Orbital Space Settlements
Unlike astronauts in space stations, people will consider their orbital space colony home, although they may visit Earth from time to time. A colony is a place to live, work, play, raise a family, argue with the neighbors, and grow old in low-g comfort.Since space colonies are for permanent living, not just a few month's work, they must be much larger. In this article we will discuss several space settlement designs, all include habitable areas a kilometer (nearly a mile) or more in size. This is much larger than the space stations described above, which are all a few tens of meters long at best. Thus, orbital space settlements are expected to be about 100 times larger than today's space stations.
Space stations have small crews, usually only a few people. Supplies of air, water, and food can be brought from the ground with limited recycling. The colony designs we examine here are for thousands of people, and this means there are a few minor problems to solve :-). For example, space has no air, no water, no food and it's way too difficult to bring enough up for thousands of people. In addition, in the sun it is far too hot and in the shade it's unbelievably cold. There's also quite a bit of radiation. All the great stuff the Earth provides more-or-less for free: air, water, reasonable temperatures, radiation protection, and even gravity need to be provided by the space colony.
The first priority is something to breathe. The colony must be filled with air and the hull (the outermost shell) must be air-tight. If the hull leaks much, the colony will lose its atmosphere and become unlivable. Since outside the hull is a vacuum, the atmosphere on the inside will push outwards on the hull, which must be strong enough to withstand the air pressure. At sea level on Earth air pressure is about ten tons for every square meter (roughly a square one yard on each side). However, people can breathe perfectly well in the mountains where the air pressure is much less, so less dense air pushing with six to seven tons per square meter should be adequate. Fortunately, we have been building small, air- tight spacecraft for over forty years now, we just need to make them bigger. Unfortunately, just being strong enough to hold the air in isn't enough. To get something resembling gravity we need to spin the colony and that puts even more stress on the hull.
Astronauts in the ISS don't weigh anything. They just float around, which is great fun. However, even though astronauts exercise for hours every day, weightlessness causes muscles and bones to atrophy. Our bodies only stay strong when they work, and removing the stress of Earthly gravity reduces the workload a great deal. Some astronauts become so weak they can't stand up when they get back to Earth.
No one has any idea what would happen to children raised in weightlessness, but it's a safe bet they will never visit Earth, even for an all-expense scholarship to Harvard. Without the stress placed on a growing body by Earth-normal gravity, bones and muscles will have no reason to develop strength and power. Fortunately, we can get something that feels a lot like gravity by rotating our colonies. This is called pseudo-gravity. Living in Earth-normal pseudo-gravity should allow adults to keep their strength and children to develop it. The need for pseudo-gravity puts additional stress on the hull, but, as luck would have it, we already know how to make materials that are plenty strong enough.
Currently available materials are perfectly adequate to build colonies up to a few kilometers across or so. This is about the size of a California beach town. A population of ten or twenty thousand will fit quite nicely. Credible designs even exist for colonies many tens of kilometers across with populations in the millions, but the first colonies will almost certainly be smaller.
Orbital space colonies can come in many sizes, but the need to rotate to get pseudo-gravity means that only a few shapes work really well. Specifically: balls, cans and donuts; more formally called spheres, cylinders, and toruses. It's also possible to combine shapes; for example, you can attach many toruses together - like stacking donuts - to get a colony shape called the 'crystal palace'. Each shape has its advantages and disadvantages and different groups of colonists may be expected to choose different shapes depending on their needs and desires.
The simplest shape is the ball or sphere. This has the advantage of having the minimum surface for any fixed volume. This is an advantage because the exterior must stop the radiation so common in space. The simplest way to stop the radiation is just to pile enormous amounts of material, anything will do, on the outside. You need roughly five tons for every square meter of surface. Since the sphere has the smallest surface area for whatever volume one needs, the amount of radiation shielding is minimized. Since all the shielding must be imported from the Moon or NEOs, this can save a lot of work.
NEO stands for Near Earth Object. NEOs are asteroids and comets that pass near Earth. There are about a thousand of these one kilometer across or bigger, and many millions of smaller ones. Some NEOs are actually easier to get to than the Moon, so they may be a great source of materials.
Note: this section includes 3D models of space colonies that you can view if you have a VRML plugin for your web browser. One such plugin is the Cortona VRML Client, which is free. All the VRML for this page was produced by Nittin Arora and Ankur Bajoria.
Bernal Sphere
The best developed spherical colony is the Bernal Sphere (click here for 3D VRML model). The Bernal Sphere features a sphere for the living area and stacked toruses for the agricultural modules. Mirrors direct sunlight into the colony. Click on the thumbnails for larger drawings of the Bernal Sphere. |
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Any spherical colony, including the Bernal Sphere, has a significant disadvantage. Orbital colonies must spin to create pseudo-gravity to keep the inhabitants strong. The amount of pseudo-gravity you experience depends on how far you are from the axis of rotation. In a sphere, the axis of rotation must pass through the center of the sphere. The closer to the axis of rotation you are, the less pseudo-gravity you experience so the less you weigh. The further from the axis the more you weigh. No fair saying your diet is working simply by moving closer to the center.
In general smaller colonies are easier to build than larger ones, so you generally want to make colonies just big enough to get 1g at the furthest point from the axis of rotation. In a spherical colony, only a single great circle on the hull is the maximum distance from the axis of rotation. Everyplace else will be at a lower pseudo-gravity level. Unless you are very careful how you move about, your weight will be constantly changing. This can be fixed by placing a flat floor across the sphere on both sides of the axis of rotation, but then the space below the floor must be at higher than 1g pseudo-gravity levels.
Stanford Torus
A second popular shape for space colonies is the donut shape, also called a torus. The torus shape is popular with movies and television shows; for example, the space station in the movie "2001 - A Space Odessy" and the TV show "Star Trek: Deep Space Nine." The best developed toroidal colony design is the Stanford Torus (click here for 3D VRML model of two Stanford Toruses). The Stanford Torus has a diameter of about a mile and rotates roughly once per minute. It features a large mirror to reflect natural sunlight into the colony. There is also a sphere in the center for weightless recreation and light industry. Click on the thumbnails for larger drawings of the Stanford Torus. |
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There are disadvantages as well. First of all, there is far more surface area than other shapes, so much more radiation shielding is needed. This substantially increases the mass. Also, a simple torus has no livable space in the center where you can have weightless recreation. This is a big one. Remember that the pseudo-gravity you feel depends on how far from the axis of rotation you are. The further away, the more you weigh; the closer the less. Right at the center you don't weigh anything at all - just as if there was no rotation. Since one of the great things about living on a space colony is weightless recreation, a simple torus is not a viable approach. Most toroidal orbital colony proposals feature spheres or cylinders at the center to provide 0g fun for the inhabitants. This however, increases the surface area of the colony even more, which requires more radiation shielding.
Cylinders
The final realistic colony shape is a cylinder. The largest colony designs are cylinders since the entire curved surface will feel the same pseudo gravity. Furthermore, unlike a torus, the center is inside the hull so 0g recreation is readily available. Almost all the shielding is directly under foot in the 1g living areas, so the shielding per unit living area is small. There are at least two good examples of cylindrical colonies in the literature: the original O'Neill Cylinders and Lewis One. There are some problems with these designs and a new set of designs is under development.O'Neill Cylinders
The O'Neill Cylinders are among the most famous of all space colony designs (click here for 3D VRML model of the original O'Neill cylinders). Looking at the VRML: the green cylinder is the living area, the brown strips are mirrors to bring natural light into the colony, and the blue modules are for power and agriculture. The colors are for pedagogical purposes only, it is unlikely colonists will paint the exterior of their colonies for aesthetic purposes (although it's possible). More likely, colony exteriors will require special coatings to protect them from the harsh space environment. To see artist renderings, click on the thumbnails for larger drawings. |
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Lewis One
The final colony design we shall discuss here is Lewis One. Looking at the VRML, the large flat surface on one side is the solar arrays, which must point at the Sun. The T-shaped flat surfaces on the other side are thermal radiators. They send waste heat out into space so the colony will not fry. The large grey cylinder inbetween is non-rotating radiation shielding. It does not need to be air tight. The small grey cylinder on top is a docking port, and the small yellow hemispheres are for viewing the heavens. |
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Lewis One has a large, non-rotating shielding cylinder with three sections inside. One is the main rotating habitat. The second is non-rotating for 0g industry and tourism. The third is unpressurized and is sized to build more rotating sections. Thus, Lewis One is specifically designed to build more colonies.