NEEP602 Course Notes (Fall 1996)
Resources from Space
2.3 UNIQUE LUNAR ENVIRONMENTAL CONSIDERATIONS
The human need for life support in an alien environment is the principal driver of EVA systems requirements. However, the design of EVA systems is influenced principally by characteristics of the environment in which they must operate. The environment of the moon presents several critical issues for lunar EVA. These critical issues guide our thinking as we develop requirements to support advanced EVA on the lunar surface. They are the guard rails that we bump as we consider design criteria and requirements.
Having been to the moon and worked there, we already understand how to adapt EVA technology (suits, rovers, tools, etc.) to this unique environment. Table 2-8 is a brief review of the characteristics of the lunar environment; Table 2-9 summarizes the lunar radiation environment. Table 2-10 presents some significant considerations for lunar EVA. Further information on the lunar environment appears in sections 3.2.15, Radiation Tolerance, and 3.2.17, Sand, Dust, and Surface Terrain.
2.3.1 Absence of an Atmosphere
The moon is void of any substantial atmosphere but surface molecules of gas have been measured at densities of 2xl05 molecules/cm3 (10-12 Torr). This value could increase modestly with major activities on the surface and subsurface mining in support of lunar base operations.
Besides driving the requirement to provide breathing air and air pressure to the EVA crew, the lack of an atmosphere on the lunar surface also affects the visual perceptions of the crew. There is no attenuation and scattering of sunlight as it arrives from the sun. Solar illumination at the moon is approximately l0,000 foot candles, and the mean albedo is about .07. Consequently, there are sharp gradients in lighting on the lunar surface, with bright light in one spot but adjacent dark shadows, as shown in Figures 2-8 and 2-9. However, solar illumination falling on the lunar surface is backscattered into shadowed areas, so it is possible for the crew to see and work there. Actual contrast is not as crisp as the photograph (Figure 2-8) implies.
At close quarters, this chiaroscuro can affect task lighting; at longer ranges, it masks surface terrain features and compromises the crew's ability to judge the size, depth, and distance of craters. The textural gradient component of our learned distance estimation is affected, and distances are estimated with error due to the lack of feature softening with increasing distance. This is especially true for new crews working on the lunar surface. Visual research suggests that after two or three days of experience in the new visual environment, humans will accommodate to the new visual cues, provided they have sufficient opportunity to learn distance estimation in the stark environment. Artificial lighting, even in full sun situations, may be required in order to provide the crew with full visual apprehension and comprehension of the environment. While the Apollo films show the crew benefiting from reflections of sunlight off their suits and the down-sun lunar surface to illuminate shadowed areas (see Figure 4-3), an active illuminator that does not depend on the sun angle and crew position is a more predictable approach, particularly for lunar night.
The lack of atmosphere means that there is no natural help in cleaning surfaces of lunar dust contamination. One possible remedy is to use some form of canned air to blow surfaces clean, provided this technique does not abrade those surfaces.
The absence of an atmosphere also means that there is no overhead protection from space radiation and no atmospheric friction to slow or burn up micrometeoroids. Consequently, precautions must be taken against exposure to these hazards.
2.3.2 Reduced Gravity
The lunar gravity is about 1/6 that of Earth (0.165 g), or 1.62 m/s2 gravitational acceleration at the lunar equator as compared to 9.78 m/s2 at the Earth's equator (Bufkin, 1988). This lowgravity environment produces a kinesthetic and proprioceptive perception of up and down; it causes things to "fall down" but at rates different than on Earth. Nonetheless, in the design of lunar equipment, the center of gravity must be considered, especially in equipment worn by the EVA crewmember.
The one-sixth gravity on the moon provides humans with a visceral sense of up and down and can be used to keep tools and equipment in place, unlike the floating environment of microgravity, but it also permits humans to fall down in the regolith should they lose their balance. It permits humans to handle larger masses with ease and reduces the energy required to move these masses. It enables humans to leap and stride, but it also permits soil to be kicked in long trajectories above the surface. Human sensitivity to radiation may be affected by the reduced gravity environment. We should take advantage of this environmental feature in our designs for equipment and procedures to support long term lunar activity, just as we design to take advantage of microgravity in orbit.
2.3.3 Dust and Soil
The lunar regolith is mostly composed of extremely fine debris. (See section 3.2.17 for detailed properties.) This dust penetrates very small openings, clings to equipment, and loses its natural bearing strength and cohesiveness along routes and paths with repetitive traffic. Dust is an omnipresent fact of life on the moon; it is the most serious environmental problem for routine operations.
The dust and soil must be kept from the living spaces and shirt-sleeve environment of the main base and remote stations. It must be kept out of joints and off fabric, out of tools, and off radiators. Where it cannot be eliminated, it must be controlled; and where it can be used to benefit humans, it should be used, as a source of oxygen and other gases and as radiation protection piled up over shelters.
Dust carried into living spaces soon settles to the floor or is trapped in filters and represents only a temporary respiratory irritant. Nonsmokers are little affected by dust in terrestrial environments due to natural respiratory clearing processes. Unprotected bearings and other parts moving in contact, however, soon lose their functional characteristics.
The issue of how to control and compensate for the soil must be the subject of a thorough series of investigations. Can it be precipitated electrostatically? Can it be washed by water or other fluid? Can it be vibrated, blown, or brushed off effectively? Can it be isolated by the use of protective covers and garments? Can we derive design solutions from our clean room experience and use slight positive pressure, forced air circulation, grid floors and the like? What are the cumulative consequences of living and working in the regolith?
The lunar surface terrain is divided into two characteristic regions: the smooth maria that account for about 17% of the surface, and the highlands that make up the remaining 83% of the surface. In the maria regions, the slopes are from 0 to 10 angular degrees with a standard deviation (SD) of 3.7 degrees; in the highlands, the terrain slopes from 0 to 23 degrees with a SD of 4.5-6 degrees and higher. Sloped, crater-pocked, and boulder-strewn terrains are shown in Apollo photographs (Figures 2-10, 2-11, and 2-12, respectively).
The ridges, craters, slopes, blocks, and regolith present some design constraints for equipment and life support systems. During the Apollo missions, the limited mobility afforded by the EMU ankle design posed problems in negotiating the crater rims and slopes found on the moon. However, crews worked for an hour or more on slopes up to 20 degrees. The absence of a light diffusing atmosphere made the identification of subsurface craters very difficult in the down-sun direction. The large blocks pose problems for some line-of-sight communications, such as visual and microwave, while the smaller blocks pose problems for lunar surface vehicles and ambulatory EVA crewmembers. These are not insurmountable problems, and our equipment for the long duration exploration of the moon must account for these features of the terrain.
The lunar day and night period is approximately 28 Earth days. The sidereal period is slightly longer than 27 days and the synodic period is slightly longer than 29 days. The day/night periods offer some cyclic protection from solar non-ionizing radiation and also make artificial lighting for EVA a requirement.
The variable lunar surface temperatures are a function of solar illumination and shadows. The range of temperatures has been reported to be from 102 oK to 384 oK by Bufkin (1988), and from 102 oK to 407 oK by Bova (1987). The roughly 300 degree temperature differences can be experienced at the same time on a piece of equipment depending on its orientation with respect to solar illumination and deep space.
The radiation environment of the moon is harsh. The lunar surface is exposed to the continuous flux of galactic cosmic radiation (GCR) and to infrequent periods of intense solar energetic particle activity. Particle fluxes on the lunar surface are about 1/2 of their intensity in free space because they are blocked below the horizon. Crewmembers are not protected from these ionizing particles by either an atmosphere or a magnetosphere.
The GCR flux is between 1 and 2.5 particles cm2 s-1, depending on solar activity. It consists of about 90% protons, 9% helium nuclei, and 1% heavier nuclei. GCR dose is difficult to shield; approximately 5 to 10 m of lunar soil reduces the GCR dose to terrestrial levels.
Solar protons pose a significant risk to inadequately shielded crewmembers. Very large energetic particle events, which can cause acute radiation effects, occur at intervals of 7 to 10 years. Intermediate events, which can limit mission activities, occur several times each year. For nominal flares, build-up to peak radiation intensity occurs within a few hours or less. Monitoring of X-ray precursors may provide 30 minutes to one hour of additional warning.
We must contend with life threatening radiation hazards on the lunar surface. The galactic cosmic radiation and the intense particle radiation from solar flare events are potentially significant problems for the EVA crews exploring the lunar surface, remote from the main base. It has been suggested that the radiation hazard may be aggravated by other factors in space, including stress and low gravity. In addition to the natural radiation environment, we must consider the introduction of non-ionizing radiation associated with communications systems. High atmospheric nuclear explosions on Earth, currently banned by international treaty, might also contribute to the radiation hazard on the lunar surface. (Radiation hazards and shielding requirements are considered in detail in sections 3.2.15 and 4.5)
2.3.8 Range of Mobility, Navigation, and Communication
How far we can go beyond the protection and comfort of the main lunar base will depend on the portability of our life support systems and the distribution of our communication systems. With portable and distributed safe havens having medical and life support capabilities and with a communication system that allows full-time contact with the main base, EVA remote expeditions should be able to explore anywhere within walking or driving distance of a safe return to shelter. Lunar orbiting communications and navigation satellites could provide freedom to set up remote sites anywhere on the lunar surface. A network of distributed safe havens could permit us to leap-frog great distances on the surface, much as we did in exploring the American West, going from fort to fort and then establishing new forts at the "end of the line," or as we have done in the Antarctic with distributed shelters. (Section 4.7, Shelters, and sections 3.1.7 and 4.9, Communications, address these matters in more detail.)
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