Space habitats 101
In today’s tutorial: What is a space habitat?
Last week’s article about getting to space might have seemed a bit underwhelming. At least that was Tove’s opinion about it. I sort of agree. But, as the article tried to convey, getting to space is the first step towards being in space. And being in space is what it is all about.
Wood from Eden was originally the blog of both me and Tove. Then some of Tove's posts went viral, attracting people who like the kind of stuff that she writes. That means that most of you readers are here because you read and liked one of Tove’s posts about anthropology or gender relations. You are probably not that interested in technical posts about things in space.
But maybe you should be. Another of Tove’s favorite topics, that might have lured a minority of you, is pronatalism. Pronatalism is good for a number of reasons. But it only works as long as there is room for expansion. In other words pronatalism, and growth in general, requires space. And space is literally what space is made of.
If humanity can take the leap away from the planetary surface we are currently inhabiting the possibilities are vast. Both the raw materials from the asteroid belt and the energy from the sun are practically infinite. We will have room for several millennia of exponential growth. But in order to do this we need to find a way to actually live in space. We need to figure out what a space habitat is.
What is a space habitat?
A space habitat is, as its name suggests, any type of human habitation in space. Some people include habitats on other planet surfaces than Earth's in this definition. But personally I think that is to stretch the phrase way too far. In this article we are only interested in habitats in outer space, that is, constructions floating around in the emptiness between celestial bodies. A location characterized by weightlessness and vacuum and not much else.
Skeptic readers might wonder why one would want to live in such a strange place. It is a good question that mostly boils down to the fact that it is better than the alternatives. Had there been another planet just like Earth just waiting for us to come live there, that would have been great. But there are not and the closest candidate, Mars, is a terrible option (which I argued for in this article).
Outer space also comes with some advantages all of its own. Complete emptiness gives complete freedom to construct whatever we want. The natural world seldom constructs something that is perfectly suited for humans, just ask the average northern European what they think of their world on a rainy November morning. Outer space is a clean slate in which we can do whatever we want, suited exactly to our needs.
Finally, outer space is not really empty. There is something very valuable lurking there, namely energy. The sun is always shining bright in outer space and due to the weightlessness it is comparatively easy to build large solar plants. The result will be that the arguably most important resource of all, energy, will be very cheap and perfectly clean. That is at least something.
Overcoming outer space
There are four fundamental problems that have to be solved in order for space habitats to be inhabitable for humans.
1. The first is gravity, humans need gravity and that has to be created artificially.
2. The second is radiation protection, space is full of cosmic radiation that is lethal or at least unhealthy for humans.
3. The third is light, neither humans, nor the plants we depend on, can live without light. Space is full of solar light, but due to 1. and 2. it is a bit complicated to get the light to where it is useful.
4. The fourth is cooling. This might sound surprising since outer space is cold and empty. But due to 2. and 3. humans will have to live in heavily insulated containers into which a lot of heat-generating light is inserted. Without cooling systems the space habitants would quickly be toasted.
The solutions to each of these problems are pretty straight-forward. Artificial gravity is easily created by rotating which gives rise to a centrifugal force that can stand in for gravity. Radiation is blocked by mass, simply making the walls thick enough will do. Light can be shifted around using mirrors and cooling is no more difficult than the HVAC systems most habitats on Earth have.
The challenge is to combine the solutions in an optimal way. Several of the problems listed above also amplify each other or create wholly new problems when combined. For example the radiation protection is simple in theory, just make the walls 10 meters thick and you should not have a problem. Of course, 10 meter thick walls will be very heavy when they are rotated to create centrifugal force. That can be solved by designing the radiation protection as a non-rotating outer shell. But then you have a rotating object within a non-rotating outer object which will make it very difficult to transfer excess heat from the interior to the exterior.
Space habitat design is filled with trade-offs like these. They are not very well understood since the whole business of space habitat design is purely hypothetical. Most probably, a whole new host of problems will materialize once humanity actually starts building something habitable.
The shape of things to come
The need to rotate space habitats to generate artificial gravity means that we can be pretty certain of the shape of these future constructions. They will be round in some form. The pseudo-gravity created by rotation is dependent on the rotational speed which in turn depends on the rate of rotation and the radius of the object rotating. A greater radius means less rotations per minute which is generally believed to be a good thing. Centrifugal force is not a perfect substitute for gravity. The rotational movement means that a Coriolis force will be created, which means that when you drop something in the rotating space habitat it will not fall straight down but rather move in an arc. Although no human has ever experienced this it is believed to be nauseating. A greater radius lessens the Coriolis force and makes things appear more normal.
There are three basic shapes which are plausible for space habitats: a sphere, a cylinder and a torus (a donut shape). Each shape has advantages and disadvantages and all three have been suggested for future space habitats.
The sphere is the most basic and also the most efficient in creating volume per unit of surface area. This means it is possible to get a lot of volume for limited amounts of building material. Since building materials are hard to come by in outer space this is a good thing. However, volume is not that useful in a space habitat. Liveable area is more important, and in this regard the sphere is lacking since its interior area is sloping, which will give wildly differing gravitation.
The cylinder is more efficient than the sphere in that it creates a greater area with a uniform radius and thus a uniform gravity. The flipside is that the construction requires more enclosing for the same volume meaning higher material requirements. In fact, the cylinder is not only geometrically inferior to the sphere, it is also structurally inferior, requiring a sturdier shell in order not to be deformed when rotated. If it is not strengthened it will start to bulge outwards in the middle of the sides and get a more sphere-ish shape.
When talking in terms of space habitats, a torus could be viewed as a cylinder with a roof. Instead of enclosing the entire volume of the cylinder one only encloses a small volume over the liveable areas. This obviously saves on the amount of air one needs to fill the habitat with. More importantly, it also saves radiation shielding. Not for a long and thin cylinder like the Kaiser Science space habitat pictured above. For such a shape it would make more sense to just cover the end caps with radiation shielding than put a roof on the entire interior. But for a short and wide cylinder, the type with a large radius that gives lower Coriolis forces, it is very economical to put a roof over the liveable area instead of covering the gigantic sides with shielding.
The state of things
I have to be honest here. The likelihood of any space habitats, no matter the shape, being built in the near-term is practically zero. Since space got out of fashion (and Nasa’s budget shrinked vertiginously) in the 1970s, hardly any serious public thinking has been spent on space habitats.
What little effort has been made has gone into space stations, shelters in space where astronauts can stay for a time while doing research or just waiting for the next phase of their mission to commence.
Space stations have hitherto been simple vessels orbiting close to Earth. They have lacked gravity, radiation shielding and natural light, all things that are required for real space habitats. But at least a handful of space stations have actually been built and sent into space, which is more than anyone can say about space habitats.
The border between space station and space habitat is a bit fuzzy. Some people call space stations a type of space habitat. I cannot say they are entirely wrong. The upside here is that space stations can become more space habitat like. If space stations are large enough and expensive enough they will more or less have to become real space habitats. In the real world that progress seems to be going so-so. But dreams are cheap and planning too, so a number of such boundary pushing space stations have at least been contemplated.
Fantasies of large space stations
Fantasies of large space stations have a long history. Very long actually. In the 1950s the Space Age had not even begun and the American space program was only just taking shape. To be fair, it was not really the American space program. Rather it was a German space program financed by American taxpayers.
The most famous, and arguably the most ingenious, of the hundreds of German engineers that would build the rockets and space crafts that would take American astronauts into space, was Werner von Braun. Best known for designing the Saturn V rocket that took astronauts to the moon, von Braun had many other bold ideas for space.
The one we are interested in here is the von Braun wheel, a torus-shaped space habitat that von Braun designed in 1952. Being less than 100 meters in diameter, the von Braun wheel is not a space habitat as much as a space station with artificial gravity.
It was supposed to be made of heavy-duty nylon, that way overcoming one of the main problems of rotating space stations, namely that they have to be of a certain size that is invariably bigger than the cargo compartment of orbital rockets. These size restrictions were overcome by making the habitat in a soft material. The entire wheel was supposed to be folded and packed in the cargo of a large rocket. Once in orbit it would be inflated with air and the gas pressure would make the habitat take on its circular shape.
Jumping to more contemporary times there are no shortage of concept papers describing potential space habitats, some even written by real space engineers and financed by real space organizations. One such is by Peter Curreri, a Nasa engineer, who presented the Curreri space habitat in a 2007 paper:
The Curreri habitat is designed to be as minimal as possible but still enable human habitation. To achieve decent artificial gravity without constructing giant structures Curreri envisages a bola shape, that is two dumbbells connected by cables and rotating around a shared center of gravity. To go from one dumbbell to the other one has to use an elevator-like transport that runs on the cables connecting the two dumbbells. With the cable running transporter one can also reach the zero gravity hub of the habitat.
The smallness of the habitat has the downside that it requires a lot of material, especially shielding material, for a very limited habitat volume. The upside is that the whole construction is small and therefore inexpensive (compared to large space habitats, it will still be expensive compared to anything else humanity has constructed).
The Curreri habitat is mostly designed as a habitat during space travel (that is why there is “propulsion” written on the image). Above all it could be used to shelter astronauts on their year long trip to Mars. For such a use case it would probably work decently. But it is not much of a habitat.
Fantasies of small space habitats
Since dreams are cheap, and plans too, there are of course people dreaming of real space habitats too. When dreaming about space habitats it is hard to avoid the 1970s, the time of peak space dreams. I have already mentioned the Bernal Sphere space habitat when talking about spherical shapes. It could be worth investigating it in more depth.
The Bernal Sphere was designed by Gerard K. O’Neill (and his students, who he put to good use doing structural calculations and the like) in 1975 but the concept of a rotating sphere in space was published by J. D. Bernal, a British mathematician, already in 1929 (hence the name). The Bernal sphere was designed in several different sizes. The one pictured below, and earlier in this text, is the smallest, called Island One by O’Neill, with a habitat module about 500 meters in diameter.
The main part of the habitat, the actual habitation part, is the spherical object in the middle of the image above. It is covered by a heavy radiation shield which makes it look quite unimpressive. Light is let into the habitat through narrow slits close to the “poles” of the sphere. Two rings of mirrors, one outer by the equator and two inner by each pole, shuttle light to the slits and into the interior of the habitat.
Apart from the actual habitat there are also rotating rings used for agriculture. These are separate from the main habitat since they do not need radiation shielding or 1G gravitation, since short-lived plants are not at high risk to develop cancer and other radiation-induced ills. With lesser gravity and minimal radiation shielding they can be made much cheaper than the main habitat.
Along the center axis of the colony are also gigantic cooling panels used to radiate excess heat out in space. Exactly how this heat is to be collected in the habitat and transferred to the cooling panels is not specified in O’Neill’s texts (at least not as far as I have seen). Presumably there will be some sort of heat exchanger inside the habitat that heats a cooling liquid that is pumped to the panels where it is cooled down and returned to the habitat.
At the ends of the center axis are industry and infrastructure, communication antennas and docking ports for visiting spacecraft.
The interior of the Bernal sphere habitat should look something like in the image below (original 1970s space art by Don Davis, source). The light slits are clearly visible close to the axis of rotation. The spherical shape means that the whole habitat will be sloping gently towards the equator, where the artist (realistically or not) has placed a small river.
The Bernal sphere is in many ways utopian. But it also has nice solutions to a number of engineering problems that all space habitats encounter. The spherical shape is one such solution that gives maximum strength with minimum material. The light slits are another nifty solution, the outside mirrors concentrate the light through comparatively small slits, letting in more than sufficient light while at the same time limiting the amount of harmful cosmic radiation that is allowed in.
O’Neill also had solutions to some more obscure problems. One such is the issue of angular momentum, a rotating object will keep its direction, the property that makes a gyroscope always point in the same direction. You do not want a space habitat to always point in the same direction. This is because you want the space habitat to always point at the sun and since the habitat is in orbit, rotating around something, the habitat itself need to slowly rotate in order to keep pointing at the sun.
There are a few ways to solve this problem. It is possible to steer an object with angular momentum, but it is somewhat complicated and expensive in terms of energy. Another solution is to let the rotating habitat always point in the same direction and use non-rotating mirrors to make sure light is always hitting the right places of the habitat, no matter what direction it is pointing in. This is feasible but may entail some complicated mirror set-ups.
The Bernal sphere is using a third method: removing the angular momentum. This can be done if the rotating habitat is connected to something rotating in the opposite direction making the whole construction rotationally neutral. O’Neill solved this by using the radiation shield. The radiation shield on the Bernal sphere is very heavy, much heavier than the actual habitat. The Bernal sphere design envisages the radiation shield rotating in the opposite direction from the actual habitat. Since angular momentum is a product of both mass and velocity and since the shield is much heavier than the habitat the shield only has to rotate at a fraction of the speed of the habitat in order to cancel out the angular momentum. The end result is a space colony with no angular momentum which can easily adjust its direction to always point towards the sun.
Modern fantasies
The Bernal sphere was designed in the 1970s together with many other optimistic designs. Since then there have been significantly fewer space habitat optimists and hence fewer prospective designs. One of the few is called Kalpana One (outlined in this academic paper) designed by Nasa engineer Al Globus.
The Kalpana One is a simple cylinder, 325 meter in length and 500 meter in diameter. The dimensions are not arbitrary, the 500 meter diameter is the minimum diameter one can have while having 1G gravity and less than 2 rotations per minute, an admittedly slightly arbitrary number believed to be where the Coriolis force begins to feel nauseating. The 325 meter length is the maximum length the cylinder can have without becoming unstable.
Kalpana One’s main selling point compared to other designs is in stability, an area previously more or less ignored. It turns out that simple rotation is, in practice, not that simple. Due to imperfections in the weight balance, rotating objects in space will sooner or later start to wobble. This is not good since rotation is what gives gravity. Wobbling will make the centrifugal force, and hence gravity, uneven. In the extreme scenario the whole habitat will turn over and start rotating around another axis making a wall become the floor and the floor become a wall. This is probably what would happen to a fairly long and slender habitat like the Bernal sphere without some sort of intervention.
Kalpana One is wider than it is long in order to increase rotational stability. It also has the radiation shield integrated in the structure, meaning that it rotates with the rest of the habitat. This further increases stability, albeit to the cost of a much sturdier and more complex hull. The Kalpana One also has an active system for wobble control. This consists of accelerometers that measure rotational stability and weights attached to the outer hull with winches. By moving these weights closer to or further away from the hull the center of gravity of the whole habitat can be adjusted in real time, parrying all wobbling before it even happens.
The problem with angular momentum that the Bernal sphere worked hard to solve is completely sidestepped by Kalpana One. Instead of always pointing towards the sun, Kalpana One has its axis of rotation pointing upwards and downwards compared to the plane of the solar system. This way it always shows a rotating side towards the sun and does not need to ever change direction.
The original design for Kalpana One envisaged dome-shaped mirrors on the end caps that locked in sunlight and directed it into the habitat. That way the inhabitants got diffuse light coming in from the sides of the habitat. Later iterations of the Kalpana One design, like the one in the image above, have dropped natural light entirely in favor of artificial lights powered by external solar power plants beaming their electricity to Kalpana One.
The perfect space habitat
Living in space will never be like living on Earth. But in the right space habitat it can come quite close. Enter the perfect space habitat:
The image above shows a pair of O’Neill cylinders. Below is how it looks from the inside:
Let me explain the images. The O'Neill cylinder is a very large cylinder, approximately 32 km long and 8 km in diameter. The cylinder is divided longitudinally into six segments, three land segments and three window segments. Outside the window segments are 32 km long mirrors that direct sunlight into the cylinder and onto the opposite land segment.
This is a wonderfully elegant design. The window segments of the cylinder are covered in about ten meters of water, making artificial oceans in the habitat. This water gives radiation protection to the very large window areas. Having as much window area as land area is wasteful but makes the radiation balance approximately the same as on Earth. No active cooling will be necessary, instead the cylinder will cool itself during the night when no solar light is coming in while heat is radiating out, just as on Earth's surface.
The large window sections will even give a decent view of the night sky, stars and all. And the long mirrors can be opened and closed slowly, imitating sunrise and sunset. The cylinder's volume is so great that there will be some sort of weather inside it, complete with wind and rain. It is truly an Earth beyond Earth, a marvel of space habitats.
Back to Earth again
O’Neill cylinders are in many ways ingenious. They are always built in connected pairs, two cylinders that rotate in different directions, thereby neutralizing the angular momentum. The ring of small buckets around the top of each cylinder in the upper image are agricultural cylinders. These are small habitats in their own right but with less gravity and no radiation shielding thereby saving the main habitat for habitation only.
Despite small savings like these O’Neill cylinders will most probably never be built. For the very simple reason that they are ludicrously expensive. The ocean covered windows are nice, but spending ten tons of water for each square meter of space habitat is just nuts. Water is a slightly limited resource in space and the oceans of a single O'Neill cylinder would require 4 billion tons of water. That is clearly not going to happen, at least not anytime soon.
This is not only a problem with O’Neill cylinders but rather with space habitats in general. Who will build them? And how will they afford them? The private space launch industry I wrote about in my last post has lowered the cost of bringing things to space. But it still costs millions of dollars to place a ton of mass in orbit. Since the mass of even a small space habitat like Kalpana One is millions of tons the cost will be counted in the trillions.
And off to space again
It is clearly not possible to build a space habitat using materials lifted from Earth. Luckily, there is plenty of raw materials in space. Aspiring space colonizers of the 1970s imagined taking material from the moon. With only a sixth of Earth’s gravity and no atmosphere generating air resistance it is considerably less expensive to lift things from the moon’s surface than from Earth’s.
Today’s wannabe space colonists are mostly looking at the asteroid belt. The asteroids are vastly more distant than the moon, but the peculiarities of space travel means that distance is not that important. What matters is the difference in velocity, called delta-v in space parlance, which determines the cost in terms of fuel that is required to get to another object. It turns out that many asteroids, despite being very distant, require less delta-v to reach than the much closer moon.
Asteroids also have the virtue of being small (most of them at least), which means that they in practice lack gravity. You can just move up close to an asteroid, take a chunk of it and zip off again (or just zip off with the entire asteroid). This, combined with the fact that there are all sorts of asteroids containing all types of elements, means that the asteroids are the prime candidate for raw materials if space habitats will ever be built.
This is a big if. No one has ever utilized resources in space, much less constructed anything useful with them. Neither are there any serious attempts to do so. Space habitats are still the natural next step for humanity. In fact, it is the necessary next step. But as long as the resources of space lie unused, the dreams of space habitats will remain precisely that: dreams.
Space has a 0 to 1 problem of incomparable magnitude. It is a lot easier to imagine the economic value of space transportation, manufacturing, and research to break even once we have a long-term presence in space. Until we get there, progress is driven more by ideology and the personal interests, though that only gets us so far. In that sense, if your goal is to make humanity multi-planetary, which it should be, then short-term economic value produced by advancements in space technologies such as Starlink are under-explored and underestimated. Would be very interested in more articles on this topic!
Personally I came for the space and had the pleasant suppise of great anthro essays. I also lived the last article, but alas I'm a Mega space nerd