey Goo[4] and Paper Clip Apocalypse problems[5] At least decades, probably longer

All of these threats have horrendous consequences but a vigorous and capable space settlement based society in Earth’s vicinity can reduce some threats (discussed below) and convert existential threats to catastrophic ones. This is hardly desirable but far better than allowing humanity to be wiped out.

Deudney’s threats created by space settlement

This article will now focus on the threats Deudney derives from the application of geopolitics to space settlement.

Table 10.3 of Dark Skies lists six serious, catastrophic, or existential threats to humanity and Earth created by space settlement. The terms in parentheses are the terms used in Dark Skies. They are:

  1. War (Geopolitical Malefic): strong tendencies towards interworld and interspecies wars. Catastrophic.
  2. Deliberate Asteroid Attack (Natural Threat Amplification): weaponizing asteroids. Catastrophic.
  3. Weakening of Treaty Obligations (Restraint Reversal): abandoning the Outer Space Treaty, constraints on nuclear weapons, and treaty-like constraints on other dangerous technologies. Serious.
  4. Totalitarian World Government (Hierarchy Enablement): pressure on Earth towards totalitarianism due to security threats from cislunar space. Serious.
  5. Ubermensch (Alien Generation): human speciation as humanity moves out further and further from Earth. Existential.
  6. Unknown and Unknowable Threats (Monster Multiplication): unknowingly triggering unknown disasters. Existential.

War (Geopolitical Malefic)

Argument: Space settlement creates an endless frontier extending for millions of light-years into the cosmos. Frontiers tend to be violent places, creating wars not only at the frontier but between the polities that support the expansion. The vast size of the cosmos means that settlers are widely separated for much of the time, perhaps even evolving new species. When they come close enough to interact there may be little fellow feeling and little reluctance for the stronger to exterminate the weaker.

Counter-argument: With space settlement development there are a number of factors inhibiting violence and warfare. For one, the vast energy and materials resources available will tend to make resource wars obsolete. The fragility of space settlements, particularly free-space settlements in orbit, mandates that settlers avoid pointless provocations and chest-beating exercises. The enormous size of the space inhabited, up to and including the entire galaxy, makes it extremely unlikely that war will consume more than a small fraction of the population and resources available. It is difficult, if not impossible, to predict whether space settlement will lead to an increase or decrease in the odds that any given individual or group is involved in warfare or not. Preventing space settlement may be more or less dangerous than allowing it to proceed; it’s impossible to say.

Comparison with no space settlement: It is reassuring that since World War II warfare has decreased substantially and rarely involves the great powers directly killing each other’s citizens. That is left to proxies. However, not all wars are intentional. Consider World War I and the Cuban Missile Crisis. These suggest that there is a possibility—some would say probability—of an accidental humanity-ending nuclear war.

Space settlement or not, we would be fools indeed not to find, track, and deflect asteroids headed towards Earth.

Space settlement could reduce this probability a bit by exposing large numbers of people to the Overview Effect created by the view of Earth from space, where some astronauts have come to value Earth and the unity of Earth’s people much more than before. More substantively, a sufficiently developed space settlement society surviving a war can repopulate Earth and restock other species if prevention fails. Thus the chance of a humanity-ending nuclear war is much lower with a sufficiently advanced space settlement society.

Deliberate asteroid attack (Natural Threat Amplification)

Argument: a sufficiently large asteroid impacting Earth can exterminate homo sapiens; indeed many species have been destroyed this way. It is possible to build a surveillance system to track dangerous asteroids and develop means to deflect them. However, this very system could be used to deliberately target Earth and this may be a more extensive threat than untampered asteroids.

Counter-arguments and counter-counter arguments:

  1. Without intervention, multiple major Earth strikes by large asteroids are certain, although the timing is unknown. Space settlement or no, we must find and monitor existing asteroids and develop deflection technology sufficient to make asteroids miss Earth.
  2. Targeting Earth is much more difficult than making an asteroid miss Earth. Hitting Earth requires a much greater ability to manipulate and predict trajectories accurately. Missing Earth given an on-target asteroid only requires the asteroid to be delayed or advanced by a few minutes. Hitting Earth requires accurate prediction of thrust added to the asteroid, which is dependent on composition, and understanding the non-Newtonian effects (e.g., light pressure) for the time between thrust and collision.
    1. However, given time and a vigorous asteroid mining industry that deliberately deflects asteroids routinely, targeting asteroids will someday be practical.
    2. It is likely that the mining will be done by a factory that goes to the asteroid, and only refined products sent to Earth or elsewhere. Thus, the ability to accurately retarget asteroids will likely not get a boost from mining.
  3. Any reasonable program to protect Earth from asteroids will track and predict the future motion of all the large asteroids, and many of the smaller ones, coming near Earth’s orbit. Thus, tampering with the natural trajectory of a dangerous asteroid will likely be detected long before it is an imminent threat, providing time for deflection.
    1. A trusted but rogue element (i.e., a spy) inside the tracking organization may be able to cover up the new trajectory and true location of the asteroid until it is too late.
    2. A potential solution is multiple redundant tracking operations such that penetration of all of them by rogue elements is extremely unlikely.
  4. Asteroids are inferior weapons.
    1. Aiming at the desired target at any particular time is often impossible. Asteroids travel in predictable orbits and are only a threat to settlements near that trajectory. There may not be an opportunity to hit a particular target with a large asteroid for hundreds of years or more.
    2. Time constraints make a coordinated attack by multiple asteroids all but impossible to design much less execute.
    3. Aiming to hit something as small as Earth is difficult, much harder than trying to miss Earth. Small perturbations can cause an asteroid to miss Earth entirely, whereas small perturbations do not matter much when trying to miss Earth.
    4. Changing the trajectory of large asteroids is difficult. The velocities and masses are huge. Trajectory change must often be done years or even decades in advance to hit the target.
    5. When attacking orbital settlements the target can, at least in principle, maneuver, making the difficult task of hitting the target much harder.
  5. Nuclear weapons are smaller, easier to precisely maneuver, and much easier to hide than asteroids. Consider that, unlike asteroids, nuclear weapon locations won’t be in a solar-system-wide database. Nuclear weapons are far easier to use for surgical attacks that are not intended to exterminate humanity. They are also suitable for whole-planet destruction by designing the attack to create a nuclear winter. Given the superiority of nuclear weapons, it is unlikely that a deliberate asteroid attack will be attempted much less be successful.

Comparison with no space settlement: Space settlement or not, we would be fools indeed not to find, track, and deflect asteroids headed towards Earth. However, without settlement it will be more difficult to accomplish this. Without space settlement, our in-space industrial capabilities will be less advanced, and removing the threat entirely by dismantling the appropriately asteroids and selling the materials will not be an option. The asteroid threat will continue more or less forever and vigilance will be easier to maintain if settlements dot the solar system. Finally, once a large number of space settlements are independent of Earth, a successful asteroid attack on Earth would not exterminate humanity.

Weakening of treaty obligations (Restraint Reversal)

Argument: Nuclear-tipped ICBMs are a well documented existential threat at least to civilization and possibly humanity. Also, there are other technological developments, such as nanotechnology, biotechnology, and artificial superintelligence, that have the potential to create very serious problems. A series of treaties inhibits the further development of nuclear weapons and, at least in principle, it might be desirable to have similar constraints on other fields. Military pressure on Earth by cislunar space settlements has the potential to loosen treaty obligations and create a greater threat than would otherwise be the case. Space settlement changes the environment and may make restraining ICBMs and other dangerous technology more difficult and therefore less likely to succeed.

Counter-argument: It is possible that there may be military tensions between Earth and cislunar space. It may be that these tensions prevent successful treaty development. Other factors might instead play a dominant role in treaty development. It’s also possible that treaty development is counter-productive if one or both sides cheat. It is extremely difficult to know centuries in advance how well treaty development will fare. To give up the survivability of a space-settlement-based society on the highly uncertain and possibly unimportant improvements in the environment for treaty negotiations predicted by geopolitics does not appear to be wise.

So far, civilian space development has been a huge boon to humanity in Earth observation, communications, geolocation, treaty monitoring, and more, and there is every reason to believe this will continue.

Comparison with no space settlement: Nuclear-tipped ICBMs are here and it is many decades before the first space settlement will be built. The main comparison is that in the space settlement case the use of large numbers of nuclear weapons will not wipe humanity out. Also, hundreds, thousands, or even millions of kilometers of vacuum will separate settlements from Earth and each other, making a barrier to proliferation that even advanced artificial superintelligence may have difficulty penetrating.

Totalitarian world government (Hierarchy Enablement)

Argument: According to Deudney, “The further large-scale expansion of human activity into solar space is likely to facilitate the emergence of a highly hierarchical world government on… Earth that could then be prone to become totalitarian” due to military pressure on Earth.

Counter-argument: The hypothesized facilitation of highly hierarchical world government is due to the hypothesized threat of attack specifically:

  1. Bombardment by asteroids. But as we have seen, asteroids make inferior weapons.
  2. Attack from low Earth orbit. In this scenario one entity controls Earth and another controls the orbital space near Earth, which can then be used to launch attacks. However, if a single entity controls both, this threat becomes moot.

With regard to turning totalitarian, it should be noted that none of the classic totalitarian states (Soviet Union, Germany, Italy, North Korea, and China) were subject to significantly more threat than other countries which did not turn totalitarian (e.g., the United Kingdom, France, Switzerland, etc.), suggesting that external pressure is not necessarily the driver towards totalitarianism. Indeed, South and North Korea shows that very similar countries in similar circumstance can be driven to either totalitarianism or democracy.

Comparison with no space settlement: If there is no settlement then there cannot be a threat originating from settlements so a comparison makes no sense.

Ubermensch (Alien Generation)

Argument: As humanity spreads throughout the solar system, some branches of homo sapiens may eventually evolve into new species, with or without genetic engineering, nanotechnology implants, artificial superintelligence, and/or other cybernetics. One or more of these “Ubermensch” societies may wish to colonize Earth, Mars, or other worlds with little care for the people living there. Earth may be considered particularly valuable as it is uniquely well suited to life. That may make it a target for powerful groups of free space settlements. Assuming the Ubermensch really are superior, at least in warfare, this could lead to homo sapiens’ extinction.


  1. Speciation takes a long time. Trying to predict so far into the future is a dicey business.
  2. Except for speciation, cybernetic and nanotech modification could happen even if humanity were to stay only on Earth, although keeping it hidden would be harder than in a society consisting of hundreds or thousands of orbital habitats.
  3. The problems that may come from genetic engineering or cyborg development are likely to arise on Earth well before large-scale space settlement.
  4. A sudden attack on Earth by Ubermensch living on Earth would be harder to counter than an aggressive force working its way in from, say, the Kuiper Belt, which could take years and be seen well before they posed a direct threat to Earth.

Comparison with no space settlement: With space settlement, genetic engineering, cyborg, and nanotech research can be extremely well controlled. Research facilities can be isolated from all other life by thousands of kilometers of vacuum and the entire facility obliterated if things get really out of hand. While possible without settlement, creating a dangerous new species would be much easier for a space settling civilization as the work could be tucked away in one or a few settlements.

Unknown and unknowable threats (Monster Multiplication)

Argument: By moving out into the cosmos there will be many more teams developing and deploying new technology and new social systems. Some of these may create existential threats that we do not know about. Indeed, it is possible that some generated existential threats are unknowable.

Counter-argument: This is true. Of course, one could have made this same claim when people started pounding out flint arrowheads. So far, civilian space development has been a huge boon to humanity in Earth observation, communications, geolocation, treaty monitoring, and more, and there is every reason to believe this will continue.

Comparison with no space settlement: Presumably with no space settlement there would be fewer teams and thus fewer unknowable threats created, but this does not mean we would be safe from unknowable threats. All it would take is one existential unknowable threat to wipe out Earth. It would take a much bigger, more capable one to wipe out all space settlements.


The six threats Deudney presents are less than certain and most are far in the future, whereas the threats in Table 1 are, with one exception, a real and present danger today. Indeed, we are in a pandemic now, albeit one unlikely to exterminate humanity. It should also be noted that Table 1 is far from complete and many other threats exist that can be recovered from, if we have a capable space settlement society.

Contrary to Dark Skies’ recommendations, humanity would be wise to pursue space settlement with vigor.

The central mid-term event in Dark Skies is a deliberate asteroid attack, but we find that asteroids are much inferior weapons compared to nuclear warheads. This undermines three other threats: War, Weakening of Treaty Obligations, and Totalitarian World Government, which depend on a threat to Earth to generate tension leading to adverse consequences.

Comparison of the six threats Deudney identifies and the five threats in Table 1 makes it obvious that space settlement is the future of choice, if the survival of humanity is the measure of success. This is primarily because space settlement is able to survive most threats to Earth and space settlement is uniquely able to resurrect terrestrial civilization, including many species that would otherwise be lost, after a catastrophic event.

Most of the threats Deudney warns us about are more likely to be avoided having been identified. While they are not nearly as certain as Dark Skies might have one believe, the threats are not imaginary. They are very serious, so any reduction in probability is welcome, even if the overall effect on the likelihood of humanity’s survival is fairly minor compared to the massive survival benefits of space settlement.


Contrary to Dark Skies’ recommendations, humanity would be wise to pursue space settlement with vigor. It can contribute to preventing some worst case scenarios and is central in recovering from the others.

This does not mean we should ignore Deudney’s work, which makes many good points that deserve attention. In particular, Deudney rightfully takes the space settlement movement to task for not carefully examining the potential negative effects of space settlement. This is fairly easy to remedy and should be undertaken in the very near future. In addition, some of the threats Deudney identifies can be avoided, and his warnings give humanity something fairly specific to watch out for.

Dark Skies is difficult to read and understand. There are also a few major missing pieces, such as quantification and a direct comparison with space settlement cases. Dark Skies does, however, still provide an intellectual basis that could sway thought leaders to reject space settlement. This could be fatal in many ways, and not just for the reasons discussed here.

It is encouraging that space settlement is viable enough to motivate a well-respected academic like Deudney to take the time and energy to write an intelligent and knowledgeable, if flawed, book about it. Space settlement is a potentially vast and important movement that can and should be the focus of a society-wide debate.

Maybe not so dark skies after all.


  1. Space settlement refers to living beyond Earth on planets (such as Mars), moons (such as Earth’s Moon), or inside of gigantic spacecraft that rotate to provide artificial gravity (also known as free space settlements).
  2. Introduction to Geopolitics by Colin Flint. Different authors appear to mean different things by “geopolitics.” It is not clear that Flint and Dudney agree on the definition as Dark Skies never defines the term.
  3. Space Weapons Earth Wars, Bob Presoton, Dana J. Johnson, Sean J. A. Edwards, Michael Miller and Calvin Shipbaugh, by the Rand Corporation prepared for the U.S. Air Force, 2002.
  4. The Grey Goo problem is from the nanotechnology tradition. In it, a “universal catalyst” embedded in reproducing molecular machines turns everything it can reach into, literally, grey goo.
  5. The Paper Clip Apocalypse problem is from the artificial intelligence tradition. It involves the construction of a very capable intelligent machine that is instructed to produce optimal paper clips. The machine then turns everything within reach into paper clips, including people, biospheres, and whole planets.

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