The concept of space colonies capable of providing an Earth-like living environment to thousands or even millions of people has been extensively studied since the 1970s. IST shares the dream of one day constructing "McKEndree Cylinders" thousands of kilometres long, using carbon nanotubes as the main structural material. However, along with the question of how such colonies can be constructed, comes the question of where they will be placed. Although it may be possible to position a McKendree Cylinder in a stable orbit somewhere in the Inner Solar System, it is doubtful that such a construction will ever take place. The natural place for space colonies is in the Outer Solar System, and generally speaking the larger the colony structure, the further from the Sun it should be. Enormous space colonies located in the Oort Cloud and even beyond that in interstellar space are a distinct possibility for the future.
Space colonies located in the Inner Solar System are largely an engineering exercise and will be limited in size to O'Neill cylinders tens of kilometres long and these will be placed in solar orbits intermediate between that of Earth and Mars, and of Earth and Venus. Smaller prototype colonies will be placed in wide orbits around Mars or Venus, and will be useful as transport hubs and acclimatisation facilities for persons moving to larger space colonies. Similarly in the future, the larger space colonies in Solar Orbit in the Inner Solar System will act as transport hubs and acclimatisation centres for persons moving to the Outer Solar System colonies.
Standard designs of increasingly larger space colony structures based on spheres, torii and coupled rotating cylinders respectively are well known and extensively studied. The number of such structures envisaged for the Inner Solar System is not huge. At most about 200 O'Neill cylinder pairs and about the same number of smaller designs will be constructed. More could be built, but this would only increase the chances of collisions either between the colony structures themselves or between colonies and the Inner Solar System planets and local infrastructure. A population of about 100 million persons in space colonies should be adequate to accommodate persons in transit to and from the Outer Solar System, space tourists and persons who wish to stay on these colonies for various socio-economic or personal reasons.
In that sense the figure of 200 O'Neill cylinders in the Inner Solar System is a long term limit. The needs of a Solar System economy with transit to and from the Inner Solar System via space colonies in solar orbit could easily be taken care of using torii type designs which accommodate about 10,000 persons and which can be self-sufficient for a few years at a time. O'Neill cylinders can accommodate about one million persons for a cylinder about 6 Km long, to 10 million persons for a cylinder about 32 Km long. The total mass of such cylinders would be over 100 million tons for the smaller structure and over one billion tons for the larger. About 40 percent of that mass would be the air inside the cylinder.
It is not really practical to lift that amount of material from the Inner Solar System planets or the Moon. It would be very desirable to use asteroid materials. The construction equipment and infrastructure could be built up around Mars and Venus using material from these planets, but the bulk mass for the large colonies themselves would really have to come from asteroid mining. If, as is proposed here, the cylinders themselves are constructed from carbon and possibly silicate composite materials, then there is no shortage of such materials in the asteroid belt to create thousands of such large cylindrical colonies if required. Oxygen and is plentiful in the asteroid belt, carbon also is found in quite adequate quantities. Hydrogen probably exists in sufficient quantities on some asteroids, but the amount of hydrogen required for a space colony is quite small by weight compared to the overall weight, hence this could be imported from Earth or Venus or even Mars itself if necessary. However, one element which is required in bulk and which is not so plentiful in the asteroids is nitrogen.
This problem comes up from time to time in advanced space economy planning, namely that the Solar System as a whole is somewhat deficient in nitrogen. This is a genuine stumbling block for building O'Neill cylinders in the Inner Solar System. There exists many trillions of tons of nitrogen in the atmosphere of Venus, Earth and even the atmosphere of Mars, whose total weight is only 25 teratons is still about 2 percent nitrogen. Unfortunately there is no foreseeable way to lift billions of tons of this material off these planets and into solar orbit. The carbonaceous "C-type" asteroids may contain about 0.1 % to 0.3 % nitrogen by weight, but this would require the processing of trillions of tons of asteroid material to get the billions of tons of nitrogen required for several large O'Neill cylinders.
Nitrogen is found in quantity in the Outer Solar System, but once again not much of it is in a particularly useful place. The atmosphere of Titan and the surface of Triton contain many trillions of tons of nitrogen. This would be difficult to obtain from the Titanian atmosphere, however with Triton, it is possible that large rugged mass drivers could lob solid blocks of nitrogen into orbit at about 10 tons at a time. One hundred or so of these drivers could lift about one million tons per day into orbit. In this way the nitrogen required for a large O'Neill cylinder could be acquired in less than 5 years. It is possible that nitrogen obtained from Triton and other Outer Solar System resources could be exported back to the Inner Solar System, but this would required a huge transport infrastructure, although there is no real limit on how large you can make solar sail type craft for example.
It is more likely the Outer Solar System will use its own nitrogen to develop colonies out there in the vicinity of Neptune in particular. Looking to the long term future if nitrogen is most readily found on large Kuiper Belt type objects, the most likely location for super-large colonies of the McKendree type is in the Kuiper Belt itself. Even though the Oort Cloud may be a very interesting proposition for colonies in terms of the scientific and interstellar potential of Oort Cloud colonies, seeing as the vast majority of Oort Cloud objects are comet-like and probably less than 1 % nitrogen by weight, these colonies may be initially restricted to smaller scale structures of the torus or spherical type.
In the the long term the nitrogen problem is bound to be solved, even by nucleosynthesis if necessary. Advanced physics will allow colonies to be placed in more and more remote regions, which will be attractive for their potential for fundamental physics research. This could see a migration pattern of colonies that moves not towards the noisy centre of the Galaxy, but outwards, eventually into inter-galactic space and ultimately into the super-cold regions between galactic "walls". Such colonies would need advanced physics and advanced engineering based on that science, but ultimately if technology can make use of or harvest vacuum fluctuations of virtual particles, then they will have no need or desire to live in the noisy baryonic cluttered galactic regions of the universe. Of course eventually the "technology" will probably include life itself which will have mapped itself into virtual forms that exist purely as virtual particle patterns. They will probably interact with real particles only in ways that are required to avoid interference in their inhabited regions of space, and perhaps in subtle other ways that can effect change in the normal tangible world where they feel it is merited.
Other aspects of space colony engineering are fascinating also, but fairly well understood. Power supply can be solar panel generated and include thermal gradient generation methods to supplement this. Electric propulsion will probably suffice for attitude and manoeuvring control, albeit at very slow speeds. Overall thrust for O'Neill cylinders will probably be measured in micro-Newtons, hence changes in orbit can only be effected over a very long period of time. In the case of counter-rotating O'Neill cylinder pairs, some attitude control can be affected using the cylinders themselves as control wheels. A load on the connecting bearing will cause slight precession of the system which can serve to keep the cylinders pointed towards the sun as they move around each solar orbit.
As regards timetables of phased deployment of space colonies in the Inner Solar System, the rate of nitrogen availability in solar orbit will be very much a limiting factor. Currently, a pipeline into space using reusable technology that would take one million tons of nitrogen to LEO would require about 200 High Altitude Launch Platforms lifting an average of about 15 tons per day to LEO. To have any hope of building large O'Neill cylinders, this would require about 1 million tons per day, which would provide enough nitrogen for a large pair of rotating cylinders about every 5 years. This could be lifted from Venus, Earth of Mars or extracted from certain asteroid minerals and tholins materials. It is possible that on Mars, very large spaceplanes could be operated in the low gravity, thin atmosphere environment. They could be capable of lifting 1000 tons per flight to LMO and be strong enough to be heavily used in a long repetitive cycle regime, carrying up to 20 loads per day to LMO. About 50 such craft would be needed to meet the entire demands of continuous large O'Neill cylinder construction. The Martian atmosphere is estimated at about 25 Teutons of which about 2 per cent is nitrogen. 200 large O'Neill cylinder pairs would require about 400 billion tons of nitrogen which is about the entire mass of nitrogen in the Martian atmosphere. On Earth 60,000 High Altitude Platforms would probably cause too much environmental concern. On Venus, it is possible that the space pipeline could be stepped up. High Altitude Platforms on Venus could be constructed for vehicles that could lift about 10 tons at a time in a continuous cycle. We would need about 5000 of them. This would be difficult and it would take some time to get going, but not impossible. And of course Venus has plenty of nitrogen, the Venusian atmosphere is about 500 million teratons of which about 3.5 percent or 17.5 million teratons is nitrogen.
Venus then will be an extremely valuable source of nitrogen for the initial phase of large O'Neill cylinder construction. This will aim to build 1 such rotating pair every 5 years and to place them initially in solar orbit fairly close to Venus. These will drift slowly to an orbit more intermediate between Venus and Earth. Similarly on Mars the aim will be to build several large rotating cylinder pairs over a period of some 20 years, however a limit will be placed on the amount of nitrogen that can be removed from the Martian atmosphere, since this is a valuable asset for Martian surface and subterranean habitats into the very long term future. A limit of about 50 billion tons of nitrogen from Mars should be sufficient. After that it must come from Venus, the Asteroid Belt or the Outer Solar System.