Space Habitats: O’Neill Cylinders, Stanford Torus’ and the Future of Space Habitation

March 08, 2025

Space Habitats: O’Neill Cylinders, Stanford Torus’ and the Future of Space Habitation

The Evolution of Human Space Exploration

We have looked up at the stars for millennia. They have inspired us to dream, they have been the subject of our gods. They were perfect and free from the chaotic mess of human existence – seeming to symbolise the greatest and most divine order. Yet as time went on our knowledge became more accurate, and we began to understand the stars for what they are. Abiding not by the laws of the divine, but by the same universal laws of physics that describe the movements and forces on our own planet. Potentially the greatest discovery of all was that which led to the end of our human-centric picture of the stars. The stars and planets do not evolve around us, instead we are just one speck amongst countless others, with nothing special beside the life that inhabits it. And it is that life that has now recognised the stars as the next great realm of human exploration. In 1969, the first man walked onto the Moon, proclaiming it as “one small step for a man, and one giant leap for mankind.” For a while after that we moved much of our focus away from space exploration, yet in recent years it has re-emerged. Powered by the visions of companies like SpaceX and Blue Origin, we are investing further and further into space exploration, with plans for constructing sustainable colonies on both the Moon and Mars within this century.

If we can persevere to maintain and advance our space exploration technologies, then one hopes that we may propel our civilisation into the stars. And if that is the case, where will we live, and what will be our means of exploration? Elon Musk has set his sights on colonising Mars, while Jeff Bezos aims to move heavy industry off of the Earth. Yet to peer into the more distant future, where we may be involved in deep space exploration, there is another means that arises. Space habitats are artificial environments created in space to allow humans to live, just as we do on Earth, enabling long-term extra-terrestrial presence both in-between planets, and in the great depths of space. Yet what will these look like, how will they be built, and what role will they serve?

O’Neill Cylinders and Stanford Torus’

O'Neill Cylinder - NSS

Many distinct, albeit fairly similar, forms of space habitat have been proposed, the first being in 1929 by physicist John Bernal. Yet there are two primary habitats that appear promising as homes of future space civilisations: the O’Neill cylinder and the Stanford Torus. One key similarity between both of these is that they are in constant rotation around a central axis, creating a centrifugal force that simulates the effects of gravity. As Einstein demonstrated in his equivalence principle, gravity and acceleration are fundamentally linked, meaning that the acceleration due to rotation can imitate the experience of gravity. The other similarity is the interior, which are both cylindrical. They would be filled with a thin layer of dirt (which a variety of plants could then grow on top of) and would consist of a wide range of infrastructure, no different to that found in a modern city. These are the general features that one would expect to be common among all potential space habitats, yet there are some key differences between the two.

Stanford Torus - Wikimedia

The O’Neill cylinder is a fairly straightforward design that will need little explanation. It is simply a regular cylinder, generally with dimensions of around 30 kilometers long and 6 kilometres in diameter, and rotating around its longitudinal axis (meaning that the axis lies in the centre of its length). On the other hand, the Stanford Torus is a donut-shaped (or torus-shaped) habitat that rotates (at 1rpm) around a central axis that lies in the centre of the donut’s hole. The central axis consists of a docking module and an array of solar panels (to power the habitat). To receive optimal sunlight, the solar panels take in light that has been heavily focused by a large mirror, as shown in the accompanying diagram. Furthermore, while the O’Neill cylinder is generally considered large enough to act as a state, or small country, the Stanford Torus is designed to house only around 10,000 people. Overall, these are the two primary space habitat options that may house our future civilisation among the stars, and we must now investigate the economics of such habitats.

Building Space Habitats: The Economics

Building a space habitat, particularly at the present, would be an enormous undertaking with costs likely in the trillions. The raw materials and power systems would cost hundreds of billions of dollars, which is potentially doable, yet the main problem lies in transportation of materials, as well as assembly, which would likely cost tens (even close to hundreds) of trillions of dollars. These prices clearly show us that space habitats are still a fair while off, and will demand much more established space infrastructure, yet – even in 200-300 years – space habitats will still prove costly. So how will they be built, and what from? One of the biggest cost and time factors in building a space habitat is launch and transportation. Since celestial bodies like the Moon, Mars, and asteroids have much lower gravity than Earth, launching materials from them is far easier and cheaper. Additionally, they are likely closer to the construction site than Earth, making them more practical sources of materials. Thus it seems that the habitat will likely not be built on Earth but will instead make use of in-situ resource utilization (ISRU) – meaning that materials will be mined and processed directly from the construction site or nearby celestial bodies. So we have now outlined the how, but we must next look at the what – the material and energy sources.

The space habitat hulls have two primary demands, namely a strong and light structure, and effective radiation shielding. The most promising structural materials are graphene and carbon fibres, which are extremely strong and lightweight, while other alternatives include titanium and steel alloys. On the outer shell of the hull, to provide radiation protections, the key options would be polyethylene and (lunar or asteroidal) regolith. Furthermore, there may be a desire for natural sunlight, which would demand strong silica glass windows, although gigantic LED lights could also serve as an alternative to natural light. Beyond materials there is also a significant energy demand, similar to that of a city (maybe more), which would need to be accounted for. The most likely energy source for this would be solar, as solar panels could receive light 24/7 and in much more focused quantities (with the help of a mirror), although compact nuclear reactors could also be used if necessary. We have now discussed all of the technical details of these space habitats, but we still have one more question to ask: who will live in them?

There are many potential purposes of space habitats, some temporary and some permanent. Temporarily, one use case could be tourism, as large-scale space habitats such as these, if successfully constructed and maintained, are clearly impressive and I think many people would be interested in checking it out for a couple weeks, while another potential purpose would be as a pit-stop for space travel to more distant space stations and celestial bodies. Yet I think the primary purpose would be a permanent (or long-term) one, and I have identified three distinct manifestations of this. The first possibility would be that we have simply grown too populous for the Earth and need to expand somewhere, although I believe this to be unlikely as it is more likely we would expand to lunar and martian colonies. Another potential would be as a home for industrial workers in areas such as the asteroid belt, yet I see this also as unlikely as, by the time we’ve built these habitats, we will almost certainly have AI doing industrial work. The final reason, and in my view the most likely, would be as a temporary, albeit long-term, home for explorers. If we had decided to make a trip, likely an interstellar one, that would take decades, maybe even centuries, how would we do it? We would need a home for the trip, and I see space habitats as the most likely candidate for this, although the fuel demands will be tremendous (but that’s a different problem). Overall, space is the next great frontier, and space habitats offer themselves as a great home for our future civilisation.