¹ Synthetic Carbohydrates, Summer Study Report, NASA Ames Research Center.
Choices of food sources within the general realm of terrestrial agriculture become a compromise between preference and diversity on the one hand and efficiency on the other. For the colony, efficient use of area (even at expense of efficiency measured in other terms, i.e., as energy) is a critical factor to be balanced against a varied and interesting diet. For example, the almost exclusive use of rabbits and goats for animal protein previously proposed (ref. 15) for space colonies is rejected as being unnecessarily restrictive and seriously lacking in variety.
Recycling Wastes
High costs of transportation place great emphasis on recycling all the wastes of the colony. Because in the near future Earth appears to be the only practical source of elements fundamental to agriculture — carbon, nitrogen, and hydrogen — they must initially be imported from Earth. To avoid having to continually import these elements, all wastes and chemicals are recycled with as small a loss as possible.
Waste water can be treated biologically as in most terrestrial communities, physiochemically, by dry incineration, or by some more advanced technique such as electrodialysis, electrolysis, vapor distillation or reverse osmosis (ref. 20). Each of these alternatives is ruled out for various reasons. Biological treatment provides only incomplete oxidation and produces a residual sludge which must then be disposed of with attendant risks of biological contamination. Physiochemical treatment has no organic conversion, and is chemically a difficult process. Dry incineration requires an external energy source to maintain combustion and it produces atmospheric pollutants. All the advanced processes are incomplete in that the resulting concentrates require further treatment.
Wet oxidation (Zimmerman process) has none of the foregoing defects. Operating at a pressure of 10.7 MPa (1500 lb/in.²) and a temperature of 260°C, wet oxidation with a total process time of 1-1/2 hr produces a reactor effluent gas free of nitrogen, sulfur and phosphorous oxides; a high quality water containing a finely divided phosphate ash and ammonia. Both the reactor gas and the water are sterile (refs. 21, 22). At solids concentrations greater than 1.8 percent the process operates exothermally with an increase in the temperature of the waste water by 56° C (personal communication from P. Knopp, Vice-President, Zimpro Processing, Rothschild, Wisconsin). These definite advantages lead to the choice of this process as the basic technique for purification and reprocessing within the space colony.
Composition and Control of the Atmosphere
The desired composition of the atmosphere is arrived at as the minimum pressure needed to meet the criteria for atmospheric safety stated in chapter 2. This results in the atmospheric composition detailed in table 4-3. Its outstanding features are: normal terrestrial, partial pressure of oxygen, partial pressure of carbon dioxide somewhat higher than on Earth to enhance agricultural productivity, and a partial pressure of nitrogen about half of that at sea level on Earth. Nitrogen is included to provide an inert gaseous buffer against combustion and to prevent certain respiratory problems. Because nitrogen must come from the Earth, its inclusion in the habitat's atmosphere means there is a substantial expense in supplying it. This fact, in turn, suggests that it is desirable to hold down the volume of atmosphere in the habitat, a factor taken into consideration in the discussion of the habitat geometry given earlier. The total atmospheric pressure is thus about half that at sea level on Earth.
Atmospheric oxygen regeneration and carbon dioxide removal are by photosynthesis using the agricultural parts of the life support system. Humidity control is achieved by cooling the air below the dewpoint, condensing the moisture and separating it. Separation of condensate water in zero gravity areas (such as the manufacturing area and hub) by hydrophobic and hydrophilic materials offers the advantage of a low pressure drop and lack of moving parts (ref. 23) and is the preferred subsystem.
TABLE 4-3 — HABITAT ATMOSPHERE T = 20 ± 5° C Relative humidity = 50 ± 10 percent
| Gas | Partial Pressure, kPa | | :--- | :--- | | Oxygen ($O_2$) | 22.7 | | Nitrogen ($N_2$) | 26.7 | | Carbon Dioxide ($CO_2$) | 0.4 | | Water Vapor ($H_2O$) | 1.0 | | Total | 50.8 |
1 standard atmosphere = 101 kPa
Trace contamination monitoring and control technology is highly developed due primarily to research done in submarine environments. The habitat environment is monitored with gas chromatograph mass spectrometer instruments (ref. 24). Trace contamination control can be effectively accomplished by sorption (e.g., on activated charcoal), catalytic oxidation, and various inert filtering techniques.
SATELLITE SOLAR POWER STATIONS: NO ALTERNATIVES
An important goal for the design for space colonization is that it be commercially productive to an extent that it can attract capital. It is rather striking then that the study group has been able to envision only one major economic enterprise sufficiently grand to meet that goal. No alternative to the manufacture of solar power satellites was conceived, and although their manufacture is likely to be extremely valuable and attractive to investors on Earth, it is a definite weakness of the design to depend entirely on this one particular enterprise. A number of valuable smaller scale manufactures has already been mentioned in chapter 2 and, of course, new colonies will be built, but these do not promise to generate the income necessary to sustain a growing space community.
There is some choice among possible satellite solar power stations (SSPS). Two major design studies have been made, one by Peter Glaser of Arthur D. Little, Inc. (ref. 25), and the other by Gordon Woodcock of the Boeing Aircraft Corporation (ref. 26). Conceptually they are very similar, differing chiefly in the means of converting solar power to electricity in space. Woodcock proposes to do this with conventional turbogenerators operating on a Brayton cycle with helium as the working fluid; Glaser would use very large arrays of photovoltaic cells to make the conversion directly.
There is not a great deal to argue for the choice of one system rather than the other, except perhaps that the turbogenerator technology proposed by Woodcock is current, while Glaser relies on projections of present day photovoltaic technology for his designs. In the spirit of relying on current technology, the Woodcock design seems preferable, but a definite choice between the two is not necessary at this time. A more detailed description of the SSPS alternatives with a discussion of microwave transmission and its possible environmental impact is given in appendix H.
WHERE THE COLONY SHOULD BE LOCATED
Chapter 2 surveyed space and described what is there and how space is shaped in terms of distance, propulsive effort and gravitational attraction. These aspects of space together with the location of needed resources are important to choosing a site for the habitat. The community should be located for convenience with respect to its resources — sunlight, weightlessness, and minerals — and also with access to and from its principal market, Earth. The site should be chosen by balancing the needs of production against the needs of marketing the product.
Near to but not on the Moon
The minerals of space are to be found in the distant outer planets, the asteroids, the nearer and more accessible planets like Mars, the moons of other planets, or our own Moon. Of course the Earth is a primary source of mineral wealth too. It seems reasonable to place the colony near one of these sources. For reasons explained in the next section, the Moon is chosen as the principal extraterrestrial source of minerals, hence the habitat should be near the Moon.
But where should the habitat be placed in the vicinity of the Moon? At first glance the Moon's surface seems a good choice, but any part of that surface receives the full force of the Sun's radiation only a small fraction of the time. Moreover, on the Moon there is no choice of gravity; it is one-sixth that of Earth and can only be increased with difficulty and never reduced. Space offers both full sunshine and zero gravity or any other value of simulated gravity one might choose to generate. An additional difficulty with a lunar location is related to the major product of the colonies, SSPS's. Transporting them from the Moon to geosynchronous orbit is not economically viable. For ease of exploitation of the properties of space, the habitat should be located in free space.
In Free Space at L5
Although there is no stable location at a fixed point in space in the Earth-Moon system, the colony could be located in any one of a number of orbits in free space. These orbits can be around the Earth, or the Moon, or both the Earth and the Moon. Those near either the Earth or the Moon are rejected because of the frequency and duration of solar eclipses which deprive the colony of its light and energy. Large orbits around the Earth make it difficult to deliver the large mass of material needed from the Moon, while large orbits around the Moon become orbits in the Earth-Moon system about which little is known at the present time. These last two options, while not chosen, present interesting alternatives which should be examined more closely.
There remain the orbits about the five libration points. Three of these, L1, L2, and L3, are known to be unstable, and to maintain orbits around any of these three points for long periods of time requires appreciable expenditures of mass and energy for station keeping.
There do exist, however, large orbits around both of the remaining libration points, L4 and L5. These have been shown to be stable (refs. 27, 28). A colony in either of these orbits would be reasonably accessible from both Earth and Moon. One of these libration points, L5, is chosen for the location of the first space colony. This choice is somewhat arbitrary for the differences between L4 and L5 are very slight.
MINING, TRANSPORT, AND PROCESSING IN SPACE
From where will come 10 million tonnes of matter needed to build a colony? And where and how will it be processed, refined and shaped into the metals, glass and other necessary structural material? The topography of space shapes the answer to the first question; human ingenuity offers answers to the second. A major problem only partly solved is how to transport large quantities of matter from mines on the Moon to space. Some possible solutions to that problem are suggested.
Sources
As noted previously, lunar materials have been chosen to supply the great bulk of mass necessary for the first colony, including the shell and internal structure, passive shield, soil, and oxygen. As indicated in figure 4-10, only a small percentage of the mass, including initial structures, machinery, special equipment, atmospheric gases other than oxygen, biomass, and hydrogen for water, comes from Earth.
This decision has been made for a variety of reasons. Of the bodies in the solar system which might supply materials, the other planets are eliminated by the expense of transportation from their surfaces, and the moons of the outer planets by transport times of years and by costs. This leaves the asteroids, comets, and the moons of Mars.
While the composition of the moons of Mars is unknown, both the comets and asteroids are apparently abundant sources of organic materials in addition to rock and possibly nitrogen and free metals as well. For immediate future applications, however, the Moon's position makes it attractive and, compared to the asteroids, the Moon has advantages of known properties, a distance suitable for easy communication, and it allows perhaps simpler overall logistics.
However, when the space colonization program is begun, technical and economic imperatives seem likely to drive it quickly toward exploitation of asteroidal rather than lunar materials and toward much less dependence on Earth. Long before the results of mining activity on the Moon became visible from the Earth, the colony program would be obtaining its materials from the asteroids. Given that source, the "limits of growth" are practically limitless: the total quantity of materials within only a few known large asteroids is enough to permit building space colonies with a total land area many thousands of times that of the Earth.