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.
Figure 2-4 — Average compositions of rocks and soil returned by Apollo missions, excluding oxygen (~45%) and elements present in amounts less than 1000 ppm.
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Figure 2-5 — Impact rates of meteoritic material.
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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 (1 AU) 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.
Figure 2-6 — Energy spectra from several moderate size solar flares (dotted curves) compared with galactic cosmic ray spectrum.
<!-- image -->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.
Figure 2-7 — Distribution of energies of galactic cosmic rays. This is a graph of the more abundant nuclear species in cosmic rays as measured near the Earth. Below a few GeV/nucleon these spectra are strongly influenced by the Sun. The different curves for the same species represent measurement extremes resulting from varying solar activity. (Taken from Physics Today, Oct. 1974, p. 25.)
<!-- image -->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 tonnes¹ 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.
¹ A metric ton, or tonne, is 10⁶ g and equals 0.98 long tons and 1.10 short tons.
APPENDIX A: METEOROIDS AND SPACE HABITATS
The risk of damage by collision with meteoroids can be assessed if the flux of meteoroids as a function of mass values can be determined. Data to do this come from three sources:
- Photographic and radar observations from the Earth of meteors entering the atmosphere,
- Measurements from spacecraft of meteoroid fluxes,
- Lunar impacts measured by lunar seismometers.
In the meteoroid mass range from 10⁻⁶ to 1 g spacecraft sensors provide abundant data, and for masses above 10 kg the lunar seismic network is believed to be 100 percent efficient in assessing the flux. Earth based data are subject to large corrections but agree with space data at the 10-g value.
Figure 2-5 shows the distribution law (integral flux) for meteoroid masses of interest to the problem of habitat protection. The Prairie Network data are not shown because they are subject to large corrections of an uncertain nature. The type of meteoroid structure most commonly found in space is a conglomerate of dust bound together by frozen gases. This has been described as a "dirty snowball" as opposed to a stony or nickel-iron rock that remains at the Earth's surface after a meteorite survives passage through the atmosphere.