Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
The Human Exploration of Space http://www.nap.edu/catalog/6058.html RESEARCH FOR MISSION OPTIMIZATION 47 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. 3 Research for Mission Optimization This chapter describes several issues that are relevant to the health and well-being of humans but that appear, at present, to represent less critical threats to the lives of astronauts than those discussed in the previous chapter. They are, however, no less important as related to optimum human performance during exploration missions. In addition, increased knowledge of the physical aspects of the Moon and Mars is required to ensure that human explorers perform efficiently. As new information accumulates, and as implementation decisions are made, the significance of any or all of the areas where research is needed to ensure mission optimization could increase to the point that they become critical issues. SENSORIMOTOR INTEGRATION Changes in the gravito-inertial environment during a space mission may lead to disturbances of sensorimotor function.1 The consequences may include impaired spatial orientation, instability of position and gaze, and motion sickness. Fortunately these problems are of short duration because the central nervous system adapts to those changes within a few days provided a constant environment is maintained . There are, however, two caveats to this assessment of relative risk. First, gravito-inertial changes occur at the most critical times during a mission: takeoff and landing. Second, the crew of a spinning spacecraft (possibly used to counter the problems associated with prolonged exposure to microgravity) might suffer repeated changes in Copyright © National Academy of Sciences. All rights reserved.
The Human Exploration of Space http://www.nap.edu/catalog/6058.html RESEARCH FOR MISSION OPTIMIZATION 48 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. their gravito-inertial environment when moving to different parts of the craft or if the spin rate is changed. Fortunately, no known long-term health risks are associated with sensorimotor adaptation to microgravity. Although both the National Institutes of Health and NASA are studying vestibular function and its interaction with other sensorimotor modalities, the etiology of motion sickness in general, and space adaptation sickness in particular, is still not known. The extent to which adaptive responses can be shaped or overridden by appropriate training in sensorimotor strategies is also unknown. Studies of vestibular function and its neuronal substrates in appropriate animal models are needed both on the ground and in a microgravity environment. Parallel studies of human sensorimotor performance in both environments must also be pursued. IMMUNOLOGY Can the immune system be damaged by spaceflight? This possibility stems from observations of abnormalities in the two major types of human lymphocytes, T-cells and B-cells, and in other white blood cells on the Spacelab D-1 mission. A reduction of function and disordered morphology of T-cells have been detected on some other flights. Moreover, changes in rat immunity have been observed on spaceflights conducted by the former Soviet Union. Serious infections in humans during spaceflights are rare. Thus, there have been no opportunities to systematically assess the capacity of humans or other mammals to contain and eradicate infections by various types of terrestrial microbes while in space. The potentially devastating consequences of any immune dysfunction, particularly on long-duration flights, indicate the urgent need for further studies. The possible defects already identified in lymphocytes and also other elements of immunity vital to specific and adaptive defense mechanisms in humans need to be examined. The potential effects of spaceflight on normal human immunity must be judged in terms of the antibody responses and reactions of lymphocytes, macrophages, and other white blood cells to different types of antigens. The most common antigens on Earth are proteins, carbohydrates, and complex lipids. These are presented to the immune system in soluble form and as a part of cells or other complex structures. The studies of responses to antigens in space should use both intact microbes, to mimic infections, and soluble purified proteins and carbohydrates, to simulate simple vaccines. A vital aspect of immunity is a memory of exposure to antigens. Thus, comprehensive studies should encompass both new and previously encountered antigens of each major chemical class and physical form. This diver Copyright © National Academy of Sciences. All rights reserved.
The Human Exploration of Space http://www.nap.edu/catalog/6058.html RESEARCH FOR MISSION OPTIMIZATION 49 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. sity of experimental challenges is critical for assessment of immunity in space, because the variety and intensity of antigen challenges to the immune system will be substantially different from that experienced on Earth: the unique closed environment imposed by the spacecraft offers significantly decreased opportunities for the constant bombardment by new antigens encountered on Earth. The potential problem is that the immune system could become atrophic and render an individual more vulnerable to infection (especially if sufficiently rigorous measures are not taken to control microfloral contamination of the spacecraft). If the T-cell defects are confirmed, then their effects should be delineated in relation to four factors: 1. The differences in responses to antigens and broader cell stimuli called mitogens; 2. Abnormalities in subsets of regulatory T-cells, which help or suppress activities of other immune cells; 3. The roles of diverse immune-cell-derived regulatory proteins called cytokines, which direct T-cell proliferation and functions; and 4. The functions of macrophages and other accessory white blood cells responsible for presenting antigens specifically to T-cells. Effector systems, which eliminate toxins and kill microbes targeted by antibodies, such as white blood cells of the granulocyte series and serum proteins called complement factors, also should be assessed functionally. Some in vivo studies are required to detect and understand any deficiencies or excesses in integrated human immune responses. The critical need for controlled variable-gravity studies cannot be overemphasized. Only such studies will produce data useful in identifying specific mechanisms, perceiving the impact of any immune system abnormalities on other systems, and providing clinical guidelines for preventing and countering any defects in human immune defenses. The closed environment of the spacecraft may encompass a variety of living organisms (e.g., humans, animals, and plants), many types of energy- using equipment, and a wide variety of materials. The effluent from these multiple sources will contain microflora, gases (e.g., oxygen, carbon dioxide, and methane), and other chemical contaminants that must be collected and either disposed of or channeled through the life support system. The accumulation of colonies of microflora, pockets of gases, or dispersed trace chemicals could jeopardize the health of a crew and interfere with the success of a mission.2 At this time we do not have adequate information to assess how microbial and immunological problems would affect humans during extended spaceflight. Copyright © National Academy of Sciences. All rights reserved.
The Human Exploration of Space http://www.nap.edu/catalog/6058.html RESEARCH FOR MISSION OPTIMIZATION 50 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. DEVELOPMENTAL BIOLOGY A major scientific goal of studying developmental biology3 in space is to ''evaluate the capacity of diverse organisms, both plant and animal, to undergo normal development from fertilization through the subsequent formation of gametes under conditions of the space environment.''4 Plants are key to the entire biological system that has developed on Earth. Thus, it is essential to understand the effects of gravity and its absence in order to grow plants in space for food or for use in life support systems (see next section). A considerable amount of scientific literature already exists on the biology of plants in space. However, most studies have not dealt with general questions about plant growth but, rather, have addressed the orientation and motion of roots and shoots or have focused on plant hormones and events associated with normal and gravity-stimulated cell and organ growth. Our understanding of plant signal transduction is scant and may well be enhanced by using models based on animal work. Such constituents as G-proteins, phosphoinositides, actin, and calmodulin also occur in plant cells and may have active roles. The increasing applicability of techniques of molecular biology to problems in plant growth and development will be useful in attempts to understand the responses of plants to the space environment and in developing breeding programs designed to increase plant performance in microgravity environments. A major question is whether plants are capable of producing multiple generations in microgravity. The definitive space experiment is to observe a plant's life cycle from seed to seed to seed. The first generation of "on-orbit" seeds could have ground-born flowers upon germination, and thus produce seeds with ground-born tissues, since seed has maternal material in it. These seeds, however, would produce flowers exposed only to microgravity. Thus, their offspring, the third generation of seeds, would be entirely free of any prior terrestrial gravitational influence. Another important question is whether microgravity affects the single cell or if some plant cells acclimate to gravity deprivation. Some space-based studies suggest that chromosome behavior is fundamentally changed in microgravity. Should this be the case, the consequences and their implications for cell development must be determined. The lack of thermal convection in the microgravity environment may affect short- and long-distance transport phenomena in plants. For example, the function of cell membranes, the pathways for ion uptake and nutrient absorption, plant-water relations, and the transport of organic and inorganic molecules must be investigated to determine whether any of these is affected by microgravity. For example, is the plant-supporting structure of lignin and cellulose modified in space in ways analogous to the loss of bone density? Copyright © National Academy of Sciences. All rights reserved.
The Human Exploration of Space http://www.nap.edu/catalog/6058.html RESEARCH FOR MISSION OPTIMIZATION 51 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. LIFE SUPPORT SYSTEMS Closely related to the question of plant growth in space is the feasibility of a closed-loop life support system (CLLSS). CLLSSs are integrated self- sustaining systems capable of providing potable water, a breathable atmosphere, and ultimately, food for astronauts on long-duration missions. Some such systems may be able to operate in a small enough volume to be practical in a space vehicle, while larger systems could be deployed at lunar and martian outposts. Although it is not yet clear if the initial phases of the human exploration of Mars demand a CLLSS, it is certain that without one, long-term missions will require either vast amounts of on-board stores or access to prepositioned supplies. Thus, an effective and reliable CLLSS, even if limited to generating air and water from crew waste, would greatly simplify the logistics of long-duration missions. While a first-generation CLLSS would recycle only air and water, more advanced versions would be highly integrated subsystems for plant growth, food processing, and waste management. We have very little data on the operation of individual system components under realistic conditions. A small amount of information has been gathered on the performance of a few arbitrarily chosen plant species in open growth chambers. In addition, some encouraging, but still tentative, experiments have been initiated on plant growth in closed environments. Virtually nothing seems to have been done with respect to microbial and other systems of waste recycling, soil microbes and other microflora, or pathogen control. Nor have any of the food-processing technologies for converting biomass into palatable human nutrients been developed. Green plants are critical components of even the simplest CLLSS. They can fix carbon dioxide, produce food and oxygen, and purify water. However, as noted in the previous section, we do not yet know if plants will grow in space well enough to support a CLLSS for significant periods of time. A major scientific goal is simply to grow plants in space for extended periods of timeâ over several life cyclesâwhile carefully monitoring their performance. This goal is related to the more general need, outlined in the previous section, to investigate how diverse organisms undergo development in the space environment. For development of a CLLSS, this overall scientific goal assumes immediate practical importance. As we have already seen, processes such as reproductive development, fluid transport, and photosynthetic gas exchange may be adversely affected in low-gravity and microgravity environments. Even small effects may have serious consequences when performance is integrated over long time periods. Many other components of a CLLSS must also receive attention. Diverse plant, animal, and microbial species must be evaluated, environmental parameters optimized, and procedures developed for food processing and Copyright © National Academy of Sciences. All rights reserved.
The Human Exploration of Space http://www.nap.edu/catalog/6058.html RESEARCH FOR MISSION OPTIMIZATION 52 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. for recycling liquid and solid waste materials. In many cases, we do not know enough to produce a suitably sized CLLSS on the ground, much less in space. Obtaining the required scientific knowledge and engineering experience will require extensive experimentation under actual conditions in space. MICROMETEOROID FLUX ON THE MOON Long-duration activities on the surface of the Moon increase the potential risk of experiencing lethal impacts by micrometeoroids. The use of average collisional fluxes may give a false sense of security as excursion times outside protective habitats increase. The occurrence of periodic terrestrial meteor showers related to comets is well known. Recent reanalysis of lunar seismic data reveals that lunar impacts are neither temporally nor spatially random. Moreover, not all observed meteoroid showers on the Moon correlate with known terrestrial meteor showers. The potential dangers meteoroids pose to a long-duration presence on the Moon are twofold. First, there is an increased risk of direct hits during peak activity. Second, there is a risk of high-velocity impacts from secondary and ricocheting debris. The potential for lethal damage depends on the actual flux, the size distribution of the impactors, and the effect of spatially clustered impacts. These unknowns need to be studied over a sufficiently long period not only to assess the short-term risks (day to month), but also to recognize annual events and possible catastrophic swarms during orbital passage of newly discovered comets. Lunar seismometers have proven their usefulness as meteoroid impact detectors. Establishing a seismic network on the Moon to characterize the flux, size distribution, spatial clustering, and possible directional anisotropies of impacts over a multiyear period is essential to evaluating the hazards posed to astronauts by meteoroids. The potential dangers of unexpected meteoroid storms can be assessed through continued monitoring and evaluation of newly discovered comets. Experience gained from seismic monitoring of small impactors will be important for assessing risks over even greater durations en route to, and in orbit around, Mars. SURFACE AND SUBSURFACE PROPERTIES Humans exploring the Moon and Mars will require knowledge about their proposed landing sites not only to ensure a safe touchdown and subsequent departure, but also to identify regions of potentially high scientific interest. Prime questions to be answered for candidate sites involve the mechanical properties of the landing zone and the surrounding terrain to be explored and sampled. Size distributions of rocks at potential landing sites Copyright © National Academy of Sciences. All rights reserved.
The Human Exploration of Space http://www.nap.edu/catalog/6058.html RESEARCH FOR MISSION OPTIMIZATION 53 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. are required for three reasons: first, to ensure sufficient clearance for the landing vehicle; second, to allow reasonable leveling of the lander; and third, to certify that the terrain is sufficiently benign to be traversed by astronauts on foot and with rovers to carry out mission objectives. Of equal importance is a priori knowledge of the mechanical or bearing strength of the surface, particularly at the precise landing site but also over the region to be explored by the astronauts. The distribution of rock size can be obtained by precursor flights using remote sensing and in situ robotic exploration. Imaging with a resolution of less than 1 meter is necessary for selecting the landing sites themselves. Information on bearing strength is more difficult to obtain remotely. Significant estimates can be made of the near-surface soil densities using radar reflection and microwave emission techniques. Robotic landers may be required to achieve sufficient confidence to certify sites for human landings unless the areas selected are familiar (e.g., Apollo or Viking sites or demonstrably similar ones). In addition to rocks, the lunar surface is blanketed with unconsolidated debris generated by meteoroid impacts. This material, called regolith or soil, contains broken mineral and rock fragments, impact-produced glasses, and rocky glass-bonded aggregates. On average, about 20% of the regolith is composed of particles smaller than 20 microns in size. These properties, coupled with the hard lunar vacuum (10 -12 to 10-14 torr), make the regolith extremely abrasive. This will affect the longevity of all moving parts it comes in contact with. To make matters worse, regolith tends to cling to surfaces, leading to additional wear and tear on mechanisms such as gears, habitat airlocks, and spacesuit joints. Further in situ and remote sensing of the lunar surface and subsurface, together with studies of the abrasive and adhesive properties of lunar soil under hard vacuum conditions in terrestrial laboratories, will help in designing equipment to operate on the Moon's surface. Large-scale simulation facilities might also be needed to conduct long-duration, full-scale tests on engineering equipment and transport vehicles. The nature of the lunar subsurface at depths of 1 to 10 meters is poorly known. Although the size distributions of surface blocks in the regolith are known for typical mare and highland regions, there is little knowledge of how these distributions may change with depth. In most regions, bedrock occurs at depths of just a few meters, but the nature of its interface with overlying fragmental debris is unknown. Moreover, subsurface discontinuities, including interbedded lava flows, bedrock ledges, and voids, may pose additional hazards to landing craft, rovers, and excavation equipment. The elimination of such hazards may require active seismic imaging. Like the lunar regolith, the martian surface material may also be hazardous, but for different reasons. Existing data show that it contains highly Copyright © National Academy of Sciences. All rights reserved.
The Human Exploration of Space http://www.nap.edu/catalog/6058.html RESEARCH FOR MISSION OPTIMIZATION 54 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. reactive components in sufficient concentration to have oxidized the organic compounds used in one of the Viking life-sciences experiments. Such compounds may perhaps be responsible for the complete absence of any organic compounds in samples examined by Viking's gas chromatograph/mass spectrometer. Toxicity analysis could probably be carried out by a precursor robotic mission and might not require the analysis of martian material in terrestrial laboratories. Based on current knowledge, the oxidizing material is likely to be associated with fine, windblown, particulate material. Thus, specific precautions against this dust will have to be built into the airlock system on a lander. Moreover, spacesuits will have to be decontaminated as astronauts reenter the lander after completing extra-vehicular activities. Perhaps the spacecraft itself will have to be "cleaned" prior to its return to Earth. The data required to certify landing sites for safety may be highly desirable for other purposes such as planning surface construction, instrument installation, and the layout of extended surface traverses. Construction, prospecting, and mining operations will require subsurface sampling around the landing point. This can be carried out by the astronauts if the site has been selected on the basis of good information from precursor flights. That is, good measurements of surface rock distributions can be used to infer the subsurface geology. For Mars, such information is particularly critical because broad regions of the planet were not emplaced as primary geologic units, but, rather, have undergone episodic resurfacing tied to atmosphere-surface interactions. Astronauts can locate regions free of subsurface hazards for construction and mining using seismic and electromagnetic sounding devices on their rover. The need for some of these data could be partly alleviated through the use of a robust and forgiving design for excavation and construction equipment. For example, if the capability to efficiently crush and remove rock is a requirement for a lunar bulldozer, the need for knowledge of the sizes and locations of subsurface boulders is diminished. POTENTIAL MARTIAN HAZARDS Potential hazards posed by martian weather and climate, volcanic and seismic activity, and a number of other factors need to be considered in the context of concern for astronaut safety and the major investment of resources in any program of human exploration. A mission failure due to lack of adequate assessment of all plausible and sensible potential hazards, however unlikely, would be inexcusable. Following appropriate studies, some of the potential hazards may be realized; others may turn out to be either non-existent or of such low probability that they can be dismissed. Severe martian weather (such as dust storms, dust devils, and other Copyright © National Academy of Sciences. All rights reserved.
The Human Exploration of Space http://www.nap.edu/catalog/6058.html RESEARCH FOR MISSION OPTIMIZATION 55 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. vortices) may pose hazards to man-made structures or to field operations. Data on near-surface winds, including local wind shear and vorticity, are available only for the two Viking lander sites. Winds may affect descent vehicles by posing a hazard to, for example, parachute deployment or the spacecraft's ability to land precisely at a desired site. Ascent vehicles may also be affected by strong wind shears or turbulence. Variations of atmospheric density with local time, with solar activity, and with variations in the lower atmosphere (e.g., dust storms) may affect the operations and lifetimes of near-Mars support spacecraft, such as site-reconnaissance orbiters and communications satellites. Long-term meteorological measurements of temperature, pressure, wind velocity, and dustiness from orbit and at a variety of surface sites are required to assess these hazards. The current Mars Observer mission is directly relevant to this need. Large dust devils and clouds associated with local storms have been observed. Although dust storms may occur in any season, one or more may grow to regional and, on occasion, even global scale during southern spring and summer. Dust storms reduce surface visibility and insolation, thus affecting, for instance, the efficiency of solar cells. Moreover, the movement of sand-sized particles near the surface may pit, scratch, and erode surfaces, and may foul joints. Continued remote sensing of the martian atmosphere will help define this hazard. As is the case on the Moon and in free space, components of solar radiation reaching the surface of Mars may pose hazards to field workers and equipment (e.g., ultraviolet degradation of plastic material). Unlike the lunar surface and space, however, the total flux and the spectral distribution will change with variations in atmospheric aerosols and the seasons. Information on the diurnal and seasonal variation of atmospheric temperature, density, and wind speeds is needed to design a martian outpost. Other factors such as local and regional topography can present additional hazards (e.g., strong winds on steep slopes or in canyons, or regions of local fogs). Certification of landing and base sites in regions of large interannual variability (mainly at mid and high latitudes) may require observations spanning several martian years or longer to characterize the complete range of conditions likely to be experienced. Practically nothing is known about electric fields on Mars. The presence of moving dust particles in an atmosphere nearly as dry as Earth's stratosphere, however, could produce significant electrostatic charging. Besides being a nuisance (e.g., fine dust clinging to optical surfaces), such charging and discharging could severely affect crucial electrical equipment, such as computers. Large dischargesâsuch as lightningâmay also occur. Although the hazard posed by meteorites falling on Mars is small, the impact flux could range from a nominal lunar value to one larger by as much as an order of magnitude. The circum-martian meteoroid flux could Copyright © National Academy of Sciences. All rights reserved.
The Human Exploration of Space http://www.nap.edu/catalog/6058.html RESEARCH FOR MISSION OPTIMIZATION 56 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. be determined by a spacecraft akin to NASA's Long Duration Exposure Facility in orbit around Mars, by detectors of meteors passing through the martian atmosphere, and by seismic networks on the martian moons. The long-term safety of a martian outpost also requires assessment of the hazards due to seismic or volcanic activity. Insufficient data currently exist to make confident statements about martian seismicity. Volcanic activity has been widespread on Mars in the past. We do not know, however, if there has been any recent volcanism or if near-surface thermal activity or magma chambers exist. A network of seismometers and heat-flow measuring devices could provide the information to measure current activity. Other geologic hazards, including slides and slope failures, need to be assessed. Areas of scientific interest in potentially dangerous locations, such as deep martian canyons or close to known volcanic vents, may require precursor visits by robot landers or rovers. Such sites may be especially important in deciphering the history of Mars, particular the role played by liquid water in both geological and biological contexts. AEROBRAKING AT MARS Aerobraking, or aerocapture, is a technique using atmospheric drag to reduce a space vehicle's orbital energy. It can thus cut down on the amount of propellant needed to achieve orbital insertion. Indeed, aerocapture may significantly reduce (perhaps by a factor of three or more) the mass that must be delivered into Earth orbit for a Mars exploration mission. Aerocapture could be critical to the feasibility of such a mission, and a proper understanding of the atmospheric structure of Mars and its variability should be considered part of the enabling science for such a mission. Successful aerobraking requires a detailed knowledge of not only the mean density structure of the martian atmosphere but also its temporal and spatial variations. The Viking 1 and 2 landers, for example, measured vertical density profiles differing by more than 20% as they descended from an altitude of 100 kilometers to the surface. Most of the atmospheric variations at aerobraking altitudes on Mars (20 to 70 kilometers) are due to gravity waves. These are thought to be generated by thermal tides and by high-speed winds flowing over surface topography. Further understanding of the statistics of density variations in the martian atmosphere is required before human landings using aerobraking are attempted. NASA's Mars Observer mission should answer many of the outstanding questions on this issue. However, a longer mission (with greater seasonal coverage) and some in situ measurements of the atmosphere will be required to calibrate remote observations. A better understanding of the temporal and spatial variations of atmospheric dust is also needed and should Copyright © National Academy of Sciences. All rights reserved.
The Human Exploration of Space http://www.nap.edu/catalog/6058.html RESEARCH FOR MISSION OPTIMIZATION 57 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. be obtained either from direct atmospheric measurements or by ground-based opacity observations. These concerns are addressed in considerable detail in a recent report.5 This NASA document likewise concludes that mission safety requirements lead to a significant need for understanding the statistical behavior of the martian atmosphere. Remote spacecraft monitoring of atmospheric properties should be carried out both before and during the arrival of humans at Mars. MICROGRAVITY SCIENCE AND TECHNOLOGY Human exploration will require more understanding of fluid flow and transport under reduced (and sometimes increased) gravity conditions. In order to support extended space travel, we must know more about the processing of materials, thermal management, and the handling of fluids. Microgravity studies must be viewed as more than the advancement of science and technology for its own sake or as a means to obtaining potential benefits for society on Earth; these studies are essential to the advancement of spaceflight. Many examples of challenges associated with a modified gravity field can be found: producing needed materials from available raw materials; washing and drying of clothing, equipment, humans, and animals; handling of hazardous and obnoxious wastes; improving and ensuring spacecraft fire safety; and achieving temperature control for humans, animals, plants, and electronics. The challenges occur predominantly in the life support areas but extend well beyond them. For example, modern electronics are becoming so compact that, in the near future, volumetric heat-generation rates are expected to rival those values for controlled nuclear fission. Also, there is overlap with the life sciences since fluid transport is essential to life itself, as, for example, the transport of liquid from the roots to the leaves of plants. There is a strong need to address the underlying science as well as the technology. The relevant technology for related Earth-gravity-level processes is often based on empirical methodology. Therefore, engineering extrapolations cannot be readily made. EXOBIOLOGY ISSUES While there may be little chance that life exists on Mars today, this may not always have been the case. Thus, many of the science requirements relating to exobiological exploration of Mars revolve around technologies for detecting and analyzing fossil organisms or the chemical precursors to life. Closely related is the question of the history and present occurrence of liquid water and ice on Mars. Some specific questions include: Copyright © National Academy of Sciences. All rights reserved.
The Human Exploration of Space http://www.nap.edu/catalog/6058.html RESEARCH FOR MISSION OPTIMIZATION 58 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. 1. How to detect indigenous martian microorganisms and assess their biological activities; 2. How to recognize and analyze fossil remains of such indigenous microorganisms; 3. How to search for the presence of chemicals that might relate to past activities of life forms or that might relate to prebiotic chemistry; 4. Where to seek evidence for past life or prebiotic chemistry; and 5. How to detect the current, and understand the past, distribution of liquid water and ice. Beyond laboratory studies, answering these questions will involve acquiring a more detailed knowledge of Mars and its history. The location of ancient lake beds and of possible wind- and water-emplaced sediments will surely play a major role in selecting martian sites of interest to exobiologists. The development of new organic analysis instrumentation with perhaps a 1000-fold improvement in sensitivity over the Viking mass spectrometer is likely to be needed. This needs to be coupled with a flexible "wet" chemistry input. If we are to adequately investigate the possible prehistory of biology on Mars, we need to answer whether or not there are any organic compounds of either abiogenic or biogenic origin on the surface or below the surface. Determining the ratios of different stereoisomers of amino acids will help distinguish between those of biogenic or abiogenic origin. RESOURCE UTILIZATION Long-term human exploration of Mars may require or greatly benefit from landing sites in close proximity to exploitable resources. If, for example, water needs to be acquired on Mars, it might be extracted from the air, from surface materials containing chemically bound water, or from sub-surface ice or permafrost. Which reservoir should be tapped depends on trade-offs between various extraction technologies available and detailed knowledge of the martian environment. The atmospheric abundance of water is known adequately for this purpose, but the location (particularly the depth) of subsurface ice is not. If there is a requirement to mine water at the landing site, then precursor flights should be designed to locate regions where subsurface ice may exist. Similarly, detailed knowledge of the local mineralogy should be obtained on precursor flights for in situ extraction of water from mined minerals. If habitation is chosen as a long-term goal of Mars exploration, then the technology necessary to locate subsurface water or permafrost will probably need to be developed. Copyright © National Academy of Sciences. All rights reserved.
The Human Exploration of Space http://www.nap.edu/catalog/6058.html RESEARCH FOR MISSION OPTIMIZATION 59 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. NOTES AND REFERENCES 1. Space Science Board, A Strategy for Space Biology and Medical Sciences for the 1980s and 1990s, National Academy Press, Washington, D.C., 1987, Chapter 4. 2. For an assessment of this problem in the context of Space Station Freedom, see Board on Environmental Studies and Toxicology, Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants, National Academy Press, Washington, D.C., 1992. 3. See Ref. 1, Chapter 2. 4. See Ref. 1, p. 32. 5. Mars Atmosphere Knowledge Requirements Working Group, SEI Engineering Requirements on Robotic Missions, Roger D. Bourke (ed.), JPLD-8465, NASA, Jet Propulsion Laboratory, Pasadena, Calif., May 1991. Copyright © National Academy of Sciences. All rights reserved.
