When worldbuilding any outer-space setting, from galactic-scale space opera to the earliest space colonies, one must consider demographics: the magnitude, distribution, and composition of the setting’s population. While other galaxies or far-future settings could easily have a very wide range of outcomes, a near-future in our own solar system or in an interstellar civilization spanning only the nearest stars is much more constrained when it comes to plausible outcomes. Since this will be the earliest stage of the history of any space opera set in humanity’s future we will explore the demographics our own civilization may have when it becomes spacefaring within its own solar system.
The current situation is a one-planet civilization in the middle of the “demographic transition”, the transition in question being between a high-mortality-high-fertility world and a low-mortality-low-fertility world. Fertility (normally) lags mortality in this transition, so Earth has experienced a population explosion in the past two centuries or so, the human population having expanded from 1 billion to 7 billion in that time. Human population growth has steadily slowed and will likely plateau sometime in the 21st century. The most recent United Nations projections for the year 2100 range between a high projection of over 12 billion and still growing (albeit much more slowly) and a low projection of 9 billion and slowly shrinking; the medium projection is for a population of 11 billion and (very) slowly growing. For whatever reason the low projections have been the most accurate historically, and it’s not unreasonable to assume that will continue to be the case. In any event all reasonable projections agree that the human population will not be too far from the 10 billion mark for the duration of the 21st century.
Ribbons of Space Settlements
Of the almost 8 billion humans alive today, not all of them live on Earth; a very small number, normally in the single digits, reside even if only temporarily on board the International Space Station, which has been continually occupied by rotating crews since November 2, 2000. If plans for new space stations post-ISS, new spaceflights, and the establishment of space colonies on the Moon and Mars go forward November 1, 2000 may well go down in history as the last day no humans were in space. Also if plans go ahead for space colonization, even if only on an experimental basis, the number of humans in space will expand by several orders of magnitude. Space tourism will also drive population numbers up, though in this case it will still be a temporary population. Any sufficiently desirable tourist destination, however, eventually acquires permanent residents, and there is no reason to believe free space won’t follow the same route.
Thus it seems likely that research bases and more-or-less experimental colonies on the Moon and Mars will be the first centers of permanent human population in space, followed by the centers of space tourism. Space mining and manufacturing facilities will also host whatever number of employees may be necessary to operate them; more importantly, the owners of these facilities might move to these places themselves along with their entourages. Owners of mines and factories on Earth in desirable locations often build their homes near these facilities, and there’s no reason to believe space will be any different. Naturally, these speculations assume spaceflight later in the 21st century will be relatively cheap compared to today; this is a prerequisite for spacefaring on a mass scale in the 21st century, so is baked into the premise of this post. Also much progress is being made toward this goal even now, so in any case it’s not an unreasonable assumption.
These population centers will be the seeds and the nexuses of space colonization; their population will increase over the years, and they will spawn new colonies and abodes across the solar system. Near-Earth space, the Moon, and Mars will be the centers of the off-world population for the earliest stages of spacefaring civilization. Venus is an underrated possibility for space colonization; eventually, probably not long after Mars is colonized in earnest, floating installations and cloud cities will be established in the upper atmosphere. In time Venus may approach or overtake Mars in popularity. Mercury will be colonized much like the Moon, along with the asteroid belt.
The outer solar system then beckons, with the fascinating and resource-rich moons of the gas giants, some of which may have the ultimate draw of subsurface liquid water oceans and alien life. The atmospheres of Saturn, Uranus, and Neptune have similar pressure and gravity as the Earth, albeit with cold temperatures; the immense relatively Earth-like expanse of these worlds may attract many colonists in time. The Kuiper Belt is much like the gas giants’ moons and the asteroid belt in one, offering the potential to become a population center over the very long term. Even further beyond, the Oort cloud may someday host a significant fraction of the human population, though it will almost certainly remain a secondary player. Its primary draw will be as a scientific center once the “telescope cloud” is created to take advantage of the Sun’s gravitational lensing effect for communication and observation.
So there are many contenders to host large human populations across very different parts of the solar system. Free space near Earth, Mars, and to a lesser extent the Moon are currently attracting the most interest in terms of any actual plans being made; it therefore stands to reason that these regions will have the most momentum in terms of immigration and will retain a majority of the off-world population for most if not all of the 21st century. How high will this population be, though?
Space Population in the 21st Century
Elon Musk has a goal of settling a million people on Mars by 2050 or so, which is certainly ambitious but probably achievable. Assuming the first people reach Mars in 2025 or so, then without taking into account births, deaths, or emigration, 1 million by 2050 would require an average of 40,000 immigrants per year. Since Starship is designed to transport 100 people between planets, this works out to 400 flights per year, or a bit more than one flight a day between Earth and Mars. At that point larger and more efficient vehicles will be used, but it does demonstrate the idea. Assuming a comparable rate of immigration is sustained for the rest of the 21st century Mars would have a population closer to 3 million by 2100, probably more since the higher population at this point could more easily absorb a greater raw number of immigrants. Taking this into account the Martian population may well be 10 million or so.
10 million out of a human population of 10 billion (give or take a few billion) in 2100 is still only 0.1% of the whole human population. However, 0.1% is still large enough to be noticeable on a pie chart, a pretty good result for what would even then be a recently-settled frontier region; consider that in 2100 a few of the first settlers in the 2020s would still be alive. A population in this range is quite plausible, considering this would imply a planetary population density similar to Greenland’s. Even Antarctic population density would still yield a planetary population of 50,000; much less than 10 million, but still much greater than nothing. Mars’s population density will over the long term greatly exceed Antarctica’s (after all, there’s more interest now in Mars to Stay than there ever was in Antarctica to Stay), but Antarctica provides a useful baseline for what sort of population density unattractive planets and asteroids that still have scientific value will acquire.
Mars won’t be the only off-world colony, of course; for every colonist on Mars there’s likely to be at least one other colonist somewhere else in the inner solar system. Any civilization that can sustain daily flights to Mars will have easy and affordable access to Earth orbit and near-Earth space; space tourists alone will account for a non-negligible fraction of the human population. A peak number of 600,000 to 900,000 people are in airplanes at any one time today, or 0.01% of the world population. Much of this is business travel but later in the 21st century with more economic development tourists and other vacationers will likely account for at least this many people; assuming that these people are spending ten to a hundred times as long at their destinations as they are traveling, 0.1% to 1% of the world’s population then would be engaged in tourism at any one time. If 1% of the world’s population is engaged in tourism, then if off-world tourism’s market share expands to only 10% of the whole industry by 2100 then space tourists would outnumber Martian colonists.
People who live at space tourist destinations permanently would inflate this figure, but temporary tourists at such locations usually outnumber permanent residents, so at most this would probably add another 0.1% to the off-world fraction of the human population. All other planets, asteroids, space stations, and other habitats might account for as much as the space tourists and Martians combined by 2100, so at most they will be 0.3%, or 30 million people. Even that might be pushing it, but the upshot is that the off-Earth population by 2100 might very plausibly be 0.6% of the human population, or 60 million people. Now, it could easily be much more or much less than that figure, but it’s worth noting that 60 million put together is equal to some of the world’s more populous countries.
Human Population into the 22nd Century
Much more interesting is what will happen in the 22nd century. What will be the population growth rate once the demographic transition is complete? If the more-developed countries today are any indication fertility will drop below the replacement rate of 2.1 children per woman. Even some developing countries are slipping below replacement levels; the only major region still significantly above replacement fertility is sub-Saharan Africa, and even there fertility is dropping as economic development advances. If the whole world economically converges with the more developed countries, global fertility will be perennially below replacement; unless death rates drop to far below levels seen in today’s more developed countries, human population will start to shrink at this point.
How fast will it shrink? This primarily depends on how low the fertility rate is over the course of the 22nd century. Low fertility rates are often considered an economic problem, but actually they are only a problem for government budgets and pension systems, not economic growth. A 2014 study found that the fertility rate that maximizes the standard of living for every person (as opposed to revenues flowing to the state) is around replacement, 2 per woman; after accounting for the larger capital costs of developing a larger workforce the per-capita-wealth-maximizing rate drops to “moderately below replacement”. This amounts to 1.6 per woman, possibly even modestly lower than that according to the authors. Perhaps not coincidentally the average fertility rate for developed countries is not far from that 1.6 per woman mark to maximize per-capita living standards.
If we assume a gradual drop to 1.6 per woman by 2060, human population peaks at 9 billion or so in the early 2050s. More to the point if the fertility rate stays at 1.6 the population stands at 7.6 billion in 2100 and by 2200 drops to 3.72 billion, less than half the peak population. This 51% rate of decline per century if maintained indefinitely then leads to 1.82 billion in 2300 and 930 million in 2400; going much further out, in the year 3000 only 13 million humans would remain.
Projecting out 1000 years is an exercise in pure speculation, as even minor changes in the assumptions have very large effects when compounded over that long. For comparison, a milquetoast fertility rate of 2.4 (the current world average) leads to a 32% increase per century; sustained to the year 3000 it would lead to a human population of 134 billion. Just for fun, a Baby-Boom-style rate of 4 per woman, the highest seen in a demographically modern context, would produce a population of 109 quadrillion by the year 3000; by contrast a Shanghai-style 0.7 per woman rate, the lowest seen in a demographically modern context, would produce a population of 168 (yes, less than two hundred people) by the year 3000. Such is the power of compounding.
Space Population in the 22nd Century
Moving back to the 22nd century, the situation as of 2100 in our exploration is 0.6% of the human population, or approximately 60 million people, are in space; many of these are space tourists or temporary inhabitants, but in space they are all the same. Once these colonies are established, what sort of immigration rate will they have? Obviously this can vary widely.
The United Arab Emirates have a predominately immigrant population (albeit mostly guest workers rather than colonists) and growth there over the past 40 years has averaged 6%. If an off-world population of 60 million experienced such growth over the 22nd century, by 2200 the off-world population would stand at 20 billion, much more than the entire human population; obviously migration-driven growth would fade out long before it absorbs Earth’s entire population, but if space proves as attractive as the Emirates as a percentage of the people who live there, off-world humans may be the majority as early as the 22nd century.
A better analogue might be mid-to-late 19th century America, which unlike the Emirates already had a fairly high population but also had a high per-capita immigration rate: 6-12 per thousand people per year, which would’ve showed up as even higher had it not been for high fertility rates (almost 3 per woman after adjusting for infant mortality). Taking the high end of that range, we come to an off-world population of 198 million in 2200. This would constitute 2% of a total human population of 10 billion, but 5% of a population of 4 billion. If off-world colonies collectively garner double 19th century American rates, their population would come to 642 million in 2200, constituting 6.4% of a population of 10 billion or 16% of a population of 4 billion. In this case an off-world majority isn’t achieved so soon but space-based populations do nevertheless constitute a large minority.
As for what part of space these people will be in, over the 22nd century more objectively attractive parts of the solar system than the Moon and Mars, assuming there are any, will start to catch up with the earlier-settled worlds. The atmospheres of Venus, Saturn, Uranus, and Neptune could easily each approach Mars in population; the moons of the gas giants could easily collectively equal the Martian population. Space habitats will be an old and venerable tradition by then, and their advantages may compel a large majority of off-world humans to locate themselves in free space rather than planetary surfaces. By 2200 most space habitats and space infrastructure will probably still be located near Earth, as Earth will still have the solar system’s largest economy and population, but all the other major planets will have a whole ecosystem of space habitats near them by then.
As a very rough guess, I would say 70% (420 million) would be in space habitats and stations, with Mars, Venus, Saturn, Uranus, Neptune, and the gas giants’ moons getting 5% each (30 million people). This might sound like an enormous number in space habitats, but at an average density of 2000 people per square mile it amounts to the equivalent of only 1700 O’Neill cylinders (Island Three design). Over a century or two an average construction rate of only 10 per year would be needed.
If a more rural density of 100 per square mile is employed then the equivalent of 34,000 cylinders would need to be constructed, or 200 per year; of course in this event it’s likely the average cylinder would simply be bigger. Wild nature may be desired by space colonists more than is sometimes thought, however.
Our Wild Future
In the future agricultural productivity will rise as the population shrinks, leading to the crops needed to feed the population taking less and less land; over the course of the 21st and 22nd centuries most land even in fertile regions like the United States, Europe, and China will no longer be used by agriculture. The vast majority of this land will be reclaimed by forest and revert to a more or less natural state, though much of it will be used for ranching and tree-farming as food farming retreats. Today’s woodlands will have aged into old-growth forests by the year 2200. Wild animals extirpated by farmers from these regions will return; the wolf, for example, was once found as far south as the Gulf of Mexico, and if uncontrolled could easily return. In the 22nd century most Americans may be able to hear wolves howling in the night.
Advances in technology and infrastructure very likely to be obtained by the 22nd century will allow the now-urban masses to live in these wilder lands. Personal aircraft or advanced automobiles traveling at 300-600 mph mean people will get around 6-12 times faster than the urban norm today; 300-600 miles then would be as easy a commute as 50 miles is now. 6-12 times the distance for sprawl equals 30-150 times as much area; 7-35 acre lots are the new quarter-acre lots. Urban sprawl becomes rural sprawl. This is actually likely, since it is merely a more advanced stage of the population de-concentration we’ve already been seeing in some form since the 19th century.
So by the later 21st century and certainly the 22nd century it may be standard to provide Earth-to-space-habitat immigrants the rural and natural environment they are accustomed to, rather than an urban and artificial environment. This will be more expensive than a more crowded habitat, but the marginal cost of shipping more material off an asteroid (where most space habitat material will come from due to lower launch costs) won’t amount to much; in a future era of cheap spaceflight “every gram counts” is as obsolete as “every byte counts” in our present era of cheap computing.
It is an open question how a large Earth-like natural environment fits in on a planetary surface; it would be more difficult to construct, and is one reason why free space probably will prove a more popular destination for colonists by the 22nd century. Large pressure domes could be constructed on Mars or the Moon, albeit not as large as the ones in free space, but the main attraction may be the natural environment of the planet itself. Spacesuits by the 22nd century might be advanced enough to permit almost as much movement and ease of wear as shirt-sleeve clothing does today, enabling full enjoyment of places like Mars.
An interesting feature of this more rural future is that fertility rates would, all other things being equal, rise compared to the early to mid 21st century; urban life induces a significant drop in fertility and birth rates, likely due to being a more crowded environment. Humans tend to have more children when living in a more open space. Not a great deal more; after all, fertility is generally significantly below replacement even in the most rural areas of developed countries, but enough to make some difference. In fact it’s among the strongest factors influencing population growth. Another one is the number of years of formal education, which has a strong enough effect that it might explain most of the gap between modern fertility rates (and the wealth-maximizing fertility rate as well) and replacement fertility rates. The next two centuries might see a decrease in formal education; simply accelerating the legions of students already performing above grade level would accomplish this, let alone anything more radical.
A world where the population (both on- and off-world) is predominately rural and formal education is completed not long after puberty will have significantly higher fertility than today. Religious sects that promote larger families will also constitute a larger share of the population, assuming their fertility rates and retention rates don’t decrease substantially, which will also tend to increase the fertility rate. These factors would combine to increase total human fertility to around replacement at most, but it seems likely population decline over the next two centuries will be slower than the constant fertility of 1.6 scenario would suggest. The lack of growth in the total human population combined with migration off Earth suggests that Earth’s population is very likely to shrink in the solar system of the near future.
Similar considerations as we’ve covered in this post apply to the settlement of other solar systems as humanity expands across interstellar space; earlier-settled solar systems will have a massive initial advantage over later-settled solar systems, evening out due to natural growth and migration. As long as interstellar transportation remains constrained by near-future technology, journeys by people will take decades and thus will be undertaken exclusively by those seeking to move between stars on a permanent basis. Dispersion of the population across the stars will take correspondingly longer than dispersion across the planets of our solar system. The patterns of interstellar colonization and migration, and the likely course that would take in the early stages of that chapter of space colonization, is a subject for another post.
We have seen here a realistic outline of what the demographics of the human population will be in the next couple of centuries in the context of space colonization and especially off-world migration. When realistically worldbuilding the solar system of the near future the maximum population off-world colonies will have is quite constrained compared to the gigantic visions of the typical space opera. Space opera authors, artists, and worldbuilders to their credit are usually aware of this, and thus usually set their works in another galaxy or an era much later than 2200.
The demographic issues raised in this post, and some historic issues not raised here, are the reasons for this, not the technology being unrealistic. High technology, great wealth, and rapid affordable interplanetary or interstellar travel could certainly occur by the end of the 22nd century, but vast densely-populated city-planets spanning the galaxy filled with human cultures completely alien to our own that care as little about 21st century history as we do about the ancient or medieval world is not something that will realistically happen that soon. Being cognizant of these concepts and their rather substantial implications for how settlement of the solar system will play out in the next few centuries is fundamental for any worldbuilder, artist, author, or futurist seeking to create a plausible human future in the solar system.