Space Settlements - A Design Study 1977

The hazard of meteors is not necessarily that of a single collision. Because meteors occasionally occur in clusters or "showers," they could, by a series of hits, initiate a chain of failures otherwise impossible.

On an average night an observer will see about 10 meteors an hour. During the most intense of annual showers the observation rate rises to about 50 an hour. Thus the existence of annual showers causes temporary flux enhancements by perhaps a factor of 5. In the year 1833 the hourly rate over Paris from the shower of Leonids rose to 35,000 an hour — an increase in flux of many thousand times. Thus the meteoroid flux can at times increase enormously to constitute a qualitatively different kind of hazard from the usual situation. A detailed analysis of what risk such a shower would entail must await a final system model for space habitation and extensive computer simulation. Even so it is apparent that the risk from showers would only occur on a time scale of hundreds of years.

The second piece of knowledge needed to assess meteoroid risk to space habitation is the damage caused by a meteoroid of a given size. There are three mechanisms of destruction. First, a mass traveling at the typical meteoroid velocity of 40 km/s will create a crater in any material object with which it collides. McKinley (ref. 22) quotes Whipple to the effect that the depth of penetration is related to the incident energy by:

$$d = (3E / \pi \rho \zeta)^{1/3}$$

Whipple is also quoted as saying that a thin metal sheet a meter or so away from the main hull acts as a "meteor bumper" by vaporizing any incident meteoroid and thus minimizing blast loading on the hull through 1/r² attenuation of the blast wave.

The second damage effect is shock wave destruction of interior structures if a meteoroid penetrates the main hull. Such an event is equivalent to creating an explosion at the point of entry with 200 g of TNT for every gram of meteoroid traveling at 40 km/s. The overpressure in Pa of a strong explosion shock wave is given roughly by

$$P_{OVER} = 0.34 E/R^3$$

E = total energy released, J R = distance from shock center, m

As a point of reference, as little as 5 psi (34.5 kPa) overpressure suffices to knock down buildings and kill an average human being.

The third effect of meteoroid impact is the loss of internal atmosphere through the hole created. The repairing of such a hole is not a difficult problem since air flows, though supersonic in the hole region, fall to gentle values a few hole diameters away. The main operational problem for a habitat is efficient detection and repair of any small holes that occur.

TABLE 2-1 — SCALE OF DAMAGE FROM COLLISIONS WITH METEOROIDS AS A FUNCTION OF METEOROID MASS

| Mass (g) | Energy (J) | TNT equiv (g) | Crater depth (cm) | $R(2\text{ atm})$ (m) | Flux (km⁻² yr⁻¹) | | :--- | :--- | :--- | :--- | :--- | :--- | | 1 | $8 \times 10^5$ | 200 | 10 | 1.1 | 0.1 | | 100 | $8 \times 10^7$ | 20,000 | 46 | 5.1 | $2 \times 10^{-4}$ | | 10,000 | $8 \times 10^9$ | 2,000,000 | 215 | 23.6 | $1 \times 10^{-5}$ |

Table 2-1 presents the risk factors for a space habitat due to meteoroid impacts. In this table $R(2\text{ atm})$ is the radius at which any shock wave created has two atmospheres of overpressure² — a high value for a "kill radius!" Obviously the hazards of meteoroids pose little danger to kilometer-sized habitats.

² 2 atmospheres overpressure = 202 kPa.

APPENDIX B: IONIZING RADIATION IN SPACE

Radiation which deposits 100 ergs of energy³ per g is said to deliver a dose of 1 rad. Because different forms of radiation may deposit this energy at different rates and with different intensities along the track, the biological damage of a dose of 1 rad varies with the type of radiation. To correct for this effect the radiation dose in rads is multiplied by the "relative biological effectiveness" (RBE) of the particular kind of radiation. The product is then a measure of danger of the particular kind of radiation, and that product is described in units of rems. Thus, 1 rem of neutrons and 1 rem of X-rays represent the same amount of biological danger. (For X-rays 1 rem results from the exposure of 1 roentgen.) The RBEs of most of the common kinds of radiation are given in the table.

The principal ionizing radiations to be found in space are summarized in table 2-2. Ionizing radiation endangers humans because it is capable of breaking chemical bonds in tissue. The damaging power depends upon the amount of energy deposited per unit volume, the rapidity with which the energy is transferred, and its concentration along the track of the particle of radiation.

³ The most commonly used unit to measure energy of radiation is the electron volt (eV). This very small unit is defined as equal to the energy imparted to a particle with unit electric charge when it is accelerated through a potential difference of 1 V, or $1.6 \times 10^{-12}$ ergs. Because of the small value of this unit super multiples are more common — keV for $10^3$ eV, MeV for $10^6$ eV, and GeV for $10^9$ eV.

TABLE 2-2 — IONIZING RADIATIONS IN SPACE

| Type | Source | Energy | RBE | | :--- | :--- | :--- | :--- | | Protons | Sun, Galaxy | 1 MeV - 10 GeV | 1.0 | | Electrons | Sun | 1 keV - 1 MeV | 1.0 | | Alpha particles | Sun, Galaxy | 1 MeV - 10 GeV | 10.0 | | Heavy ions | Galaxy | 100 MeV - 100 GeV | 20.0 | | X-rays | Sun | 1 keV - 100 keV | 1.0 |

Figure 2-8 — Ionizing power of protons in SiO2 vs. energy.

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The damaging power of heavy charged particles with charge numbers equal to or greater than 3 is most conveniently described in terms of their ionizing power. This measures how many chemical bonds per unit of body mass are broken and thereby gives a rough measure of the tissue damage sustained.

Figure 2-8 plots the ionizing power of protons in silicon dioxide as a function of proton energy. Since the units of ionizing power are in units of mass traversed, the same values are reasonably accurate for all matter with a low charge number (Z), for example, human tissue. This basic curve holds for any ion species when the vertical axis is multiplied by the ion's charge number squared (Z²).

Essentially the result is that the ionizing power increases as the particle energy decreases, so as to cause the more slowly moving particles to be the most damaging. In the extreme relativistic energy region the damage effects are basically constant — at a level which is termed the ionization minimum. At the lowest velocities the charged particles are finally neutralized by picking up electrons.

REFERENCES

1. La Grange, J. L.: Oeuvres, vol. 6, pp. 262-292, Sewer and Darbaux, Paris, 1873, "Essai Sur Le Probleme bese Trois Corps" (L'Academie Royale de Sciences de Paris, vol. 9, 1772).
2. Szebehely, V. G.: Theory of Orbits, the Restricted Problems of Three Bodies, Academic Press, New York, 1967, and Analytical and Numerical Methods of Celestial Mechanics, American Elsevier Publ. Co., N.Y., 1967, pp. 227, 229.
3. Katz, J. L.: Numerical Orbits Near the Triangular Lunar Libration Points, Icarus, vol. 25, June 1975, pp. 336-359.
4. Kamel, A. A.: Perturbation Theory Based on Lie Transforms and Its Application to the Stability of Motion near Sun-Perturbed Earth-Moon Triangular Libration Points, NASA CR-1622, 1970.
5. Farquhar, R. W.: The Utilization of Halo Orbits in Advanced Lunar Operations, NASA TN D-6365, 1975.
6. Ross, H. J., Jr., et al.: Compositional Data for Twenty-Two Apollo 16 Samples, Proceedings of the Fourth Lunar Science Conference, vol. 2, pp. 1149-1158.
7. Chapman, C. R.: The Nature of Asteroids, Scientific American, vol. 232, Jan. 1975, pp. 24-33.
8. Wood, J. A.: Meteorites and the Origin of Planets, McGraw-Hill, N.Y., 1968.
9. Gehrels, T., ed.: Physical Studies of Minor Planets, Washington, D.C., 1971, NASA SP-267.
10. Dohnanyi, J. S.: Interplanetary Objects in Review: Statistics of their Masses and Dynamics, Icarus, vol. 17, no. 1, Aug. 1972, pp. 1-48.
11. Wetherill, G. W.: Solar System Sources of Meteorites and Large Meteoroids, Ann. Rev. of Earth and Planetary Science, vol. 2, 1974.
12. McCord, T. B., Chapman, C. R.: Asteroids: Spectral Reflectance and Color Characteristics, Astrophysical Journal, vol. 197, 1975, pp. 781-790.
13. Middlehurst, B. M., Kuiper, G. P., ed.: The Solar System: Vol. 4, The Moon, Meteorites, and Comets. Univ. Chicago Press, 1963, Physics and Chemistry of Meteorites, Wood, J. A., pp. 337-401, Chapter 12. Chemical Evolution of Carbonaceous Chondrites, DuFresne, E. R., and Aners, E., Chapter 14. (Especially the tables on pp. 348 and 514) pp. 496-526.
14. Ehricke, K. A.: Space Industrial Productivity, New Options for the Future, presentation before Subcommittee on Space Science and Applications of the Committee on Science and Technology, U.S. House of Representatives, 94th Congress, Serial M, vol. 2, Sept. 1975.
15. Sonett, C. P., Wilcox, J. M., Coleman, P. J., ed.: Solar Wind, a conference held at Pacific Grove, Calif., Mar. 21-26, 1971. NASA SP-308.
16. Beischer, D. E., Reno, V. R.: Magnetic Fields and Man: Where do we Stand Today? North Atlantic Treaty Organization Advisory Group for Aerospace Research and Development (AGARD) Conference Proceedings, pt. 3, Special Biophysical Problems in Aerospace Medicine, no. 95, Luchon (France), 1971, pp. C12-1 to C12-9.
17. Nakhil N'Itskaya, Z. N.: Biological Effect of Permanent Magnetic Fields, Kosmicheskaya Biologiya I Aviako Smicheskaya Meditsina, vol. 8, no. 6, 1974, pp. 3-15.
18. Comstock, G. M., et al.: Cosmic-Ray Tracks in Plastics: The Apollo Helmet Dosimetry Experiment, Science, vol. 172, Apr. 9, 1971, pp. 154-157.
19. Basic Radiation Criteria, National Council Radiation Protection and Measurement, NCRR Report 39, 15 Jan., 1971, pp. 88-107.
20. Martin, A., Harbison, S. A.: An Introduction to Radiation Protection, Chapman and Hall, London (England), 1972.
21. Easley, C. W.: Basic Radiation Protection; Principles and Organization, Gordon and Breach, 1969.
22. McKinley, D. W. R.: Meteor Science and Engineering, McGraw-Hill, New York, 1961, p. 274.

CHAPTER 3: Human Needs in Space

Elementary essentials such as air, water, food, and even the sensation of weight all have to be provided to the space colony. Engineering criteria to assure physiological safety and comfort are essential, but equally important is to provide for psychological and esthetic needs of the colonists.

The structure, mass, and shape of the habitat are sensitive to the choice of design criteria. Rather substantial savings in structural mass, and hence in cost and construction time, can be obtained by deviating from Earth conditions. Because the physiological effects of appreciable deviations from some of the terrestrial conditions are unknown, the living conditions in space are designed to be similar to those on Earth despite additional costs. The treatment of weightlessness is an example of this conservative approach.

WEIGHTLESSNESS: PSEUDOGRAVITY IS NEEDED

An outstanding feature of space is the absence of the sensation of weight. In vessels moving freely in orbit objects exhibit weightlessness; they are said to be in "free fall," or subject to "zero gravity" or "zero g." Weightlessness is a major potential resource of space, for it means humans can perform tasks impossible on Earth. Large masses do not require support, and their movement is restricted only by inertia. Structures can be designed without provision for support against the forces of gravity; in free space there is no such thing as a static load. Although these opportunities are only beginning to be explored, it seems likely that weightlessness will permit novel industrial processes (refs. 1,2). Moreover, in free space, levels of pseudogravity can be produced and controlled over a wide range of values. This capability should foster the development of manufacturing processes not possible on Earth. Despite these potentially important commercial advantages of life in free fall, possible physiological consequences are of concern.