Colonizing the Oort Cloud: The Final Frontier of the Solar System

Once mankind’s footprint has touched worlds across the solar system, what will be the last frontier? In the solar system of the near future, colonization will almost certainly spread outward from near-Earth space. The planets and settings closest to Earth will be settled first: Mars, Venus, Mercury, the Moon, and the asteroid belt. Although there may be surprises in store, due to the much greater distances to the outer solar system it wouldn’t be surprising to see Mars and the Moon harbor thousands or millions of inhabitants while the ice and gas giant systems hold only explorers, research outposts, and homesteader-style pioneers.

Nevertheless, eventually any place that presents any advantage at all to people living there will be settled, assuming transportation is simple and affordable. As spaceflight technology advances it will only become cheaper to leave Earth and go off-world; by extension the high-speed interplanetary flight methods needed to make flying to, say, Neptune easy, cheap, and relatively fast will also only become cheaper over time. Once this occurs colonization will proliferate across the entire space that can be easily accessed. To some extent this will happen even without any spaceflight advances; even using off-the-shelf current technology it’s rather affordable to ship cargo or personnel to Mars (especially Mars Orbit) if your starting point is Earth orbit. This was touched on in a previous post on this blog.

It is often asked why this would happen without some compelling economic motive, and indeed without an economic motive the rate of off-world migration will be slow and populations will remain light. However, the sheer thrill, novelty, and beauty of living in space will inspire some number of people to live there anyway, much as tourist, retirement, and resort communities develop on Earth in otherwise unprofitable locations. Wealthier people that have investments or business assets elsewhere, especially, can live almost anywhere that is technically feasible to reach if they so choose. This also applies to a lesser extent to anyone with a location-independent lifestyle, from digital nomads to off-grid survivalists and homesteaders. Economic development over time has supported a number of resort and tourist communities that are growing faster than the overall economy; this trend in the centuries to come could easily extend into space.

Illustrating this point, very few regions on Earth are completely devoid of people; even Antarctica has a few thousand people at any given time, adding up to a few thousand square miles per person. This very light population density on Mars would still add up to fifty thousand people worldwide. In Mars’s case, a population density similar to Greenland would add up to a planetary population of five million. An economic landscape that supports only very light settlement (by Earth standards) still adds up to a lot of people when totaled over the vastness of space.

Thus, once the technical ability exists to reach somewhere at a price people can afford, people will head out there and some will live there, as they have in every region on Earth. If the settlement intensity of the backwaters of the solar system is remotely comparable to the backwaters of Earth, there will still be a lot of people living in these sort of locations. Who would these people be? What sort of colonies would they establish?

Into the Outermost Solar System

In this post we will focus on the most distant part of the solar system from Earth: the Oort cloud. Once human settlement has reached the major planets of the outer solar system, it will expand into the Kuiper Belt, into places like Pluto, Eris, Makemake, and Haumea. The last unsettled region in the solar system after this will be the most distant from Earth. The Oort cloud is a theoretical region: no one has observed objects in this region to date because they are too small and faint for current telescopes to detect. Nevertheless, the orbits of the long-period comets that pass by Earth indicate almost all of them come from this part of the solar system, and reason suggests that the comets that are nudged into passing by Earth are only the tip of the iceberg, i.e. there is a much larger population of cometary objects out there. This is the Oort cloud.

This outermost region of the solar system is thought to start at a distance of 2000 AU (for comparison Pluto orbits at around 40 AU), and ranges outward to anywhere from 5000 AU to 50 000 AU. The outer edge is ill-defined and depends on gravitational interaction with passing stars, to the extent that it has been speculated that the outer edges of one solar system’s Oort cloud may overlap with another solar system’s.

The outer Oort cloud alone may have trillions of objects larger than one kilometer, and billions of more than twenty kilometers, adding up to a total mass of five Earth masses or so. The inner cloud is thought to hold tens or hundreds of times as many objects as the outer cloud, with correspondingly greater total mass. This is a lot of real estate; particularly since these objects are small, their collective land area in this region makes up much (if not most) of the entire solar system’s total!

Challenges of Cometary Living

There are only a few real downsides to this abundant virgin land. First, these objects are separated by great distances, for outer Oort cloud comets perhaps around 20 AU (similar to the distance between Earth and Uranus). In the inner cloud distances will be closer but still very remote by the standards of the rest of the solar system. Secondly, the Sun is very dim that far away; at 2000 AU the Sun’s apparent magnitude is around -10 (for comparison from Earth it is -27; the full moon’s apparent magnitude is -12). At 5000 AU it is more like -8 (much dimmer than the full moon), and at 50 000 AU it is -3 (not all that much brighter than Sirius’s -1!). Thirdly, Oort cloud objects are thought to be depleted in heavy elements such as uranium and most metals, instead being composed mainly of water ice, methane, ammonia, carbon dioxide, and assorted molecules made up of the lighter elements.

Of course, the remoteness itself may be an attractive characteristic for those who wish to isolate themselves from the rest of civilization, perhaps including hardcore survivalists or homesteaders, cults, and ascetic individuals or groups such as monks. This would be by far the best part of the solar system for isolationist groups or individuals to live in. Accordingly, these sorts of people will likely be the earliest colonists of the Oort cloud, aside from those scientific researchers studying the comets, searching for clues about the primordial solar system. Because of this remoteness and the fact it will be settled relatively late, it will likely remain the least populous part of the solar system for a very long time.

The dim sunlight means that illumination will be comparable to nighttime on Earth (full moon-like in the inner cloud to new moon-like in the outer cloud) even in broad daylight. Paradise for a vampire perhaps, but humans would find it very challenging to live in what would amount to constant night. Polar night on Earth is the closest comparison, and people seem to do alright in that, but one would prefer a more hospitable environment. Artificial illumination is the most obvious solution, and if it replicates the spectrum of sunlight it may prove adequate. More intriguing, though, is the idea of amplifying the natural sunlight that exists through lenses. The same sort of mirrors that concentrated solar power plants use on Earth could be used to concentrate sunlight in an Oort cloud context; existing technology is more than capable of amplifying moonlight to be as bright as sunlight, and should be suitable for the purpose of amplifying Oort cloud sunlight to be as bright as Earth sunlight.

The raw materials colonists would need for life support and basic industry are abundant enough; the most common metals are found in the needed quantities, even if they aren’t as thick on the ground as somewhere like Mercury. Carbon fiber and the like could be easily made from carbon distilled from carbon dioxide ice, which is commonplace on comets. Water and thus oxygen, in addition to nitrogen, are abundant, as are all the essential nutrients; indeed, comets have more of the raw materials needed for life as a percentage of their mass than any other kind of body. This, in combination with the industrial materials, makes comets very attractive targets in terms of space economics. The only real challenge is that the ingredients for life and industrial civilization tend to be rather raw even by outer space standards and need a lot of processing to be usable.

Power for Oort Cloud Colonies

In terms of raw materials the relative depletion of heavier metals is a larger problem. Where the heavy element crunch becomes an issue is energy: solar energy’s power density is extremely low this far from the sun, which leaves nuclear energy as the only viable power source. Nuclear fission requires uranium, plutonium, or thorium, which are all rare in cometary bodies.

Breeder reactors can recycle fuel, but new fuel (what will be needed to provide power in addition to what they start with) will need to be sourced from difficult refining of what little fissile material exists in cometary material, or imported, thus compromising the self-sufficiency colonists in the Oort cloud would likely prize. As long as nuclear fusion reactors are unavailable or remain in an experimental stage, nuclear fission breeder reactors will be the dominant power source in the outer reaches of the solar system.

Nuclear fusion reactors would make life in the Oort cloud much easier when they become cheap and abundant, as the fuel for them is light as opposed to heavy elements. Hydrogen isotopes in particular are abundant on comets, and the power density from fusion is much higher than fission anyway. Fuel for different fusion reactions can be bred much like in fission, thus enhancing power output over the long term. Ultimately nuclear fusion reactors will become the power source of choice for Oort cloud colonists, if they aren’t already during the first settlements.

This excellent post at Centauri Dreams goes over the topic of colonizing the Oort cloud, but also brings up speculation (from a 1983 Los Alamos paper by Eric Jones and Ben Finney) that starlight could be collected with advanced photovoltaics and used as a renewable source of power. A mirror array would need to be 150 000 kilometers across to power a colony of a few hundred people. This doesn’t seem to be very practical when compared to fusion; nuclear fusion fuel may not be renewable but it isn’t that scarce, either. For the foreseeable future fusion should be more than sufficient to power any colony’s needs.

Even if nuclear fusion reactors remain unavailable, however, there are hybrid fission-fusion options available which are rather pointless on Earth but may be useful in a setting with scarcer nuclear fission fuel. Perhaps the most dramatic is Project PACER, which uses hydrogen bombs (fusion bombs that are triggered with a fission reaction) exploded in an underground cavity to drive a steam turbine for electricity. More exotically, antimatter instead of fission could serve as a trigger, which would provide certain advantages but more in the area of small-scale power sources and spacecraft propulsion but not so much in something like PACER.

Another possibility would be combining this post’s idea of a solar amplifier lens with photovoltaic panels, but the area that would need to be covered with sunlight to generate sufficient power would be much larger than for personal needs, and the power density once one accounts for the cost to build the concentrating mirrors would be nowhere near a nuclear reactor. In particular the power density would be orders of magnitude poorer than concentrated solar power plants on Earth.

Between the Comets with Nuclear Pulse

In the outer Oort Cloud, transportation technology would have a far larger impact than it would in less exotic inner solar system settings that are much more commonly explored in science fiction and scientific speculation. This is because the average separation between comets (and hence each colony site) is around 20 AU. It took the Voyager probes nine years to travel a comparable distance, so obviously our colonists of the outermost solar system will want something much faster. Solar-based methods such as solar sails obviously won’t work well this far out, and this leaves nuclear power as the only viable option. Nuclear thermal rockets that we could build now could cut travel times in half, to only several years. More advanced models could achieve further reductions, but travel between two Oort cloud colonies would still require travel times comparable to a 16th century circumnavigation of the Earth.

Nuclear pulse propulsion, on the other hand, can easily reach one percent of light speed or so even with early versions of the technology, which may not be enough for a viable interstellar mission but is more than sufficient for getting around in the Oort cloud. A 20 AU trip using a constant acceleration of 1g (forward thrust for the first half, reverse thrust for the second half), thus providing artificial gravity without need for a centrifuge, would take only twelve days and reach a maximum velocity of 1.8% of light speed. This makes transit between colonies quite easy, and would be the dominant spaceflight technology in the outermost solar system.

In the inner solar system nuclear energy may be the most efficient or effective option, but there are many alternatives that are viable, particularly solar sails. In the Oort cloud there is no alternative to nuclear technology for anyone that desires a high standard of living or any capacity to interact with wider civilization. Without nuclear propulsion even links to other Oort cloud colonies will be more or less severed with the exception of radio or laser transmissions.

The concerns sometimes expressed about nuclear pulse propulsion or even nuclear proliferation on Earth will be much less relevant in the Oort cloud; sheer isolation will prevent any accidents or recklessness in one colony from affecting anyone else for the foreseeable future. It is hard to imagine an environment much more hospitable to even the nastiest or most dangerous nuclear technologies than the Oort cloud.


For the same reason it is friendly to nuclear technologies the Oort cloud would be an ideal place for scientific experiments that are for whatever reason extremely dangerous and require the highest level of isolation feasible.

The presence of such a powerful means of propulsion raises the question of interstellar travel. If a community is already used to isolation, probably by deliberately seeking it out, they may find a way to become more isolated still and undertake a pioneering journey at the same time: by traveling to the stars. The same nuclear pulse propulsion that can propel a ship at 1% of light speed could propel a whole colony, comet included, at the same velocity. At such a speed Alpha Centauri could be reached in 430 years. At 5% of light speed, which would require more advanced technology, Alpha Centauri could be reached in 86 years. Generational time scales would be required to undertake such a “slow boat” journey, but even at more normal speeds and normal colonization rates, eventually the Oort cloud civilization will expand so far out that the front line of colonization will be closer to another star than the Sun. From there, perhaps over a period of many centuries, their descendants will slowly migrate inward into a new solar system, and start inner solar system civilization anew.

In this way human civilization could expand (very gradually) to the stars even if no dedicated manned interstellar mission is ever launched. This opens up the next stage of the final frontier, that of other solar systems beyond our own. The solar system’s last frontier, the Oort cloud, ultimately proves a mere stepping stone to a wider universe of possibilities.

Nevertheless, for the reasons outlined previously the Oort cloud will almost certainly remain lightly populated for the foreseeable future and likely unimportant in the context of humanity as a whole. Eventually, however, the extremely large surface area of the Oort cloud may attract larger and larger populations over time as propulsion technology advances and makes trips of even 20 AU relatively short and cheap. The total amount of raw materials available, while not nearly as large a quantity as the gas giants or the sun, dwarfs the Earth, and may prove economically important in the very long term. Fabrication of space habitats such as O’Neill cylinders or even larger constructions comes to mind. Of course, the same propulsion technology that makes 20 AU journeys trivial also makes shipping the raw material inward to where there is daylight instead of constant night (a much more desirable area for most humans) trivial, so it seems unlikely the Oort cloud or similar environments will ever hold the bulk of the human population.

The Kuiper Belt, on the other hand, which is in the outer solar system but enjoys daylight levels of illumination, is the outermost part of the solar system that stands any real chance of being the future core of human civilization. The Oort cloud, although destined to remain a marginal player in the solar system as a whole, may someday harbor a much larger population than parts of the inner solar system. This is a future that, although only seldom explored, provides a rich setting for science fiction writers, worldbuilders, and futurists alike.

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