Note: References 5 through 30 are as cited by Salkeld, R.; Space Colonization Now? Astronautics and Aeronautics, vol. 13, no. 9, Sept. 1975, pp. 30-34.
Figure 1-1 — The colony at Lagrangian point L5.
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Figure 1-2 — The space colonization system.
<!-- image --> <!-- image -->CHAPTER 2: Physical Properties of Space
The physical properties of space are rich in paradoxes. Space seems empty but contains valuable resources of energy and matter and dangerous fluxes of radiation. Space seems featureless but has hills and valleys of gravitation. Space is harsh and lifeless but offers opportunities for life beyond those of Earth. In space, travel is sometimes easier between places far apart than between places close together.
The purpose of this chapter is to explore and understand these properties of space and the apparent paradoxes to derive a set of basic design criteria for meeting the goals for space colonization set out in chapter 1. Together with considerations of the physiological and psychological needs of humans in space, these basic criteria compose the quantitative and qualitative standards on which the design of the space colonization system is based. These criteria also serve as the basis for a discussion and comparison of various alternative ways to locate, organize and construct, and interconnect the mines, factories, farms, homes, markets, and businesses of a colony in space.
THE TOPOGRAPHY OF SPACE
For the resources of space to be tapped safely, conveniently and with minimum drain on the productive capabilities of the colonists and Earth, the peculiarities of the configuration of space must be understood.
Planets and Moons: Deep Gravity Valleys
Gravitation gives a shape to apparently featureless space; it produces hills and valleys as important to prospective settlers in space as any shape of earthly terrain was to terrestrial settlers. In terms of the work that must be done to escape into space from its surface, each massive body, such as the Earth and the Moon, sits at the bottom of a completely encircled gravitational valley. The more massive the body, the deeper is this valley or well. The Earth's well is 22 times deeper than that of the Moon. Matter can be more easily lifted into space from the Moon than from the Earth, and this fact will be of considerable importance to colonists in deciding from where to get their resources.
Libration Points: Shallow Gravity Wells
There are other shapings of space by gravity more subtle than the deep wells surrounding each planetary object. For example, in the space of the Earth-Moon system there are shallow valleys around what are known as Lagrangian libration points (refs. 1,2). There are five of these points as shown in figure 2-1, and they arise from a balancing of the gravitational attractions of the Earth and Moon with the centrifugal force that an observer in the rotating coordinate system of the Earth and Moon would feel. The principal feature of these locations in space is that a material body placed there will maintain a fixed relation with respect to the Earth and Moon as the entire system revolves about the Sun.
The points labeled L1, L2, and L3 in figure 2-1 are saddle-shaped valleys such that if a body is displaced perpendicularly to the Earth-Moon axis it slides back toward the axis, but if it is displaced along the axis it moves away from the libration point indefinitely. For this reason these are known as points of unstable equilibrium. L4 and L5 on the other hand represent bowl-shaped valleys, and a body displaced in any direction returns toward the point. Hence, these are known as points of stable equilibrium. They are located on the Moon's orbit at equal distances from both the Earth and Moon.
The foregoing picture is somewhat oversimplified; it neglects the effect of the Sun. When this is taken into consideration (refs. 3,4), stable equilibrium is shown to be possible only in particular orbits around L4 and L5, as indicated by the dashed lines in figure 2-1. The shape of space around L4 and L5 is discussed in detail in reference 4. The basic conclusion is that massive objects placed in the vicinity of L4 and L5 would orbit these points with a period of about one month while accompanying the Earth and Moon around the Sun. At the price of the expenditure of some propulsive mass, objects could be maintained near the other libration points rather easily (ref. 5). The cost of such station keeping needs to be better understood before the usefulness of these other points for space colonies can be evaluated.
Two Kinds of Separation in Space: Metric Distance vs Total Velocity Change (∆v)
The availability of resources for use by colonists is closely related to the properties of space. The colony should be located where station-keeping costs are low, where resources can be shipped in and out with little expenditure of propulsion mass, and where the time required to transport resources and people is short. These three criteria, minimum station-keeping, minimum propulsion cost, and minimum transportation time cannot be satisfied together. Some balance among them is necessary. In particular, time and effort of transportation are inversely related.
Figure 2-1 shows the distances between points in the vicinity of Earth of importance to space colonization. The diagram is to scale, and the distances are roughly in proportion to time required to travel between any two points. However, in space travel the important measure of propulsive effort required to get from one point to another is the total change in velocity required (∆v). Thus the ∆v to go from low Earth orbit (an orbit just above the atmosphere) to lunar orbit is 4100 m/s, which is only 300 m/s more than to go to geosynchronous orbit (note that these numbers are not additive). Figure 2-2 shows a schematic diagram of the ∆v's required to move from one point to another. It is drawn to scale with respect to ∆v, and shows that most of the effort of space travel near the Earth is spent in getting 100 km or so off the Earth, that is, into low Earth orbit. Note, this orbit is so close to the Earth's surface that it does not show on the scale of figure 2-1. Thus travel time to low Earth orbit is a few minutes, but the effort required to obtain this orbit is very large. Or, again revealing the inverse relation between travel time and effort, to go from low Earth orbit to lunar orbit takes about 5 days, but requires less than half the effort needed to go from the Earth's surface to low orbit. Figure 2-2 also shows that certain points that are far apart in distance (and time) are quite close together in terms of the propulsive effort required to move from one to the other; for example, geosynchronous orbit, L5, and lunar orbit.
The three primary criteria for choosing sites for the various parts of the colony — mines, factories, farms, homes, markets — are ease of access to needed resources, rapidity of communication and transportation and low cost. The topography of space can be exploited to achieve satisfactory balances among them.
Figure 2-1 — Earth-Moon libration points.
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Figure 2-2 — Velocity increments to transfer between points in space.
<!-- image -->SOLAR RADIATION: AN ABUNDANT AND ESSENTIAL SOURCE OF ENERGY
Although apparently empty, space is in fact filled with radiant energy. Beyond Earth's atmosphere this energy flows more steadily and more intensely from the Sun than that which penetrates to the surface of the Earth. Through one square meter of space facing the Sun pass 1390 W of sunlight; this is nearly twice the maximum of 747 W striking a square meter normal to the Sun at the Earth's surface. Since the Earth does not view the Sun perpendicularly and is dark for half of each day, a square meter of space receives almost 7.5 times the sunlight received by an average square meter on the whole of the Earth. Figure 2-3 compares the wavelength distribution of the Sun's energy as seen from above the Earth's atmosphere with that seen at the surface of the Earth and shows that not only is the intensity of sunlight greater in space, but also there are available in space many wavelengths that are filtered out by the Earth's atmosphere.
Figure 2-3 — Solar radiation as a function of wavelength.
<!-- image -->To live in space humans must be protected from the fierce intensity and penetrating wavelengths of unattenuated sunlight, but this same energy is one of the primary resources of space. If this steady, ceaseless flux of solar energy is tapped its value may be very large. If the Sun's energy is converted with 10 percent efficiency to electrical power which is sold at a rate of $.012/kW-hr, a square kilometer of space would return more than $14,000,000 each year.
It is important for the colonization of space that an effective way be found to use this solar energy.
MATTER IN SPACE: A MAJOR RESOURCE
Space is extraordinarily empty of matter. The vacuum of space is better than any obtainable with the most refined laboratory equipment on Earth. This vacuum may be a resource in its own right, permitting industrial processes impossible on Earth. Nevertheless, there is matter in space and it is of great interest to space colonization.
Matter in space comes in a broad spectrum of sizes — great masses that are the planets and their satellites, smaller masses that are the asteroids, even smaller meteoroids, and interplanetary dust and submicroscopic particles of ionizing radiation. The entire range is of interest to space colonization because the principal material resources must come from the great masses while meteoroids and ionizing radiation may be dangerous to the colony's inhabitants.
Sources of Matter in Space
The principal material resources of space are the planets, their moons, and asteroids. Their accessibility is determined by distance from possible users of the material and by the depth of the gravitational wells through which the matter must be lifted.
The planets of the solar system are major loci of material resources, but they are mostly very distant from prospective colonies, and all sit at the bottoms of deep gravitational wells. The effort to haul material off the planets is so great as to make the other sources seem more attractive. Of course, if a planet is nearby and is rich in resources, a colony might find the effort justified. Consequently, the Earth could be an important source of material to a colony in its vicinity, especially of the elements hydrogen, carbon, and nitrogen that are not available in sufficient amounts elsewhere near Earth.
The moons of planets, with their usually shallow gravitational wells, offer an attractive source of needed matter. The moons of Mars have very shallow wells, but they are too distant from any likely initial site for a colony to be useful. The same argument applies even more strongly to the more distant satellites of the outer planets. It is the Earth's natural satellite, the Moon, that offers an attractive prospect. The Moon is near the likely initial sites for a space colony; its gravitational well is only 1/22 as deep as that of the Earth. Moreover, as figure 2-4 shows, the Moon can be a source of light metals, aluminum, titanium, and iron for construction, oxygen for respiration and rocket fuel, and silicon for glass (ref. 6). There are also trace amounts of hydrogen (40 ppm) and carbon on the Moon, but not enough to supply a colony. Certainly the Moon's resources, supplemented with small amounts of particular elements from Earth, can supply all the elements necessary to sustain human life and technology in a space colony.