ebook img

NASA Technical Reports Server (NTRS) 19930007678: Human safety in the lunar environment PDF

23 Pages·1.2 MB·English
Save to my drive
Quick download
Download
Most books are stored in the elastic cloud where traffic is expensive. For this reason, we have a limit on daily download.

Preview NASA Technical Reports Server (NTRS) 19930007678: Human safety in the lunar environment

ORIGINALCOHTAS ST 98- 1G8 7 OLORILLUSTR E!Q I /l_,, o Human Safety in the Lunar Environment I" Robert H. Lewis Any attempt to establish a Compared to the Earth, the Moon continuously staffed base or is geologically inactive. Volcanism permanent settlement on the and internally generated seismic Moon must safely meet the activity are almost nonexistent. challenges posed by the Furthermore, water and Moon's surface environment. atmospheric processes are This environment is drastically unknown on the Moon. Other different from the Earth's, and than igneous differentiation, radiation and meteoroids are which occurred early in lunar significant hazards to human history, the main geological safety. These dangers may be process that has acted on the mitigated through the use of Moon is impact cratering. underground habitats, the piling up of lunar material as shielding, The Moon was heavily bombarded and the use of teleoperated by meteoroids throughout much of devices for surface operations. its early existence. Evidence from the Apollo expeditions suggests that the bombardment decreased The Lunar Environment significantly about 3.8 billion years ago. This early bombardment and The Moon is less dense than the subsequent impacts during the past Earth and considerably smaller. Its 3.8 billion years have pulverized density indicates that the Moon's the lunar surface into dust and bulk composition is also somewhat small fragments of rock, a layer different from Earth's, although it is referred to as the lunar "regolith." still aterrestrial (rocky) body. The The majority of the Moon's surface Moon's surface gravity is only is made up of heavily cratered one-sixth the Earth's. And, with terrain, rich in the mineral its consequently lower escape plagioclase feldspar and known velocity, the Moon cannot maintain as the lunar "highlands." The a significant atmosphere. Thus, uncompacted, upper portion of the surface is directly exposed to the highlands' regolith is 10 to the vacuum of space. Lacking an 20 meters deep in most places. A atmospheric buffer, the Moon has smaller portion of the lunar surface, asurface temperature that varies mostly on the Earth-facing side, over several hundred degrees consists of basaltic lava flows and Celsius during the course of a is known as the lunar "maria." The lunar day/night cycle. A complete maria are geologically younger than lunar day, one full rotation about the highlands and thus have been its axis, requires approximately cratered far less than the highlands 27-1/3 terrestrial days. have. The depth of uncompacted 272 regolith in the maria is roughly 4 to The solar wind is an isotropically 5 meters. distributed, neutral plasma travelling at an average velocity of The bulk density of lunar regolith 400 km/sec, tn Earth/Moon space, increases with depth. Its upper it has an average density of about surface is believed to have 45- 10 particles per cubic centimeter percent porosity (Taylor 1982, (Taylor 1982, p. 155). This plasma p. 119). The porous upper is composed of a relatively constant 20 cm of the regolith results from flux of charged particles, mainly repeated meteoroid impacts, which electrons and protons, plus ions of stir up the exposed surface and various elements. occasionally form large craters. These meteoroids represent A solar flare is similar in composition potential hazards to both manned to the solar wind, but its individual and unmanned activities. The particles possess higher energies. meteoroid hazard on the lunar A solar flare may be considered surface may be greater than a transient perturbation in the that in free space (Mansfield 1971, solar wind. Exact timing of the p. 1-4-14). In addition to the free- occurrence of a flare is difficult to space flux of meteoroids, there predict, but the frequency of flares is also ejecta from the impacts. may be related to the 11-year solar Some fragments of ejecta could cycle. Most flares can be observed have larger masses and slower at the Sun's surface some time velocities than the free-space before a large increase in the solar population of meteoroids. wind's higher energy particles is detected in the vicinity of the The Moon's surface is exposed to Moon. Not all solar flares yield three types of hazardous ionizing particles that reach the Earth/Moon radiation. The first two, the solar vicinity, but, of those which do, this wind and solar flares, are produced flux reaches a peak within hours by the Sun. The third type has its and then decreases over several origin outside the solar system and days to the previous solar wind is known as galactic cosmic rays. level. 273 Galacticcosmicraysareapparently The Earth's magnetic field and isotropically produced outside the atmosphere provide significant solar system. The average cosmic protection, lacking on the Moon. ray flux has been almost constant The cosmic ray flux per square over the past 50 million years centimeter of lunar surface per (Taylor 1982, p. 159). Cosmic rays year (during minimum solar activity) are made up of very high energy contains 1.29 x 108 protons plus particles consisting mostly of 1.24 x 107 helium nuclei plus protons and electrons, plus some 1.39 x 106 heavier ions for a total heavy nuclei (iron, for example), of 1.4279 x 108 particles per cm 2 positrons, and gamma rays. per year.* Fortunately, as the Both the Earth and the Moon energy of the radiation increases are exposed to these cosmic rays, from solar wind to cosmic rays, the but the Moon's surface receives a frequency of encountering that higher intensity of cosmic rays radiation decreases. than does the Earth's surface. "Countingonlyparticleswithavelocitygreaterthan10MeV pernucleon. Informationfrom D.StuartNachtwey,MedicalSciences Division,LyndonB.JohnsonSpace Center,Houston. 274 Lunar Public Works should be able to obtain these elements concrete. [These manufacturing processes by heating the soil. Once people have are also discussed in the accompanying Thedry, barren Moon mightnot seem like a provided them with lunar water, carbon Materials volume.] The unprocessed soil promising land for settlement. But, with the dioxide, oxygen, and nitrogen, plants itself can serve as shielding against the eyes of a chemist, a pioneer settler may see should be able to extract nutrients directly diurnal temperature fluctuations and, lunar conditions as advantages and lunar from the lunar soil. more importantly, against the hazards of soil as a bountiful resource. radiation unscreened by an atmosphere and undeflected by a magnetic field, as Lunar Filling Station "_ discussed by Rob Lewis in this paper. Lunar Water Works ,_ x ll/ U I I The lunar covered wagon will be a chemical f-'_1 The first concern of a lunar pioneer must be water. There may or may not be water, as rocket, its horsepower hydrogen and Lunar Lightand Power _(_.'- trapped ice, at the lunar poles, but there oxygen. Hauling these propellants from certainly is an abundance of its chemical Earth will be expensive. Itmay prove The Sun shines on the Moon plentifully and components, oxygen and hydrogen. cheaper to provide them from the lunar soil. predictably, but only half the time. Storing Oxygen isthe most abundant chemical Forty tonnes of hydrogen, a reasonable solar energy over the 2-week-long lunar element (45% by weight) in'the lunar soils, estimate of the amount needed for all nightseems difficult and may have to be from which it may be extracted by various transportation from low Earth orbit for a done in the form of hydrogen, metals, and processes. In contrast, the concentration year, could be obtained from just 0.3 kin2 of oxygen whose extraction was powered by of hydrogen in lunar soil is very Iowl but the soilmined to a depth of 1m. Alternatively, energy from the Sun. Thus, initially, lunar total quantity available is nevertheless lunar transport vehicles might burn a metal power is likely to come from an imported great. The lunar surface has been bathed such as iron, aluminum, or silicon, even nuclear power plant. But electrical power for billions of years in the solar wind, a flux though these are less efficient rocket derived from the Sun is alikely lunar product of ionized atoms from the exterior of the fuels than hydrogen. All three are major and may even be the first major export to Sun. These ions embed themselves in constituents of lunar soils, in chemical the Earth from the Moon (once the souvenir the surface of grains of lunar topsoil. combination with oxygen, from which they market has been satisfied). Eventually, Furthermore, meteorites, unimpeded by an can be extracted. In fact, each is a the solar cells will probably be derived atmosphere, continually plow under the old byproduct of one or more processes for from lunar silicon, a byproduct of oxygen solar-wind-rich grains and expose new producing oxygen. [Several techniques extraction, or from lunar ilmenite, recently grains. In this way, large amounts of for extracting oxygen from lunar soils are shown to be photovoltaic. Conversion need hydrogen have become buried in the soil, proposed in the Materials volume of this not be efficient if a local material, simply enough to produce (if combined with lunar Space Resources report./ obtained, is used as the photovoltaic. More oxygen) about 1million gallons (3.8 million futuristically, lunar hefium-3 has been fiters) of water per square mile (2.6 km2) of proposed for use as a fusion fuel superior to soil to a depth of 2 yards (1.8 m). This Lunar Lumberyard tritium in that it is not radioactive, does not hydrogen can be extracted by heating the have to be made Fnnuclear fission reactors, soil to about 700°C. Supplying the Lunar Better than burning the iron, aluminum, and and yields a proton instead of a more Water Works is a matter of technology and sificon produced as byproducts of oxygen destructive neutron when it fuses with economics, but not a matter of availability of extraction from lunar soils might be to use deuterium. them to construct lunar shelters. Iron and oxygen and hydrogen on the Moon. Lunar soilcontains In abundance the aluminum can be fabricated into beams. materials required for fife support, Theboards of space construction may well transportation, construction, and power. Lunar Community Farm __ be made of glass. Molten lunar soilcan be With proper understanding and new ideas, cast into silicate sheets or spun into lunar pioneers should be able to turn the Thenext concern of a lunar pioneer will be fiberglass. These may have greater lunar environment to their advantage. food. Like hydrogen, carbon and nitrogen strength than similar products on Earth are available in large quantities from the because of the lack of water to interfere with Taken from Larry A. Haskin and Russell O. lunar soil,although they are present in very their polymer bonds. Partially distilled in a Colson, 1990, Lunar Resources- Toward low concentrations, having been placed solar furnace, soil residue may take on the Living Off the Lunar Land, in Proc. there, like hydrogen, by the solar wind. All composition of a good cement, which when Ist Symp. NASA/Univ. of Arizona Space the other nutrients necessary to life are combined with locally produced water and Engineering Research Center (in press), likewise present in the soil. Pioneer settlers the abundance of aggregate would become ed. Terry Triffet (Tucson). 275 The Human Factor Meteoroid impacts may have effects ranging from long-term erosion of the surface materials of In order to develop permanent pressure vessels and space suits human settlements on the Moon, all the way to penetration and we must understand how the local subsequent loss of pressure and environment influences the settlers' injury to personnel (see fig. 9). safety and health. The lack of More serious impacts could result atmosphere and the extreme in destruction of equipment and temperature range mandate the loss of life. use of sealed and thermally insulated enclosures. These enclosures--the colonists' first line of defense--will range from individual space suits to buildings. The next line of defense must protect the colonists from both meteoroids and radiation. Figure 9 Crisis at the Lunar Base Aprojectile has penetrated the roof of one of the lunar base modules and the air is rapidly escaping. Three workers are trying to get into an emergency safe room, which can be independently pressurized with air. Two people in an adjoining room prepare to rescue their fellow workers. The remains of the projectile can be seen on the floor of the room. This projectile is probably a lunar rock ejected by a meteorite impact several kJIometers from the base. A primary meteorite would likely be completely melted or vaporized by its high-velocity impact into the module, but a secondary lunar projectile would likely be going slowlyenough that some of it would remain intact after penetrating the roof. Detailed safety studies are necessary to determine whether such a meteorite strike (or hardware failure or human error) is likely to create a loss-of-pressure emergency that must be allowed for in lunar base design. Artist: Pamela Lee 276 In 1971, the Rockwell Lunar Base being penetrated within 100 days. Synthesis Study investigated A logical next step would be to several strategies for dealing with add more layers of material to the the meteoroid hazard. They took tent. This, of course, increases a probabilistic approach to the the weight of the tent and its problem of safety and examined associated transportation costs. several options. Rockwell was interested in providing portable The next option that Rockwell shielding for short-term surface considered was identical to that activities as well as more permanent just described but with an fixed shielding. The shielding might additional layer of material filling be needed many times during an the gap. In theory, the tent would expedition covering large distances. serve to fragment a meteoroid and the underlying material would On Earth, mobile expeditions which impede and absorb the fragments require temporary environmental before they reached the pressure protection that is lightweight and vessel. On the basis of their easy to redeploy often use tents. surface meteoroid flux model, The Rockwell study examined the the gap filter would need to have use of a tent-like structure which a density of 16 kg/m3 to provide could be erected over an inhabited a 0.9999 probability of no pressure vessel. The tent could penetration within 100 days. A be constructed of a lightweight design of this type may prove to material such as aluminum foil or be practical as portable meteoroid nylon. The Rockwell investigators shielding for short-term surface anticipated that such a structure activities. would act as an extra outer layer of protection against meteoroid However, these measures would impact. For their calculations, the be completely inadequate for any tent had an area of 46 m2 and the long-duration habitat (for a stay insulated wall of the pressure vessel of over 100 days), so the addition had a density of approximately of shielding material seemed 8 kg/m3. A small gap between the desirable. If lunar regolith were tent and pressure wall was initially used as a gap filler, significant considered. This arrangement protection could be added without could provide a 0.9999 probability increasing transport costs from of no penetrations in 100 days if Earth. Rockwell concluded that only a free-space meteoroid flux a gap of approximately 15.2 cm was assumed. However, assuming (6 inches), filled with lunar regolith, also the secondary ejecta hazard, would reduce the penetration risk they found that the tent system to less than one chance in 10 000 had only a 0.1 probability of not over a 2- to 5-year stay. 278 PRECEDING PPlGE BLANK NOT FILMED Althoughmeteoroid impacts may Individual responses to radiation be a serious problem on an exposure vary somewhat and there infrequent basis, the effect of is controversy over safe limits for ionizing radiation on human health long-term, low-level exposures. is continuous and cumulative over Currently, the maximum permissible an individual's lifetime. A brief whole-body dose for radiation discussion of radiation dosimetry workers is 5 rem/year and for the is now in order. The fundamental general public 0.5 rem/year (CRC unit of radiation transfer is the tad; Handbook of Tables for Applied 1 rad represents the deposition of Engineering Science 1980, 100 ergs of energy in 1 gram of p. 753). Both of these doses are mass. The characteristics of the larger than the dose of background deposition mechanisms vary and radiation at sea level that humans additional factors must be are normally exposed to. Just as considered. One conversion radiation workers must accept a factor is the quality factor, Q, greater risk than do members of the which is conservatively based on general public, so astronauts are the experimentally determined prepared to accept a greater risk than radiation workers. Table 3, relative biological effectiveness, RBE. When Q is multiplied by the provided by Stu Nachtwey, lists the doses and health risks that the rad exposure, the result is a unit Medical Sciences Division at the of dosage corrected for the type of radiation; this resulting dosage Johnson Space Center estimates is measured in a unit known as an astronaut on a Mars or lunar base the rem. mission would be exposed to during a period of minimum solar activity. 279 TABLE3. Approximate Radiation Doses and Health Risks for an Astronaut on a Mars or Lunar Base Mission During Minimum Solar Activity [From D. Stuart Nachtwey, Johnson Space Center] Radiation source Representative Skin dose Deep organ (5 cm) Excess lifetime shielding equivalent dose equivalent cancer incidence in a 35-year-old male* Chronic exposure < 0.1% Trapped belts 2 g/cm2AI < 2 rem < 2 rein (one-way transit) Free space 4 g/cm2 AI 75 rem/yr 53 rem/yr 1.2%/yr of exposure On lunar surface 4 g/cm2 AI 38 rem/yr 27 rem/yr 0.6%/yr of exposure On martian surface 16 g/cm2CO2 (atm.) 13.2 rem/yr 12 rem/yr 0.3%/yr of exposure Acute exposure to large (e.g., Aug. '72) solar particle event Free space 2 g/cm2AI 1900 rem 254 rem ~ 5.7% On lunar surface 4 g/cm2AI 440 rem 80 rem 1.8% + shielding 15 g/cm2AI 19 rem 9 rem 0.2% 0.1% On martian surface 16 g/cm2 CO2 (atm.) 9 rein 4.6 rem + shielding 60 g/cm2AI < 1rem < 1rem < 0.03% *The excess cancer incidence for a 35-year-old female is roughly twice that for a 35-year-old male. 280 The rate of irradiation per unit time Silberberg et al. (1985) have and the age and sex of the suggested that a compacted individual at irradiation are also layer of lunar regolith at least important. Younger people are 2 meters thick should be placed more sensitive to the cancer- over permanent habitats. With inducing effects of radiation than shielding of this thickness, the older people, and females are more colonists' yearly exposure could be sensitive than males because of held to 5 rem per year if they spent cancer induction to the breast and no more than 20 percent of each thyroid. Other serious radiation Earth month on the surface. In effects include cataracts, genetic order to provide an overall level of damage, and death. Radiation protection of no more than 5 rem exposure is considered cumulative per year even in the event of an over an individual's lifetime. extreme solar flare, such as occurred in February 1956, the Solar flares and cosmic rays are the depth of shielding would have to most dangerous radiation events be doubled. that lunar pioneers will be exposed to. The cosmic ray For the sake of completeness, it dosage at the lunar surface is should be pointed out that some about 30 rem/year and, over an lunar regoliths contain a naturally 11-year solar cycle, solar flare radioactive component material particles with energies greater than known as KREEP. KREEP, 30 MeV can deliver 1000 rem probably a product of volcanism, (Silberberg et al. 1985). Solar contains radioactive potassium, flares deliver most of their energy uranium, and thorium. Material periodically during only a few days containing a high concentration out of an 11-year cycle; whereas, of KREEP should not be used for the cosmic ray flux is constant. shielding, and care should be taken to avoid concentrating it Although the lunar surface radiation as shielding is prepared. The flux is too high to spend much time concentration of KREEP in most in, it is definitely possible to alleviate regolith material would add an the radiation danger with shielding. amount of radioactivity no more When colonists are removed from than that in the granite used in continuous exposure to surface buildings here on Earth. If the radiation, long-term settlement small contribution by KREEP to becomes possible. As in the case radiation dose is considered of meteoroid protection, the when exposures are calculated, simplest solution is to use locally it should not pose any significant available regolith for bulk shielding health problem by itself. of habitats. 281

See more

The list of books you might like

Most books are stored in the elastic cloud where traffic is expensive. For this reason, we have a limit on daily download.