Space Colonization

Abstract

The purpose of this paper is to study the design of space habitats that are practical and feasible. It includes analyses of the reason for a space habitat, as opposed to planet colonies. It also includes analyses of existing designs and a detailed analysis of one proposed design.

Space Colonization

The first concept we need to analyze is the idea of space colonization. When this subject is discussed on the Internet or any other group, space colonization is understood to be when a group of colonists embark in a ship and travel to the Moon, Mars or any other place and establish a colony there. It is hard to understand why this idea is so entrenched. Consider the following points:

• All the bodies of the Solar System, the Earth included, are hostile places. If some group of persons comes to Earth from space, they would be subject to the harassment of the authorities, they would be charged taxes, they would be subject to medical examinations and who knows what else. Consider only what happens when a group of persons passes from one country to another. Landing on the Moon, Mars or any other body of the Solar System requires space suits, supply of air, water, food, energy sources, and many other conveniences.
• The next point to consider is that a group of colonists traveling to the Moon, Mars or anywhere else in the Solar System would require the design and construction of a ship to transport them there. Such a ship needs to have all the necessary life support systems. It needs air, water, food, heat as well as all other conveniences. Further more, even a trip to the Moon takes some time. A trip to Mars takes a substantial time. The colonists need something to do while they are in the ship. They also need to exercise to prevent lose of bone and muscle strength. Their ship needs to have gravity for the same reasons.
• Now consider that the colonists travel in a ship that has all they need and all additional conveniences. When they get to their destination, they must abandon these conveniences to land on whatever destination they have. They would need to start again by building a new habitat and create a way to satisfy their needs and develop their conveniences.
• Another point to consider is that the design and construction of the ship was done on Earth, where there is air to breath and all conveniences. The construction of the new habitat must be done under very hostile conditions, wearing space suits, having to carry air, water, food, power sources and who knows what else. For example on the Moon, they would need lighting equipment to work during the two weeks of lunar night.

For all these reasons, unless the colonists are stupid, they would not abandon the conveniences of their ship for the uncertainties of a hostile environment. They would convert the ship into their home and remain there. Consequently, space colonization should not consider building ships to send colonists into space but it should consider building the habitats where the colonists would live in space.

Another idea that is mentioned very often when considering habitats in space, as opposed to colonies on the Moon, Mars or wherever, is that the habitat must be placed in orbit around Earth. It is very hard to understand why anybody would want to do this except when considering a space station for scientific, political or military purposes, a hotel for very rich tourists or similar applications. A habitat in orbit around Earth would have many disadvantages. The most important of these disadvantages is that some government from Earth would claim authority over the habitat and its inhabitants. That government would create taxes, police, laws, regulations and all the bad things that all governments of the world have in common. Further more, a habitat in an orbit around Earth would need to expend more energy compensating for the variations on its orbit due to the presence of Earth and the Moon. This is a minor problem when considering communication satellites. A human habitat would have a considerable mass and suffer proportionally larger attractions and displacements. There is also the problem of the shadow of Earth and the Moon, reducing the source of energy. Another problem is that since the habitat is close to Earth, it would receive more meteoric impacts than in open space. This is because the mass of the Earth and the Moon attract meteorites.

There are other considerations. A space habitat parked in open space in orbit around the Sun would have a constant source of energy, without shadows or eclipses, without day or night. Having a constant source of energy is a big step towards solving the other needs like air, water, etc. If there is abundant, clean, cheap, renewable energy, recycling the air and water is a minor problem. This can be achieved without any problem by parking a habitat in orbit around the Sun.

The final point we should consider is the L-Points. For some reason, these points fascinate the imagination of many people. The L-Points are points where the gravitational attraction of the Earth, Moon and Sun cancel out. Most people talk about the L-Points as if they were the bottom of a gravitational bowl. They consider that an object in an L-Point would be in a stable equilibrium. If this were the case, the L-Points would be full of debris. The L-Points are not the bottom of a bowl but the peak of a mountain. An object can be in orbit around an L-Point, but not park at the L-Point. An object in an L-Point is in an unstable equilibrium. Any small deviation in any direction breaks the equilibrium. Say that the center of gravity moves a small distance towards the Moon. The attraction of the Moon increases a little bit. This produces a force in the direction of the Moon that increases the displacement. The object would fall towards the Moon. Note that an object in equilibrium in an L-Point is not in orbit around any of the three bodies. Consequently, if the equilibrium is broken, it would fall to the body exerting the higher attraction.

Current Designs

The idea of a space habitat or of people living in space is not new. Since before our civilization started venturing out from Earth, man has been dreaming on this possibility. The first attempts at designing space habitats were driven more by fantasy rather than by engineering. The large number of proposals lack practicality on one side and feasibility in another. We need to note also that the space stations cannot be considered a human habitat. The basic reason is that these space stations lack gravity and they cannot be used as human habitation for prolonged periods of time.

We will study some of those designs that have received a lot of attention. See http://www.ssi.org, the home page of the Space Studies Institute, founded by Gerard O'Neill at Princeton University. This web page gives some details of the O'Neill Cylinder, the Stanford Torus, and the Bernal Sphere, among others. See also http://members.aol.com/oscarcombs/settle.htm for a description and artist conceptions of these habitats. From the information that it was possible to obtain from these web sites, the O'Neill Cylinder is a cylinder with four miles diameter and twenty miles long. The Bernal Sphere is one mile in circumference. We have not been able to find dimensions for the Stanford Torus. Judging from the artistic representations available in both of the web pages mentioned above, all these designs seem to be variations on a single idea. They all have characteristics in common. They use only the interior surface of the figure of revolution to support the population. The rest of the volume is empty. Another common characteristic is that all of these designs show an agrarian landscape, with rivers, lakes, mountains, trees and vegetation. Some of the drawings show people; none show any habitation, like houses, towns or anything. It is true that the scale of these drawing is such that a house would not be visible.

Let us consider the O'Neill Cylinder. This habitat has four miles diameter and twenty miles long. The first point that comes to mind is that an object in space whose larger dimension is larger than its smaller dimension, has a natural tendency to rotate with the smaller dimension as axis of rotation. Think on the asteroids. They tumble on a small axis. Consequently, it would be necessary to have special controls to prevent this from happening and to make the cylinder rotate on the larger axis, as it is intended. Further more, all the representations of this habitat show rivers and lakes, large trees, valleys and mountains. It is indicated that the population would live in three valleys around the periphery of the habitat.

Let us analyze this last point. The surface area of the O'Neill Cylinder is on the order of two hundred and fifty square miles. This interior surface is shown covered with soil, to form the valleys, mountains and rivers. This fact would indicate an average depth of soil of something like one tenth of a mile (500 feet). This represents a total volume of some twenty-five cubic miles of soil. This is four billion cubic feet of soil. Considering that dry soil weights only two pounds per cubic foot, we are talking about eight billion pounds, or four million tons of soil. Current rockets can carry to orbit something like two tons of payload. We would need two million trips of such rockets only to carry the soil! If we have hundred of these rockets making one trip a day, they would take fifty-five years to lift the soil to orbit! Notice that we have not considered the weight of the water to form lakes and rivers that need to be spanned by bridges like the San Francisco Bridge.

An additional problem would be filling this huge cylinder with air. A related problem is to keep this cylinder airtight. There are almost three hundred square miles of exterior surface to keep airtight! This would be a continuous problem because this cylinder was supposed to be parked in orbit around the Earth, consequently partaking of the continuous shower of meteorites that is attracted by the mass of the Earth, without having the protection of the atmosphere. Consider that one-kilometer square is crossed by one 1-gr meteorite every 10 years. This cylinder has a cross section of more than 200 square kilometers. This means that it will receive 20 impacts of meteorites 1-gr or larger per year.

Something that these designs do not consider, is the cost of such a habitat per person. Such a huge cylinder would have only some two hundred and fifty square miles of area. Farms, mountains, lakes and rivers take a good part of this area. Using the normal population density of farming areas, we can expect that only some ten thousand persons could be accommodated in this habitat. We also need to consider the complex controls required to maintain this cylinder in an stable orbit around the Earth, compensating the interference of the Moon, that would exert a substantial pull on that mass, on top of the problem mentioned above of the axis of rotation. The maintenance of this habitat would be a huge and expensive problem. Consider only the number of workers that would be needed to check that the habitat is airtight and repair the damages.

The Habitat

We are interested in designing a space habitat. Since this idea is not defined in itself, except for the fact that it describes habitation for people who live in space, we need to restrict this definition. We would consider that the habitat we would design would have a capacity of some two to three thousand families when fully occupied. We would consider that when the habitat is built and launched there would be one thousand families, with between four and five thousand persons. This number would increase as the population reproduces. Considering a normal rate of reproduction, the habitat would get to design capacity in about hundred years. The dimensions of the habitat would be generous enough that it would permit a rearrangement of the facilities to increase its capacity. The habitat would have the form of a figure of revolution so it can be rotated to produce artificial gravity.

We will consider that these families will live in a normal environment of a terrestrial city, not in an agrarian environment. This comes from the fact that now, most of the population of Earth lives in a city environment; consequently, when moving into space, they will prefer to maintain that environment. That is to say, they would live in apartments or houses with capacity for one family. We would consider that the family would be formed by a father, a mother and between two and five children. We would further consider that the population of the habitat would have normal occupations. There would be factories to manufacture what they need. There would be schools, shops, malls, restaurants, theaters, etc. In other words, the habitat would be like a small town. We would also consider that the habitat would be totally filled with construction; that is to say, that there would not be open spaces where a person can see the other side of the enclosure where the houses and people would look like hanging up-side-down. We would further assume that the colonists would develop their own independent civilization, based on their wishes and desires.

New Design

As we said in the previous paragraph, we will try to design a habitat for some two to three thousand families doing a normal city life. So, we would consider one apartment per family. The apartments would be large, larger than most houses, because we consider that the families would be large. Let us get an idea of the size. Each apartment with say thousand square feet of living space plus another thousand of free space around. Note that this permits doubling the capacity of the habitat, if the inhabitants so desire. The previous assumption gives an area of five million square feet. We probably need five times this area to consider space for parks, shopping malls, factories, and storage areas and to leave another possibility for increasing the capacity of the habitat. Say twenty five million square feet. It is only a mile square! Is a small town! The town needs to be inside some enclosure. We need to consider the external area of the figure that encloses the town. The external area has to be insulated against the cold of space and the heat of the Sun. Further more, it needs to be protected against meteorites and shielded against radiation. The cost of the colony would depend very much on the exterior surface area. The cost of the inside would be almost the same for any form, as long as the volume is the same.

There is another problem to consider. Moving from one place to another in a flat town requires some kind of transportation, which increases the need for energy. In a more compact form the need for transportation would be smaller. Consider a town of some twenty thousand persons, which is what we are considering. It requires quite an area that we computed as a mile square. If the town is cut in pieces and the pieces are put one on top of each other, forming like a cube, all the distances reduce very much. There can be ramps and stairs from one level to another. So, let us consider the volume of this town. The first condition is twenty five million square feet. Assume sixteen feet between levels, because we need to leave space for pipes, electricity, telephone and all the things that normally are installed underground and some room above to avoid a sense of enclosure. Notice that we are considering very generous dimension for all the requirements. That gives four hundred million cubic feet.

This volume gives a sphere of about four hundred and sixty feet radius. The rotating cylinder has the advantage that, even inside a rotating sphere, the levels of equal gravity are cylinders. We can also cut slices perpendicular to the axis of rotation of the cylinder, and they are all the same, which is nice for design purposes. There is also a large volume with low gravity for storage and heavy work. Finally, a cylinder has a natural axis of rotation, which the sphere does not have. Most of the designs of space habitats that work are based on a cylinder or a torus. A cylinder could be about five hundred feet in radius and five hundred feet long, so it would rotate in its natural axis. A radius of five hundred feet means something like thirty floor levels. Not all would be houses. We have considered that one fifth of the area would be houses, so half of the lower ten levels could be houses, the other half parks, shops, etc. The top levels could be used for factories, shops, and storage and for other occupations. The external lateral area of such a cylinder is three million square feet.

For those readers who want to study other choices, here is a table of habitats with different radius. The dimensions of height, volume, etc. are for a total volume of four hundred million cubic feet. This table covers both, a cylinder and a wheel (torus) with the same radius. The wheel is assumed to have a square, or quasi-square section. In both cases, the volume would be totally built. There would not be any place open, where a person can see the other side of the cylinder or the body of the torus. Note that the exterior surface gives a very good approximation of the relative cost of each habitat. The RPM values are to achieve a maximum gravity as on Mars, half the gravity of Earth, or a full gravity of Earth. This is at the surface of the cylinder or wheel. Interior levels would have less gravity.

The external form of the habitat to be studied here is, as mentioned before, a flat cylinder. Something like a can of tuna fish. It has a diameter of one thousand feet and a height of five hundred feet. One of the ends would be covered with solar panels. The axis of rotation would be pointed at the Sun. The side with the solar panels would always be illuminated, producing a constant output of energy. The area of the end is more than seventy five hundred square feet to be covered with solar panels. In the center of this area and at several points of the periphery there would be airlocks to permit servicing the solar panels.
The lateral area of the cylinder would never be exposed to solar radiation since the axis of the cylinder would be pointing to the Sun. This area should be painted white to reduce the loss of energy by radiation. There would be airlocks around this area to permit servicing the area in the case of punctures by meteorites. All the walls of the cylinder would be made of several layers, to provide protection against the impact of meteorites, thermal insulation and radiation.
The other end of the cylinder would always be in the dark. In the center of this area there would be an airlock of the proper dimensions with a window that permits the entrance and exit of shuttles. It should be understood that the shuttles would not have wings, because they would not be expected to land on Earth or any other place with an atmosphere. The window would probably be square, since the most logical section for the shuttle is a square or a rectangle. The airlock would be large enough to accommodate the shuttle inside. There would be equipment to pump the air out of the airlock when a shuttle needs to enter or leave. In this way, the air in the airlock would not be lost every time the window is open to let a shuttle leave or enter. The problems inherent of exiting and entering a world that is rotating are studied later. This side of the cylinder should also be painted white to reduce the loss of energy by radiation.

The interior of the cylinder would be totally built, as mentioned before. The fundamental idea is to prevent that there is any place inside the cylinder where the other side could be visible as some kind of a ceiling with gravity pointing in the opposite direction. Further more, the design of the streets and the position of the houses and buildings would be such that there would not be any place where the curvature of the cylinder can be detected. Streets that run parallel to the axis of the cylinder could be straight and run from one end of the cylinder to the other. Streets perpendicular to the axis of rotation would be broken at short intervals to avoid the curvature of the cylinder to be noted. For the same reason, there would not be any open spaces. The streets would be covered by the street above. The ramps and stairs between floors would be scattered to avoid open shafts. The basic idea is to give the appearance of an apartment building, probably with extra wide corridors, rather than an open space. The shopping areas, malls, parks, and similar areas would also have another on top developing the appearance of a closed mall, or similar structure. If there is a stadium, it would probable need two or three levels, to give it enough space, but it also would have a ceiling.

The core of the cylinder, where the gravity is minimal, would be used for storage areas, heavy equipment, manufacture of parts that do not need gravity or profit from low gravity, and similar uses. This space would also be divided in compartments that would prevent the view of the other side of the area. None of these compartments would include the axis of rotation. There would always be a corner of walls coincident with the axis of rotation. This is to provide that the low gravity would always have a definite direction inside any compartment. One possible use of this area would be for recreation and sports in low gravity. This can be obtained by separating areas from the axis of rotation to one side, producing a low gravity environment where the low gravity always has the same direction.

The initial capacity of the habitat would be for twenty five hundred families. Given the way the habitat has been dimensioned, it is considered that the families would be large, with up to four or five children. The houses would be large, with an average of three or four bedrooms. The estimated population of the habitat under these condition would be some sixteen thousand persons. It has been considered that only half of the area devoted to houses would be built. This permits to increase the capacity to more than thirty thousand persons by building the whole area devoted to houses. Since the area devoted to houses has been considered one fifth of the total area. Much of the area in the other four fifths would be open. Building houses in those areas, could increase the capacity of the habitat to close to hundred thousand persons, from the housing point of view.

A radius of five hundred feet gives something like thirty levels at sixteen feet per level. Since the levels do not have the same area, the bottom eight to ten levels would be devoted to houses. Houses would be only at the center half of the cylinder. The rest of those levels would be devoted to parks, shopping areas, offices, and similar uses. A good percentage of the free areas would be devoted to parks. The parks would be situated on the same location on all the levels, to reduce the cost of piping, drainage and other installations. The parks would have plants with large leaves for the purpose of oxygenating the air of the habitat. The air would be filtered, purified and circulated through the habitat, producing a gentle breeze that would prevent odors for staying in one place.

The end of the cylinder pointing towards the Sun, behind the solar panels, would be used for equipment and machinery that uses electricity. The shops and labs attached to these machinery would also be at this end of the cylinder. The offices for the personnel in charge of these operations would also be in this area. At the other end of the cylinder there would be the airlock for the shuttles. This area would also be used for the shops and labs related to the shuttles as well as offices for the related personnel.

Describing the design of the habitat in more detail is outside the scope of this paper. Describing the life of the inhabitants of such habitats is not easy in the environment of a paper. For this reason, the description of the details of the life in the habitats and many of the small details of the construction and operation of these habitats have been made in the form of a sequence of short stories. I have vacillated giving them the name of stories. They are too technical for being a simple story. Probably the classification as hard science fiction would fit. Even the human characters are used more to present the social structure and problems of the habitats that to create a real plot. In any case, the use of the storytelling approach permits not only to analyze the technical details of the design, but it also permits to analyze the social structure, the consequences of their isolation, the different problems of the design, the organization adopted by the travelers, and similar topics. These short stories have been put together in a file Ashley.

It is to be noticed that this paper studies only some of the characteristics of the cylinder. For example, nothing is said about the effect on the inhabitants of the variation of the gravity from level to level. Nothing is said of the effects of the variation of the gravity from the feet to the head, at any one level. Nothing is said about radiation and the requirements for shielding. Nothing is said about the number of families required to avoid inbreeding. For a more detailed analysis of the cylinder and the torus and the design of a practical habitat, the reader is referred to another essay, The Design of a Space Habitat, where all the conditions are considered.