Crossposted from the Global Priorities Project

Summary

I investigated this question because of its potential relevance to existential risk and the long-term future more generally. There are a limited number of books and scientific papers on the topic and the core questions are generally not regarded as resolved, but the people who seem most informed about the issue generally believe that space colonization will eventually be possible. I found no books or scientific papers arguing for in-principle infeasibility, and believe I would have found important ones if they existed. The blog posts and journalistic pieces arguing for the infeasibility of space colonization are largely unconvincing due to lack of depth and failure to engage with relevant counterarguments.

The potential obstacles to space colonization include: very large energy requirements, health and reproductive challenges from microgravity and cosmic radiation, short human lifespans in comparison with great distances for interstellar travel, maintaining a minimal level of genetic diversity, finding a hospitable target, substantial scale requirements for building another civilization, economic challenges due to large costs and delayed returns, and potential political resistance. Each of these obstacles has various proposed solutions and/or arguments that the problem is not insurmountable. Many of these obstacles would be easier to overcome given potential advances in AI, robotics, manufacturing, and propulsion technology.

Deeper investigation of this topic could address the feasibility of the relevant advances in AI, robotics, manufacturing, and propulsion technology. My intuition is that such investigation would lend further support to the conclusion that interstellar colonization will eventually be possible.

Note: This investigation relied significantly on interviews and Wikipedia articles because I’m unfamiliar with the area, there are not many very authoritative sources, and I was trying to review this question quickly.

Why did I look into this question?

• If people are likely to eventually colonize space, then it increases the potential scale and duration of civilization. This could affect arguments about the importance of trying to affect the long-run future of civilization. For example, Nick Bostrom^[1] and I^[2] have appealed to the possibility of interstellar space colonization in our arguments for the importance of reducing existential risk or otherwise affecting distant future generations.
• In correspondence, some people have tried to resist arguments for the extreme importance of the distant future by rejecting the claim that there is a reasonable chance of colonizing space in the future. I don’t believe these arguments essentially depend on the feasibility of space colonization. However, I believed that the evidence for the feasibility of space colonization was strong enough to carry that argument, and I wanted to test that assumption.

Clarifying the question

Will we eventually be able to colonize other stars? I focused on a version of this question assuming unlimited time horizons and “business as usual” (no major catastrophes or unexpected reversals of global trends), with the aim of establishing settlements which could function independently of Earth-based civilization. My version of “business as usual” could be disputed, especially by people who believe ecological constraints may spell an end to innovation and economic development in the coming century or two, leaving insufficient time for the technological developments necessary to make a project of this kind feasible. Apart from noting the potential problem, that is a discussion best left for another investigation.

Range of current opinion on this topic

A recent, lengthy report by the National Research Council discussing rationales and approaches stated that it currently isn’t known whether we’ll eventually be able to create self-sufficient off-Earth settlements,^[3] but only two of 285 pages were devoted to this issue. People who have done the most in-depth work on the feasibility of space colonization generally believe it is possible. For instance, I found several books and academic papers published on this topic, and they are all written by people who think it is possible,^[4] but I found no books or academic papers arguing for that interstellar colonization is impossible. I discuss whether this is due to a selection effect below. I found some journalistic pieces and magazine articles arguing for the impossibility of space colonization because of worries about microgravity^[5] or getting enough energy,^[6] but I generally found them very unconvincing due to lack of depth and failure to discuss obvious counterarguments. For example, the articles arguing from challenges of microgravity to the impossibility of space colonization failed to consider the most obvious solution: rotating the spaceship to induce artificial gravity. For the most part, the articles arguing from energy/propulsion challenges didn’t discuss any specific problems with existing proposals to get enough energy using alternatives to the current chemical rockets we have today, such as nuclear fission, fusion, beam propulsion, solar sails, or antimatter (more on this below). An exception was a brief essay written for an Edge.org competition by Ed Regis, which did quickly discuss some of these potential propulsion methods^[7] (in addition to several of the other challenges I discuss below). However, Regis makes the strange choice of highlighting the most speculative and unlikely propulsion methods, and does not address fission or fusion. It’s possible that the scientists quoted have more developed arguments that the journalists have failed to communicate, which would suggest that better critiques along these lines are possible, but it isn’t clear to me how these critiques would be developed based on what has been said.

Of the four people I interviewed on this topic, only one (Charles Stross) was pessimistic about our prospects for colonizing the stars. I sought out Charles Stross specifically because he had been referenced by other sources as a notable critic of the feasibility of space colonization^[8] and because his essay on the topic had the best pessimistic arguments I had seen. From my perspective, his most compelling concerns were about motivation to try in light of large expenses and scale-related challenges for getting a civilization off the ground (more on these below). However, even Stross believes that, if we develop advanced AI/robotics, we are likely to colonize the stars. I asked the four people I interviewed for more references from people who believe we can’t colonize the stars, and their comments^[9] suggest I am not missing any major critiques of the feasibility of interstellar colonization. Comments from these interviews also suggested to me that people publishing work on this topic generally believe interstellar colonization is feasible.^[10]

Perhaps only people who think space colonization is likely to be feasible get excited enough to write books and careful scientific articles on the subject. But, for a few reasons listed below, it seems this could only partly explain the distribution of informed opinion on the feasibility of space colonization.  First, according to Geoffrey Landis, informed experts in nearby fields, such as researchers at NASA, generally haven’t thought very much about whether interstellar colonization is possible,^[11]and don’t have opinions one way or the other. If there were good arguments for the infeasibility of space colonization that just weren’t being considered by the people I spoke with, I would expect that this would be the group that was aware of them. Second, I have cited a reasonable number of journalistic articles and blog posts arguing for the infeasibility of space colonization, and these articles are surprisingly bad if you think some informed people have convincing arguments for skepticism about the feasibility of space colonization. Third, some claims about interstellar colonization--e.g. by Hawking--have received significant interest from journalists and the public, and if skeptics had other convincing arguments that we probably wouldn’t ever be able to colonize the stars, I would expect someone to make these arguments more generally known.

What are the potential obstacles to interstellar colonization?

Big picture, interstellar colonization requires success at each of the following steps (though not necessarily on the first attempt):

1. Attempt to colonize space
2. Get everything you need to build a civilization into a spaceship
3. Get the spaceship going fast in the right direction
4. Have enough of what you need to build a civilization survive/remain intact during the voyage
5. Slow the spaceship down when you’re getting close enough to your target location
6. Build a civilization at your target location

This review will discuss these steps out of order because it will help with clarifying what we need to bring on a trip of this kind and what kind of motivation would be needed in order to make the attempt.

Speeding up and slowing down

Proxima Centuari—the closest star outside our solar system—is 4.2 light years away.^[12] Currently, Voyager 1 is moving away from the sun at 17 km/s, faster than any other human-made object.^[13] At its current speed, it would take over 70,000 years to reach travel that distance (though it isn’t going in that direction).^[14] The voyage would take decades even at a significant fraction of the speed of light. The longer the trip, the harder it is to ensure that all critical parts of the system (including passengers or descendants of passengers) survive the journey. These issues are discussed in the next sections. This section will focus on propulsion methods and energy requirements.

Charles Stross estimated that it would take at least 10¹⁸ Joules of energy to accelerate a 2000 kg spacecraft to 10% of the speed of light, and an equal amount to slow it down—making generous assumptions such as 100% efficient energy conversion and no reaction mass. This was equivalent to five days of human civilization’s total electrical energy production in 2007,^[15] or about one day of civilization’s whole energy production.^[16] However, there has been a long history of exponential growth in energy production and economic productivity.^[17] If these trends continue long enough, interstellar colonization will become much more affordable.^[18]

It might be objected that there are some physical reasons to be skeptical that this kind of exponential growth in energy output will continue for long enough to carry this argument. For example, Tom Murphy has argued—based on thermodynamic principles—that hundreds of years of increases in energy production at this exponential rate would have absurd consequences, such as increasing the Earth’s surface temperature to boiling temperature.^[19]  However, Murphy's calculations suggest that it would be possible to have 100 times as much energy output on Earth as we currently do without substantially affecting Earth's temperature,^[20] which would make the energy cost of an interstellar mission not unreasonable on assumptions like Stross’s. More importantly though, an interstellar mission could be launched from interplanetary space, substantially weakening the original line of argument.

Some proposed propulsion technologies—such as nuclear fission, fusion, anti-matter, beam propulsion, and solar sails—might address these challenges. I encountered disagreement about whether these proposed technologies were known to be feasible or not. Robert Zubrin claimed in our interview that there were specific technologies which were known to be physically possible, and could allow us to travel at a few percent of the speed of light.^[21] Ed Regis said we didn’t know whether some of these technologies were feasible,^[22] but he didn’t discuss fission, fusion, beam propulsion, or solar sails, which seem much less speculative than the technologies whose feasibility he questioned. I have not looked at these proposals closely, but the Orion project, which was led by Freeman Dyson, received substantial funding and attention from serious people.^[23] My intuition is that nuclear propulsion is the least speculative, and would be sufficient for bringing a spacecraft to a significant fraction of the speed of light.

Surviving the journey: challenges for human passengers

Microgravity

The human body is adapted to the level of gravitational force normally experienced on the surface of the Earth, and extended exposure to a zero-g environment has various adverse health effects, including loss of bone and muscle mass, retinal damage, redistribution of bodily fluids toward the upper half of the body, balance disorders, and loss of taste and smell.^[24] According to Wikipedia, we have very limited knowledge about the potential effects on the very young and the elderly.^[25] In addition, attempts to breed mice and fish eggs in space have come out badly, and the prevailing view is that microgravity is the source of the problem.^[26] This may be a problem because—given the distance to other stars—human reproduction may be necessary in transit.

A simple solution to this problem is to induce artificial gravity by rotating the vessel/habitat,^[27] though some other solutions have been proposed as well.^[28]

Cosmic radiation

Extended exposure to cosmic radiation damages DNA, and might cause cancer or other negative health effects. Earth’s atmosphere and/or magnetic field prevent these problems near Earth’s surface. Cosmic radiation could also damage electronic equipment.^[29]

Some proposed solutions to this problem include mass shielding and magnetic shielding. Mass shielding would definitely work, but increases the energy requirements for travel.^[30] It is less certain that magnetic shielding would work and magnetic shielding would require less mass,^[31] but it may create other health risks.^[32]

Zubrin suggested that cosmic radiation was not a significant concern for interstellar travel. In support of this, he argued that we’ve had people working close to nuclear reactors on nuclear submarines for over 50 years without major problems.^[33] There is some additional background on this obstacle on Wikipedia, “Health threat from cosmic rays.”

Human lifespan

Interstellar voyages would take decades even at a significant fraction of the speed of light, and centuries or even millennia at more modest speeds. Therefore, it may be impossible to complete the trip within a human lifetime.

Proposed solutions to this problem include “generation ships” designed to rear children and train them to continue the voyage mid-flight, extending the human lifespan, suspended animation/re-animation upon arrival.^[34]

Surviving the journey: general physical challenges

Interstellar medium

From notes from a conversation with Charles Stross:

When travelling at a few percent of the speed of light, collisions with interstellar matter could cause significant damage to a vessel. Like radiation, this is something that might be overcome with appropriate shielding, though there are trade-offs between mass from shielding and the amount of energy necessary for propulsion.

The other people I spoke with about this issue also believed that this challenge could be overcome with appropriate shielding.^[35] However, in his brief critique of the feasibility of interstellar missions, Ed Regis claimed that “a high-speed collision with something as small as a grain of salt would be like encountering an H-bomb,” suggesting he did not find it clear that this challenge could be overcome.^[36]Some simple calculations, however, suggest that Regis's claim could only be realistic under very extreme assumptions. Assuming that 100% of the kinetic energy of the salt grain were converted to an explosion, a one-milligram mass could only produce a one-megaton explosion if the spacecraft were travelling at extremely close to light speed. At 10% of light speed the impact would be equivalent to about 100 kg of TNT, which is about 10 million times smaller. At 1% of light speed, the impact would be equivalent to about 1 kg of TNT.^[37] These smaller explosions would not be negligible, but would probably be within the range of explosions that some bunkers and armored vehicles are capable of withstanding today.

In the paper I saw which investigated this issue most deeply, the authors recommended that, to be conservative, interstellar missions conducted at ≥ 0.1c prepare for the possibility of hitting dust particles of 0.01 mg. These collisions would carry only 1% of the kinetic energy discussed in the paragraph above.^[38]

Mechanical integrity over a long voyage

From notes from a conversation with Geoffrey Landis:

Interstellar colonization may require making machines that will work for hundreds, thousands, or tens of thousands of years. Few machines can do this now, but perhaps appropriate repair systems would solve the problem. This is something that has not been done, and it’s not clear that it could be done.

This concern was echoed by Anders Sandberg, though, in his estimation, this is a solvable engineering problem.^[39]As with collisions with interstellar matter, this problem could potentially be addressed through trial and error if multiple colonization or exploration attempts are made.

Voyager 1, mentioned above, was launched on September 5, 1977, has had no critical mechanical failures for over 36 years, and is expected to continue to function until 2025.^[40] Therefore, designing a spacecraft which would operate without a critical mechanical error for decades would not be wholly unprecedented.

Building a civilization on the other end

Genetic diversity

A small population establishing a space colony would risk genetic damage due to inbreeding. This challenge might be addressed by bringing tens or hundreds of people (at increased energy costs) or bringing a few people plus enough embryos/gametes.^[41]

Hospitable location

According to Wikipedia, “There are 59 known stellar systems within 20 light years from the Sun, containing 81 visible stars,” with some of the most appealing targets for interstellar travel in that range.^[42]

Stross argued that for most places on Earth throughout most of Earth’s history, the planet wouldn’t be very hospitable to humans, and this raises questions about how hard it would be to find a hospitable planet.^[43]

Potential solutions to this challenge include terraforming, searching through many planets, living off planets, and doing colonization with advanced AI, robotics, and manufacturing. Conceivably, advances in AI, robotics, and manufacturing would allow a civilization to thrive in space, eliminating the need for finding habitable planets.

Infrastructure and scale requirements

From a conversation with Charles Stross:

If the goal of space colonization is to create another civilization that is viable independently of the continued success of Earth-based civilization, there will be enormous challenges putting in place the large infrastructure necessary for an industrial civilization. Our current industrial civilization—including systems for manufacturing, science, education, entertainment, support systems for people who aren’t working, etc.—probably requires at least a billion people. It is extremely hard to see how to create a space colony capable of replacing our current industrial civilization without at least hundreds of thousands of people.

This objection has received relatively little attention from people optimistic about space colonization. An exception to this is the science fiction novel Learning the World by Ken MacLeod, which focuses on a “generation ship” which has been travelling for thousands of years.

Possible solutions to this problem include bringing a large number of people, bringing a few people with a plan to build up a large infrastructure, and creating self-replicating robots that would be capable of building a civilization.^[44]

I did not see much detailed discussion of how this could be done,^[45] but my intuition is that with advances in AI/robotics that would create machines capable of doing essentially all the tasks humans do, it should be possible for machines to build a civilization, even in a highly hostile environment. Stross expressed a similar view, though he had more uncertainty about whether such advances would ever be made.^[46] This is an area where someone with different background assumptions would be especially likely to disagree with me, and I’d have to do substantially more work to convince them of my position.

Unknown obstacles

Two people I spoke with, Sandberg^[47] and Zubrin,^[48] thought that the impossibility of space colonization would require the discovery of new physics. Landis thought it was possible that there were unknown obstacles that could make space colonization impossible, and pointed to Fermi’s Paradox as a reason that one might expect this.^[49] Some possible explanations of Fermi’s Paradox would point to other reasons that space colonization is impossible, but others would not. For example, crucial technologies could fail for some reason we can’t appreciate, we might be living in a computer simulation without real stars outside our solar system, or some aliens could be preventing interstellar colonization. On the other hand, some step we’ve already passed (e.g. the creation of the first life, multicellular organisms, intelligent life, industrialization) may happen only extremely rarely.^[50]

Will to do it

People I spoke with disagreed about whether people would ever attempt interstellar colonization. Stross argued that interstellar colonization could only have a very uncertain and very long-term economic payoff, which would make it very hard to finance by appeal to economic motives.^[51] However, Carl Shulman has forthcoming blog post in which he argues that there will probably eventually be strong economic incentives for interstellar colonization.

Other potential motives for colonizing space include curiosity and adventure, increasing civilization’s ability to survive global catastrophes,^[52] or increasing the size of civilization. More generally, in a world with great diversity of motivations, it may not be too hard to find someone who wants to try to colonize space.^[53] Historically speaking, Gerard O’Neill’s L5 Society—which had the aim of colonizing (interplanetary) space—had over 10,000 members when it merged with the National Space Institute.^[54]

On the opposing side, something that could gather the energy necessary for interstellar travel might be weaponized, and this could—imaginably—create political opposition to interstellar travel.^[55] Some further relevant notes from my conversation with Stross:

“However, other ideological perspectives—such as perspectives emphasizing the sacredness of nature—might oppose space colonization. Opposition from these perspectives could prevent an optimistic minority from colonizing space even if it were feasible.”

Implications of advanced AI, robotics, and manufacturing for the feasibility of interstellar colonization

Advances in AI, robotics, or manufacturing could reduce many of the challenges of space colonization noted above.^[56] Spelling this out a bit more, microgravity, cosmic radiation, human lifespan, genetic diversity, and habitability are either unproblematic or much less problematic with robots instead of humans. They could also reduce the amount of mass required for the mission, and therefore reduce the energy requirements. However, the use of advanced AI/robotics was rarely discussed in the materials I read, and was not emphasized by the people I interviewed, with the exception of Anders Sandberg.

Conclusions and questions for further investigation

My impression is that the most informed people thinking about these issues believe that space colonization will eventually be possible, and that they believe this for reasons that make sense to me. In my view, the most uncertain step is the part where we build a civilization upon arriving at another star system. However, my intuition—as stated above—is that advances in AI and robotics will make it possible for machines to substitute for humans in building a civilization, even in environments that would be very inhospitable to humans.

Further investigation into this topic might focus on the following questions:

1. Does this list contain all significant known obstacles to interstellar colonization?
2. How likely is it that there are unknown obstacles that might make interstellar colonization impossible?
3. Given sufficiently advanced AI, robotics, and molecular manufacturing, is interstellar colonization definitely feasible?
4. Given business as usual, is it likely that these advances in AI, robotics, and molecular manufacturing will eventually be made?
5. Do we know, as Zubrin claimed, that some physically possible propulsion technologies could get us to several percent of the speed of light? If so, which are they and how do we know this?

My process

This research draws on interviews with Anders Sandberg, Geoffrey Landis, Robert Zubrin, and Charles Stross. Charles Stross was the most credible skeptic of the feasibility of space colonization that I could find. He was cited as a critic of The High Frontier on the book’s Wikipedia page and multiple people I spoke with referred me to Stross as a skeptic that was worth speaking to.

A list of many, but not all, of the sources I considered, how I found them, and how closely I looked into them is available here. I began by looking for articles arguing for or against the feasibility of space colonization, searching Google, Google Scholar, and Amazon.com with terms like “space colonization,” “space settlement,” “possible,” “impossible,” “feasible,” and “infeasible,” and then followed up on articles referenced in the most relevant sources I found. None of the people I interviewed were aware of notable articles arguing for the impossibility of space colonization that I had missed, though I asked all of them specifically about this. I spent 36 hours on this project.

I am grateful to Robin Hanson, Pablo Stafforini, Carl Shulman, Anders Sandberg, and Toby Ord for feedback on a draft of this review.

Sources

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Beckstead, Nick. 2013. “On the Overwhelming Importance of Shaping the Far Future,” 2013. PhD Thesis. Department of Philosophy, Rutgers University. URL: http://www.nickbeckstead.com/research.

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Bostrom, Nick. 2003. “Astronomical Waste,” Utilitas 15(3): 308-314. URL: http://www.nickbostrom.com/astronomical/waste.html.

Dyson, Freeman. 1965. “Death of a Project,” Science 149:141-144.” URL: http://www.patrickmccray.com/wp/wp-content/uploads/2013/11/1965-Dyson-Death-of-a-Project.pdf

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^[1] “As a rough approximation, let us say the Virgo Supercluster contains 10^13 stars. One estimate of the computing power extractable from a star and with an associated planet-sized computational structure, using advanced molecular nanotechnology, is 10^42 operations per second. A typical estimate of the human brain’s processing power is roughly 10^17 operations per second or less. Not much more seems to be needed to simulate the relevant parts of the environment in sufficient detail to enable the simulated minds to have experiences indistinguishable from typical current human experiences. Given these estimates, it follows that the potential for approximately 10^38 human lives is lost every century that colonization of our local supercluster is delayed; or equivalently, about 10^29 potential human lives per second.

While this estimate is conservative in that it assumes only computational mechanisms whose implementation has been at least outlined in the literature, it is useful to have an even more conservative estimate that does not assume a non-biological instantiation of the potential persons. Suppose that about 10^10 biological humans could be sustained around an average star. Then the Virgo Supercluster could contain 10^23 biological humans. This corresponds to a loss of potential equal to about 10^14 potential human lives per second of delayed colonization.” Bostrom 2003, “Astronomical Waste.”

^[2] “The lion’s share of the expected duration of our existence comes from the possibility that our descendants colonize planets outside our solar system. There are many stars that we may be able to reach with future technology (about 1013 in our supercluster). Some of them will probably have planets that are hospitable to life, perhaps many of these planets could be made hospitable with appropriate technological developments. Some of these are near stars that will burn for much longer than our sun, some for as much as 100 trillion years (Adams, 2008, p. 39). If multiple locations were colonized, the risk of total destruction would dramatically decrease, since it would take independent global disasters, or a cosmological catastrophe, to destroy civilization. Because of this, it is possible that our descendants would survive until the very end, and that there could be extraordinarily large numbers of them.” Beckstead 2013, “On the Overwhelming Importance of Shaping the Far Future,” p. 57.

^[3] When listing potential  rationales for space exploration, they wrote:

“Human Survival. It is not possible to say whether off-Earth settlements could eventually be developed that would outlive human presence on Earth and lengthen the survival of our species. This is a question that can only be settled by pushing the human frontier in space.”

National Research Council, “Pathways to Exploration: Rationales and Approaches for a U.S. Program of Human Space Exploration.” p. S-2. Further discussion on pp. 2-26-27.

^[4] Examples include O’Neill 1977, Mallove and Matloff 1989, Landis 2004, Gilster 2004, and Armstrong and Sandberg 2013.

^[5] “"Giving birth in zero gravity is going to be hell because gravity helps you [on Earth]," said Athena Andreadis, a biologist from the University of Massachusetts Medical School. "You rely on the weight of the baby."

All of this means that we're not going anywhere, perhaps not even Mars, until we master either artificial gravity or some seriously speedy travel methods. Although this news won't come as a surprise to anybody who's put serious thought into interstellar travel, it is humbling to be reminded of these things from time to time. Humans are perfectly adjusted for life on Earth; as Andreadis noted, we'll have to adapt to both the journey and the destination if we're ever to leave.” McLellan 2011. “Microgravity Makes Interstellar Travel Impossible, Say Experts.”

“Hawking, Obama and other proponents of long-term space travel are making a grave error. Humans cannot leave Earth for the several years that it takes to travel to Mars and back, for the simple reason that our biology is intimately connected to Earth.

To function properly, we need gravity. Without it, the environment is less demanding on the human body in several ways, and this shows upon the return to Earth.” Piersma 2010. “Why space is the impossible frontier.”

^[6] “Already there are huge challenges facing the notion of travelling to Proxima Centauri, but in a recent gathering of experts in the field of space propulsion, there are even more insurmountable obstacles to mankind’s spread beyond the Solar System. In response to the idea we might make the Proxima trek in a single lifetime, Paulo Lozano, an assistant professor of aeronautics and astronautics at MIT and conference deligate said, “In those cases, you are talking about a scale of engineering that you can’t even imagine.”

OK, so the speed simply isn’t there for a quick flight over 4.3 light years. But there is an even bigger problem than that. How would these interstellar spaceships be fuelled? According to Brice N. Cassenti, an associate professor with the Department of Engineering and Science at Rensselaer Polytechnic Institute, at least 100 times the total energy output of the entire world would be required for the voyage. “We just can’t extract the resources from the Earth,” Cassenti said during his conference presentation. “They just don’t exist. We would need to mine the outer planets.”” O’Neill 2008, “Bad News: Interstellar Travel May Remain in Science Fiction.”

“"Human expansion across the Solar System is an optimist’s fantasy. Why? Because of the clash of two titans: physics versus chemistry.

In the red corner, the laws of physics argue that an enormous amount of energy is required to send a human payload out of Earth’s gravitational field to its deep space destination and back again.

In the blue corner, the laws of chemistry argue that there is a hard limit to how much energy you can extract from the rocket fuel, and that no amount of ingenuity will change that."” Finkel 2011, “Forget space travel: it’s just a dream.”

^[7] “But traveling at significantly faster speeds requires prohibitive amounts of energy. If the starship were propelled by conventional chemical fuels at even ten percent of the speed of light, it would need for the voyage a quantity of propellant equivalent in mass to the planet Jupiter. To overcome this limitation, champions of interstellar travel have proposed "exotic" propulsion systems such as antimatter, pi meson, and space warp propulsion devices. Each of these schemes faces substantial difficulties of its own: for example, since matter and antimatter annihilate each other, an antimatter propulsion system must solve the problem of confining the antimatter and directing the antimatter nozzle in the required direction. Both pi meson and space warp propulsion systems are so very exotic that neither is known to be scientifically feasible.” Regis 2013, “Being Told That Our Destiny Is Among The Stars.”

^[8] “It’s hard to think of notable pessimists. Charles Stross wrote a good essay arguing for pessimism about space colonies, and he might be a good person to talk to. (Nick raised Stross as the most credible pessimist he was aware of.)” Notes from a conversation with Geoffrey Landis.

“Science fiction writer Charles Stross wrote a critical essay with a similar title on the feasibility of interstellar space travel and making practical use of various moons and planets in our own Solar System: The High Frontier: Redux.” Wikipedia, “The High Frontier.”

In our conversation on this topic, Anders Sandberg recommended that I speak with Charles Stross.

^[9] “Stross has kept his eye on this literature over the years. He was disappointed to know that I didn’t find any critiques of the feasibility of space colonization that were superior to his, though he wasn’t aware of any more developed critiques.” Notes from a conversation with Charles Stross

^[10] “The people in the above groups would probably have opinions that are generally along the same lines as Dr. Landis. Some would be more optimistic, and some would be less optimistic. They would generally agree that space colonization is possible in principle, and most of the disagreement would be about how hard it is.” Notes from a conversation with Geoffrey Landis

^[11] “Most people at NASA generally haven’t thought deeply about this question.” Notes from a conversation with Geoffrey Landis

^[12] “Proxima Centauri (Latin proxima, meaning "next to" or "nearest to") is a red dwarf about 4.24 light-years from the Sun, inside the G-cloud, in the constellation of Centaurus. It was discovered in 1915 by Scottish astronomer Robert Innes, the Director of the Union Observatory in South Africa, and is the nearest known star to the Sun…” Wikipedia, “Proxima Centauri.”

^[13] “Travelling at about 17 kilometers per second (11 mi/s)[35] it has the fastest heliocentric recession speed of any human-made object.” Wikipedia, “Voyager 1.”

^[14] “Voyager 1 was traveling at 17,043 m/s (38,120 mph) relative to the Sun (about 3.595 AU per year). It would need about 17,565 years at this speed to travel a complete light year.[3] To compare, Proxima Centauri, the closest star to the Sun, is about 4.2 light-years (or 2.65×10⁵ AU) distant. Were the spacecraft traveling in the direction of that star, 73,775 years would pass before reaching it.” Wikipedia, “Voyager 1.”

^[15] “It's going to be pretty boring in there, but I think we can conceive of our minimal manned interstellar mission as being about the size and mass of a Mercury capsule. And I'm going to nail a target to the barn door and call it 2000kg in total….Now, let's say we want to deliver our canned monkey to Proxima Centauri within its own lifetime. We're sending them on a one-way trip, so a 42 year flight time isn't unreasonable. (Their job is to supervise the machinery as it unpacks itself and begins to brew up a bunch of new colonists using an artificial uterus. Okay?) This means they need to achieve a mean cruise speed of 10% of the speed of light. They then need to decelerate at the other end. At 10% of c relativistic effects are minor — there's going to be time dilation, but it'll be on the order of hours or days over the duration of the 42-year voyage. So we need to accelerate our astronaut to 30,000,000 metres per second, and decelerate them at the other end. Cheating and using Newton's laws of motion, the kinetic energy acquired by acceleration is 9 x 10¹⁷ Joules, so we can call it 2 x 10¹⁸ Joules in round numbers for the entire trip….our entire planetary economy runs on roughly 4 terawatts of electricity (4 x 10¹² watts). So it would take our total planetary electricity production for a period of half a million seconds — roughly 5 days — to supply the necessary va-va-voom.” Stross 2007, “The High Frontier, Redux.”

^[16] “In 2008, total worldwide energy consumption was 474 exajoules (132,000 TWh). This is equivalent to an average power use of 15 terawatts (2.0×10¹⁰ hp).”

^[17] “Since the beginning of the Industrial Revolution, we have seen an impressive and sustained growth in the scale of energy consumption by human civilization. Plotting data from the Energy Information Agency on U.S. energy use since 1650 (1635-1945, 1949-2009, including wood, biomass, fossil fuels, hydro, nuclear, etc.) shows a remarkably steady growth trajectory, characterized by an annual growth rate of 2.9%.” Murphy 2011, “Galactic-Scale Energy.”

^[18] “In 1968, Freeman Dyson looked at the economics of interstellar colonization, and it was prohibitively expensive. But he pointed out that there has been a long history of exponential growth in economic productivity and energy production. If these trends continue long enough, interstellar colonization will become much more affordable. On the other hand, exponential trends will not last forever.” Notes from a conversation with Geoffrey Landis.

^[19] This is not clearly summarized in the text, but it is clear in the following graph:

And the following caption:

“Earth surface temperature given steady 2.3% energy growth, assuming some source other than sunlight is employed to provide our energy needs and that its use transpires on the surface of the planet. Even a dream source like fusion makes for unbearable conditions in a few hundred years if growth continues. Note that the vertical scale is logarithmic.”

and clarificatory remarks:

“absorbs abundant energy from the sun—far in excess of our current societal enterprise. The Earth gets rid of its energy by radiating into space, mostly at infrared wavelengths. No other paths are available for heat disposal. The absorption and emission are in near-perfect balance, in fact. If they were not, Earth would slowly heat up or cool down. Indeed, we have diminished the ability of infrared radiation to escape, leading to global warming. Even so, we are still in balance to within less than the 1% level. Because radiated power scales as the fourth power of temperature (when expressed in absolute terms, like Kelvin), we can compute the equilibrium temperature of Earth’s surface given additional loading from societal enterprise.”

Murphy 2011, “Galactic-Scale Energy.”

^[20] Eyeballing the graph in the above footnote, there is not a substantial temperature increase after 200 years, and at that point we have 100 times as much energy on his model. “This post provides a striking example of the impossibility of continued growth at current rates—even within familiar timescales. For a matter of convenience, we lower the energy growth rate from 2.9% to 2.3% per year so that we see a factor of ten increase every 100 years.” Murphy 2011, “Galactic-Scale Energy.”

^[21] “He said that there are identifiable propulsion technologies which would work—given the laws of physics as they are currently known—and these could get us up to a few percent of the speed of light, allowing us to get to the nearest stars in decades rather than millennia.” Notes from a conversation with Robert Zubrin.

^[22] “champions of interstellar travel have proposed "exotic" propulsion systems such as antimatter, pi meson, and space warp propulsion devices. Each of these schemes faces substantial difficulties of its own: for example, since matter and antimatter annihilate each other, an antimatter propulsion system must solve the problem of confining the antimatter and directing the antimatter nozzle in the required direction. Both pi meson and space warp propulsion systems are so very exotic that neither is known to be scientifically feasible.” Regis 2013. “Being Told That Our Destiny Is Among The Stars.”

^[23] “Orion is a project to design a vehicle which would be propelled through space by repeated nuclear explosions occurring at a distance behind it. The vehicle may be either manned or unmanned; it carries a large supply of bombs, and nlachinery for throwing them out at the right place and time for efficient propulsion; it carries shock absorbers to protect the machinery and the crew from destructive jolts, and sufficient shielding to protect against heat and radiation. The vehicle has, of course, never been built. The project in its 7 years of existence was confined to physics experiments, engineering tests of components, design studies, and theory. The total cost of the project was $10 million, spread over 7 years, and the end result was a rather firm technical basis for believing that vehicles of this type could be developed, tested, and flown. The technical findings of the project have not been seriously challenged by anybody. Its major troubles have been, from the beginning, political. The level of scientific and engineering talent devoted to it was, for a classified project, unusually high.” Dyson 1965, “Death of a Project,” p. 141.

^[24] “Microgravity causes bone decalcification and liquid redistribution in humans.” Notes from a conversation with Anders Sandberg.

“Short-term exposure to microgravity causes space adaptation syndrome, a self-limiting nausea caused by derangement of the vestibular system. Long-term exposure causes multiple health problems, one of the most significant being loss of bone and muscle mass. Over time these deconditioning effects can impair astronauts’ performance, increase their risk of injury, reduce their aerobic capacity, and slow down their cardiovascular system. As the human body consists mostly of fluids, gravity tends to force them into the lower half of the body, and our bodies have many systems to balance this situation. When released from the pull of gravity, these systems continue to work, causing a general redistribution of fluids into the upper half of the body. This is the cause of the round-faced 'puffiness' seen in astronauts. Redistributing fluids around the body itself causes balance disorders, distorted vision, and a loss of taste and smell.” Wikipedia, “Effect of Space Flight on the Human Body.”

“Because weightlessness increases the amount of fluid in the upper part of the body, astronauts experience increased intracranial pressure. This appears to increase pressure on the backs of the eyeballs, affecting their shape and slightly crushing the optic nerve. This effect was noticed in 2012 in a study using MRI scans of astronauts who had returned to Earth following at least one month in space. Such eyesight problems may be a major concern for future deep space flight missions, including a manned mission to the planet Mars.” Wikipedia, “Effect of Space Flight on the Human Body.”

"If off-world colonization someday begins, many types of people will be exposed to these dangers, and the effects on the elderly and on the very young are completely unknown." Wikipedia, “Effect of Space Flight on the Human Body.”

^[25] "If off-world colonization someday begins, many types of people will be exposed to these dangers, and the effects on the elderly and on the very young are completely unknown." Wikipedia, “Effect of Space Flight on the Human Body.”

^[26] “One potential obstacle to human survival in space is the effect of microgravity on health and reproduction. Microgravity causes bone decalcification and liquid redistribution in humans. In addition, attempts to breed mice and fish eggs in space have come out badly; the prevailing view is that microgravity is the source of the problem.” Notes from a conversation with Anders Sandberg.

^[27] “One potential obstacle to human survival in space is the effect of microgravity on health and reproduction….However, this problem could be solved by rotating the station or vessel and thereby inducing artificial gravity.” Notes from a conversation with Anders Sandberg.

^[28] See Wikipedia, “Artificial Gravity.”

^[29] “Space radiation would damage both humans and electronic equipment left in space for a long time. This problem gets more severe for very long-distance travel at relativistic speeds.” Notes from a conversation with Anders Sandberg.

^[30] “It would be possible to prevent the negative consequences of radiation with sufficiently thick shielding on the space vessel, though this increases the mass of the vessel and the amount of resources required to travel. Error-correcting codes and other measures like shielded electronics could prevent fatal damage to computers.” Notes from a conversation with Anders Sandberg.

^[31] “This looks like a problem that could be addressed through shielding. Dr. Landis believes this problem can be solved by creating a strong enough magnetic field.” Notes from a conversation with Geoffrey Landis.

“One solution to the problem of shielding crew from particulate radiation in space is to use active electromagnetic shielding. Practical types of shield include the magnetic shield, in which a strong magnetic field diverts charged particles from the crew region, and the magnetic/electrostatic plasma shield, in which an electrostatic field shields the crew from positively charged particles, while a magnetic field confines electrons from the space plasma to provide charge neutrality. Advances in technology include high-strength composite materials, high temperature superconductors, numerical computational solutions to particle transport in electromagnetic fields, and a technology base for construction and operation of large superconducting magnets. These advances make electromagnetic shielding a practical alternative for near-term future missions.” Landis 1991, “Magnetic Radiation Shielding: An Idea Whose Time Has Returned?”abstract.

^[32] “A spherical shell of water or plastic could protect space travelers, but it would take a total mass of at least 400 tons—beyond the capacity of heavy-lift rockets. A superconducting magnet would repel cosmic particles and weigh an estimated nine tons, but that is still too much, and the magnetic field itself would pose health risks. No other proposed scheme is even vaguely realistic.” Parker, Eugene. “Shielding Space Travelers.” Pg 42.

^[33] “Dr. Zubrin didn’t think space dust was a serious concern, and argued that damage from radiation could clearly be prevented with adequate shielding. He pointed to the fact that we’ve had people working close to nuclear reactors on nuclear submarines for over 50 years without major problems.” Notes from a conversation with Robert Zubrin.

^[34] “…if interstellar travel must proceed more slowly, some possible solutions would include (i) a generation ship, (ii) freezing people and re-animating them upon arrival, and (iii) travelling with machines or uploads, possibly creating biological humans upon arrival.” Notes from a conversation with Geoffrey Landis.

See “Slow Missions” under Wikipedia, “Interstellar Travel.”

^[35] “There are good theoretical reasons to think there isn’t much millimeter-sized gravel in interstellar space. But if there are sand-sized particles, that would be a challenge for interstellar travel at relativistic or near/relativistic speeds. Shielding may be a solution to this.” Notes from a conversation with Geoffrey Landis.

“There is interstellar dust. If objects are travelling at a high speed, hitting a very small piece of dust could do substantial damage. Travelling at a high speed is desirable for interstellar or intergalactic travel. The larger the vessel, the worse the problem. Intergalactic dust is less common, and would be much less of a problem.

This problem could be averted by creating adequate shielding. At a conservative end, we know that space dust does not prevent comet nuclei from doing interstellar travel at a few tens of km/s. A vessel could simply dig into a comet and use it as shielding (for a very slow trip). Anders believes that other shields could be constructed out of metal and graphene foils which would make interstellar and intergalactic space colonization possible with at higher speeds and with less mass.” Notes from a conversation with Anders Sandberg.

“I mentioned a couple of the potential obstacles to interstellar colonization that came up in my interview with Anders Sandberg: space dust (which might damage vessels moving close to the speed of light) and radiation. Dr. Zubrin didn’t think space dust was a serious concern, and argued that damage from radiation could clearly be prevented with adequate shielding.” Notes from a conversation with Robert Zubrin.

^[36] “Even if by some miracle suitable propulsion systems became available, a starship traveling at relativistic speeds would have to be equipped with sophisticated collision detection and avoidance systems, given that a high-speed collision with something as small as a grain of salt would be like encountering an H-bomb.” Regis 2013, “Being Told That Our Destiny Is Among The Stars.”

^[37] Calculations were based on WolframAlpha’s formula for relativistic kinetic energy. I am grateful to Anders Sandberg and Carl Shulman for help selecting appropriate formulas and challenging Regis’s calculation.

^[38] “Observations over the last decade have revealed an unexpected high-mass tail to the local interstellar grain size distribution. Individual particles with masses as high as 10^-12 kg (4.5 μm radius) are almost certainly present. Moreover, radar detections of interstellar dust particles entering the Earth’s atmosphere ([27] and references therein) imply a population of interstellar grains with masses > 3*10-10 kg (corresponding to radii > 30 μm). While there are difficulties reconciling the presence of such large grains with other astronomical observations [36], a conservative planning assumption would be that particles as large as 100 μm radius (10^-8 kg), and perhaps larger, might be encountered in the course of a several light-year journey through the LISM. The kinetic energy of such large particles striking an interstellar space vehicle with a relative velocity of 0.1c are considerable (4.5*10^6 J), and some kind of active dust detection and mitigation system may need to be considered.

^[39] “Someone might question whether it’s possible to create electronic devices that would work for very long periods of time in the presence of radiation, as would be necessary for space colonization. Anders believes this is a solvable engineering problem.” Notes from a conversation with Anders Sandberg.

^[40] “Voyager 1 is a 722-kilogram (1,592 lb) space probe launched by NASA on September 5, 1977 to study the outer Solar System. Operating for 36 years, 8 months and 17 days as of 22 May 2014, the spacecraft communicates with the Deep Space Network to receive routine commands and return data. At a distance of about 127.74 AU (1.911×10¹⁰ km) from the Earth as of May 8, 2014, it is the farthest human-made object from Earth….

On September 12, 2013, NASA announced that Voyager 1 had crossed the heliopause and entered interstellar space on August 25, 2012, making it the first human-made object to do so….The probe is expected to continue its mission until 2025, when it will be no longer supplied with enough power from its generators to operate any of its instruments.” Wikipedia, “Voyager 1.”

^[41] "In 2002, the anthropologist John H. Moore estimated that a population of 150–180 would allow normal reproduction for 60 to 80 generations — equivalent to 2000 years.

A much smaller initial population of as little as two women should be viable as long as human embryos are available from Earth. Use of a sperm bank from Earth also allows a smaller starting base with negligible inbreeding.

Researchers in conservation biology have tended to adopt the "50/500" rule of thumb initially advanced by Franklin and Soule. This rule says a short-term effective population size (Ne) of 50 is needed to prevent an unacceptable rate of inbreeding, whereas a long‐term Ne of 500 is required to maintain overall genetic variability. The Ne = 50 prescription corresponds to an inbreeding rate of 1% per generation, approximately half the maximum rate tolerated by domestic animal breeders. The Ne = 500 value attempts to balance the rate of gain in genetic variation due to mutation with the rate of loss due to genetic drift." Wikipedia, “Space colonization.”

^[42] Wikipedia, “Interstellar Travel.” See the article for more detail.

^[43] “Finding a hospitable location: If you consider all the past and future locations on Earth over time, only a very small fraction of them (probably less than 0.1%) would be habitable for humans. Why? 80% of the Earth’s surface is covered by water, much of the Earth is too cold, there has only been enough oxygen for humans during the last 600 million years, a billion years from now the Earth will no longer be habitable for humans, and so on.” Notes from a conversation with Charles Stross.

^[44] For a discussion of the last possibility, see Wikipedia, “Self-replicating Spacecraft.”

^[45] While commenting on this paper, Anders Sandberg pointed me to Freitas 1981, “A self-replicating, self-growing lunar factory,” which attempted to outline something along these lines. I have not yet looked closely at this paper, and feel it would take a substantial effort on my part to tell whether the idea would hold up. But someone who wanted to explore these issues further might look at this paper and other papers citing it to get some idea of what kind of thinking has already been done on this topic.

^[46] “Given the existence of mind uploading or advanced AI, Stross sees no insurmountable obstacle to interstellar colonization. In this scenario, it seems that a near-light-speed colonization wave could occur roughly along the lines envisioned by Robert Bradbury.

In Stross’s view, it is not settled whether mind uploading or advanced AI are feasible in principle. If the mind is inherently analog or quantum, digital uploads may be impossible.” Notes from a conversation with Charles Stross.

^[47] “This would be very surprising to Anders. In his view, it would probably require learning new physics for us to learn that space colonization is in principle infeasible for some reason not listed here.” Notes from a conversation with Anders Sandberg.

^[48] ““I asked Dr. Zubrin whether he could imagine anything we could learn—consistent with everything we currently know about physics—that would mean space colonization would be impossible. He said he could not think of anything remotely plausible fitting the description. It’s just a question of how fast we can get there.” Notes from a conversation with Robert Zubrin.

^[49] “It’s possible that there are unknown obstacles. Most of the obstacles we’ve discussed seem like they could be overcome. If they can and these are the only obstacles, that makes the Fermi Paradox more puzzling. Someone could argue that this suggests that interstellar colonization is impossible for some unknown reason.” Notes from a conversation with Geoffrey Landis.

^[50] See Wikipedia, “Fermi’s Paradox” for further discussion.

^[51] “Creating a civilization of this size outside of the Earth would require a very large economic investment, and one that could (at best) have only a very long-term payoff. We currently do not have institutions which function well for highly unprecedented ventures which would require decades—or perhaps even centuries—to pay off. This issue is further explored in Stross’s novel Neptune’s Brood.” Notes from a conversation with Charles Stross.

^[52] “Professor Stephen Hawking, celebrated expert on the cosmological theories of gravity and black holes, believes that traveling into space is the only way humans will be able to survive in the long-term. He has said, "Life on Earth is at the ever-increasing risk of being wiped out by a disaster such as sudden global warming, nuclear war, a genetically engineered virus or other dangers ... I think the human race has no future if it doesn't go into space."” Sato 2007, “The "Hawking Solution": Will Saving Humanity Require Leaving Earth Behind?”

^[53] “Anders can see various reasons people might want to do this eventually: diversity of preferences, desire to do research, desire to continue civilization.” Notes from a conversation with Anders Sandberg.

“There is a lot of diversity in the world, and Landis’s intuition is that enough people would want to do it.” Notes from a conversation with Geoffrey Landis.

^[54] “In 1986 the Society, which had grown to about 10,000 members, merged with the 25,000 member National Space Institute, founded by German rocket engineer and Project Apollo program manager Wernher von Braun of NASA's Marshall Space Flight Center to form the present-day National Space Society.” Wikipedia, L5 Society.

^[55] “A possible issue is that because it might require so much energy to do interstellar colonization, anything that could produce that much energy could probably be weaponized. Conceivably, there could be opposition from a larger group of people uninterested in space colonization to creating something that could be used to create a dangerous weapon.” Notes from a conversation with Geoffrey Landis.

^[56] “Given the existence of mind uploading or advanced AI, Stross sees no insurmountable obstacle to interstellar colonization. In this scenario, it seems that a near-light-speed colonization wave could occur roughly along the lines envisioned by Robert Bradbury.

In Stross’s view, it is not settled whether mind uploading or advanced AI are feasible in principle. If the mind is inherently analog or quantum, digital uploads may be impossible.” Notes from a conversation with Charles Stross.

“Space colonization is especially likely to be possible if advanced AGI and molecular manufacturing are possible—as Anders believes they are—though he also thinks it is possible if they are impossible.” Notes from a conversation with Anders Sandberg.