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Chapter 2.

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.


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 show 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 Ll, 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 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 ()

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 (). Thus the 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 's required to move from one point to another. It is drawn to scale with respect to , 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.


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.

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.


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.

Asteroids offer some interesting possibilities. They have very shallow gravitational wells; some come closer to Earth than Mars; and some asteroids may contain appreciable amounts of hydrogen, carbon, and nitrogen as well as other useful minerals (refs. 7-14). Moving in well determined orbits which could be reached relatively easily, the asteroids may become exceptionally valuable resources, especially those that contain appreciable amounts of water ice and carbonaceous chondrite.

Comets may also be included in this inventory of material resources of space. Like many meteoroids, comets are thought to be "dirty snowballs," a conglomerate of dust bound together with frozen gases and ice. Comets are not suitable resources because of their high velocity and their infrequent penetration of the inner Solar System.

Meteoroids: An Insignificant Danger

Measurements made on Earth, in space, and on the Moon (refs. 8,10,11,13) have provided a fairly complete picture of the composition, distribution, and frequency of meteoroids in space. Near the Earth most of these travel relative to the Sun with a velocity of about 40 km/s. Figure 2-5 plots the frequency of meteoroids exceeding a given mass versus the mass, that is, it gives an integral flux. This graph shows that on the average a given square kilometer of space will be traversed by a meteoroid with a mass of 1 g or greater about once every 10 years, and by one with mass of 100 g or greater about once in 5000 years. A 10-kg meteoroid might be expected once every 100,000 years.

Danger of collision of a large meteoroid with a space habitat seems remote. But meteoroids occur frequently in clusters or showers, so that when one collision is likely, so are several more. There is a possibility of a correlated sequence of collisions with attendant damage more serious and complicated than from a single collision. This form of risk would only occur on a time scale of hundreds of years, which is the time scale characteristic of the occurrence of showers of meteoroids.

Although the probability of severe structural damage from impact of a meteoroid is negligible, blast effects of even a small meteoroid could be serious. Impact of a meteoroid with a closed vessel, for example, a spaceship or habitat, will produce a pressure wave which although quite localized will be dangerous to anyone near its origin. A one gram meteoroid, if it lost all its energy by striking a vessel, might kill or seriously harm someone standing close to the point of collision, but would be harmless to anyone more than a few meters away. Clearly it is desirable to shield a space colony against such collisions, and as is discussed subsequently, extensive shielding is also required for protection against ionizing radiation. This radiation shield would also protect against meteoroids.

Loss of atmosphere because of puncture by meteoroids is not a serious threat. In habitats of the size considered in this study, at least a day would be required to lose 60 percent of the atmosphere through a hole one meter in diameter - the size of hole that would be blasted by a meteoroid only once in 10,000,000 years. Smaller meteoroids might be responsible for small leaks, but the requirement for safe habitation under these circumstances is simply a regular (e.g., monthly) program for detecting and repairing such leaks. A more detailed analysis of the meteoroid hazard is given in appendix A.

Ionizing Radiation: Major Threat

Both the Sun and the Galaxy contribute fluxes of ionizing particles. The quiescent Sun constantly emits a solar wind (ref. 15) of about 5 to 10 protons, electrons, and particles per cubic centimeter traveling at speeds of about 500 km/s. These particles do not possess penetrating energies and therefore offer no threat to humans. However, the solar wind may indirectly affect humans because it neutralizes any separation of electric charge that might occur in space and produces a small variable interplanetary magnetic field (~5 nT at the distance of the Earth (lAU) from the Sun). Consequently, space contains essentially no electric field, whereas on Earth the electric field is 100 V/m near the surface. Given that the human body is a good electrical conductor and forms an equipotential surface in the Earth's field, and that humans live a good portion of their lives in electrostatically shielded buildings, it seems unlikely that living for prolonged times in the absence of an electric field would cause harm, but this is not definitely known. Similarly, although there is evidence that living in magnetic fields thousands of times more intense than the Earth's will harm people, the consequences of living in a magnetic field that is both 10,000 times weaker and variable with time are not known (refs. 16,17).

Solar flares and galactic cosmic rays on the other hand are direct and serious threats to life in space. In sporadic violent eruptions the Sun emits blasts of high energy protons capable of delivering dangerous doses of radiation. Figure 2-6 shows the integral flux of solar flare particles at the Earth's distance from the Sun and compares it with the galactic flux. For these moderate sized events the galactic flux is the dominant source of particles above 1 GeV/nucleon. Also shown in figure 2-6 is the most intense flare ever recorded (a class 4 solar flare) which occurred on February 23, 1956. This flare illustrates the worst known radiation conditions to be expected in space. Without a space habitat having extensive protection against extremely energetic protons such a flare would contribute many tens of rem of dose in less than an hour to moderately shielded personnel, and many times the fatal dose to the unprotected human being. (For an explanation of the "rem" see appendix B.)

The frequency of dangerous cosmic-ray flares is once in several years during a solar maximum, and once in a few decades for a flare as large as the class 4 flare. Because a significant portion of the protons originating from a large flare are relativistic (i.e., traveling at speeds approaching that of light), there is only a few minutes between optical and radio indications of an outburst and the arrival of the peak of the proton flux. People not in a sheltered place have very little time to get to one. Once a flare has begun, fluxes of energetic particles persist for a day or so in all directions.

Cosmic rays from the galaxy are a continuous source of highly penetrating ionizing radiation. Figure 2-7 shows the galactic cosmic ray spectrum and chemical abundances. The lower-energy portions of the curves show the modulating effect of the solar wind which with varying effectiveness over the 11-year solar cycle sweeps away from the Sun the less penetrating particles of the galactic cosmic rays. In the absence of any shielding the galactic cosmic radiation would deliver an annual dose of about 10 rem.

An important feature of note in figure 2-7 is the presence of heavy nuclei such as iron. In fact, heavy cosmic ray nuclei range up to heavy transuranium elements but quite noticeably peaking in abundance around iron. When a fully ionized iron nucleus is traveling below about half the speed of light its ionizing power is several thousand times that of minimally ionizing protons. (See appendix B for a brief discussion of the behavior of charged particles in matter.) At this level of ionizing power the passage of a single iron nucleus through the human body destroys an entire column of cells along its trajectory. The total amount of energy dumped in the body is small, but it is concentrated intensively over localized regions.

It is not yet known how bad this form of radiation is in terms of such things as increased rates of cancer. However, the loss of nonreproducing cells, such as spinal-column nerve cells, that any given exposure will cause can be calculated, Comstock et al. (ref. 18) estimate that the Apollo 12 astronauts during their two week voyage lost between 10-^7 and 10-^4 of their nonreplaceable cells. Such losses, although negligible in adults, might be very serious in developing organisms such as children.

The phenomenon of secondary particle production is important. When high-energy particles collide with matter, in a shield for example, they produce a great spray of particles, which in turn may produce even more particles. Consequently, the addition of a little shielding may, in the presence of highly energetic particles like those at the upper end of the cosmic ray spectrum, give rise to an even larger radiation dosage than if no shielding were used. There is also the possibility that a little shielding will slow down the rapidly moving heavy ions and make them more effective in the damage they do to tissue. Thus, for shielding that has a mass of a few tonnes (l) for each square meter of surface protected the effect will be to increase the annual dosage from cosmic rays from about 10 rem to as much as 20 rem.

But what is an acceptable radiation dose? For the terrestrial environment the U.S. Federal Government sets two standards (refs. 19-21). For radiation workers, adults over the age of eighteen working in industries where exposure to radiation is apt to occur, the standard is 5 rem/yr. For the general population, and especially children and developing fetuses, the standard is less than 0.5 rem/yr. Arguments can be sustained that these limits are conservative. There is evidence that exposures to steady levels of radiation that produce up to 50 rem/yr will result in no detectable damage (refs. 20,21), but the evidence is not fully understood nor are the consequences known of long-term exposure at these levels. For comparison, most places on Earth have a background of about 0.1 rem/yr.

Chapter 3

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