3

Physical Environmental Hazards

The committee categorized the hazards on Mars by their sources or causes. It has specifically defined physical hazards on Mars separately from the chemical and biological hazards, because physical hazards can threaten crew safety by physically interacting with humans or critical equipment, resulting, for example, in impact, abrasion, tip-over (due to an unstable Martian surface), or irradiation.

It is known that the gravitational force that will be experienced by humans on the Martian surface is approximately 3/8 (0.375) that on Earth. The committee notes that very little, if anything, is known about the long-term effects on human health from residing in a 3/8 Earth gravity environment. These long-term effects could represent a hazard to astronauts on Mars. However, since no further precursor missions are necessary to quantify the gravity on Mars, the committee has not included the low-gravity environment in the hazards discussed in this report.

The physical environments that might pose risks to crew safety on Mars fall into three categories: geologic, atmospheric, and radiation. This chapter elaborates on each of those categories in light of what is currently known about the hazards and what needs to be known in order to establish confidence in the safety of human missions to Mars.

GEOLOGIC HAZARDS

The geologic features of interest in this study are airborne dust, regolith, and terrain. Airborne dust, with an average diameter of 3.4 microns, is the smallest geological feature (see Box 3.1). The Martian regolith is the complex outer layer of fractured rock and soil on the surface of Mars. This is the material that will support astronauts and roving vehicles as they traverse the Martian surface.

The aggregate form of the regolith creates the terrain of Mars. The term “terrain” includes large-scale features such as mountains, hills, valleys, and canyons as well as smaller-scale features such as craters, dunes, and gullies. The smallest-scale terrain on Mars includes boulder and rock fields.

Terrain Trafficability

When an aircraft operates near the surface of Earth, it is imperative that the pilot know the shape and composition of the terrain beneath, be it ocean, forest, desert, or a concrete runway. The same is true for a mission descending on Mars, except in this case the terrain will not include any prepared landing surfaces. A Mars landing craft will be designed to land on a wide range of terrains, but cost and weight considerations in the design of the lander will mandate that mission planners target the most benign landing areas.

Similarly, the terrain around a landing site, specifically including the area in which astronauts will be operating, must also be studied to ensure that the astronauts are provided with the most suitable equipment for their operating environment. For example, if they will be traversing a relatively flat plain scattered with rocks 5 to 8 centimeters in diameter, a human transport rover with fairly narrow wheels, similar to the Apollo lunar rover vehicle, may be adequate. If the same plain is covered in boulders one-third of a meter in diameter,



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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface 3 Physical Environmental Hazards The committee categorized the hazards on Mars by their sources or causes. It has specifically defined physical hazards on Mars separately from the chemical and biological hazards, because physical hazards can threaten crew safety by physically interacting with humans or critical equipment, resulting, for example, in impact, abrasion, tip-over (due to an unstable Martian surface), or irradiation. It is known that the gravitational force that will be experienced by humans on the Martian surface is approximately 3/8 (0.375) that on Earth. The committee notes that very little, if anything, is known about the long-term effects on human health from residing in a 3/8 Earth gravity environment. These long-term effects could represent a hazard to astronauts on Mars. However, since no further precursor missions are necessary to quantify the gravity on Mars, the committee has not included the low-gravity environment in the hazards discussed in this report. The physical environments that might pose risks to crew safety on Mars fall into three categories: geologic, atmospheric, and radiation. This chapter elaborates on each of those categories in light of what is currently known about the hazards and what needs to be known in order to establish confidence in the safety of human missions to Mars. GEOLOGIC HAZARDS The geologic features of interest in this study are airborne dust, regolith, and terrain. Airborne dust, with an average diameter of 3.4 microns, is the smallest geological feature (see Box 3.1). The Martian regolith is the complex outer layer of fractured rock and soil on the surface of Mars. This is the material that will support astronauts and roving vehicles as they traverse the Martian surface. The aggregate form of the regolith creates the terrain of Mars. The term “terrain” includes large-scale features such as mountains, hills, valleys, and canyons as well as smaller-scale features such as craters, dunes, and gullies. The smallest-scale terrain on Mars includes boulder and rock fields. Terrain Trafficability When an aircraft operates near the surface of Earth, it is imperative that the pilot know the shape and composition of the terrain beneath, be it ocean, forest, desert, or a concrete runway. The same is true for a mission descending on Mars, except in this case the terrain will not include any prepared landing surfaces. A Mars landing craft will be designed to land on a wide range of terrains, but cost and weight considerations in the design of the lander will mandate that mission planners target the most benign landing areas. Similarly, the terrain around a landing site, specifically including the area in which astronauts will be operating, must also be studied to ensure that the astronauts are provided with the most suitable equipment for their operating environment. For example, if they will be traversing a relatively flat plain scattered with rocks 5 to 8 centimeters in diameter, a human transport rover with fairly narrow wheels, similar to the Apollo lunar rover vehicle, may be adequate. If the same plain is covered in boulders one-third of a meter in diameter,

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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface BOX 3.1 Definitions of Martian Regolith, Soil, and Dust For the purposes of this report, the committee developed operational definitions of the solid particulate materials on or near the Martian surface. The Martian regolith is the complex outer layer of loose rock and soil on the surface of Mars. Owing to repeated meteorite impacts and surface weathering processes, rock fragments of all sizes are mixed with weathered soils in various proportions to produce a regolith of unknown thickness. The composite physical properties of the regolith are likely to be different from the physical properties measured in Martian soils. The term “soil” describes deposits of fine-grained, largely unconsolidated materials on the planet's surface. The planetary, or non-Earth, usage of the word “soil” differs from terrestrial usage, which specifies that soil must have an organic component. Martian soils may be mixtures of very small particles resulting from deposits of airborne dust and coarser, sand-size particles. The chemical composition of the soil has been measured at three spacecraft landing sites separated by thousands of kilometers, and the soil analysis results are similar. 1 The mineralogy of the Martian soil is not well understood at this time, but it is commonly inferred to resemble that of palagonite, a mixture of amorphous and poorly crystalline clays, iron oxides, and other products formed from the weathering of volcanic rocks. 2 The terms “dust” and “airborne dust” are used to identify fine particles suspended in the Martian atmosphere. The average grain diameter of airborne dust, as determined from multispectral imaging, is 3.4 microns. For the purposes of this report, dust is characterized as being less than 10 microns in diameter.3 The amount of dust in the atmosphere increases during seasonal dust storms, but even in quiet times there is enough suspended dust to impart a salmon color to the sky. There is a consensus that Martian dust has been globally homogenized by the wind, so that its composition is the same everywhere.4 Although its mineralogy is unknown, magnetic experiments have demonstrated that some dust particles contain magnetic iron oxides. Its red color indicates a high oxidation state.5 1Clark et al. (1982); Rieder et al. (1997). 2Bell et al. (2000). 3Tomasko et al. (1999). 4McSween and Keil (2000). 5Madsen at al. (1999). a vehicle with very large inflatable tires capable of easily rolling over large rocks might be the appropriate vehicle in which to transport humans. Simply stated, understanding the shape and form of the terrain at the landing site on Mars is a critical requirement. Finally, knowledge of the distribution of larger rocks is needed to plan traverse routes. The rock distribution will determine if rovers have to surmount objects, that is, roll over rocks that are small enough, or maneuver around them. The Need for Measurements To ensure safe landing and operations on the surface of Mars, it is necessary for NASA to fully characterize the landing site and the topography of the anticipated surface operation zone with high-resolution stereoscopic imaging. The operation zone is the area around the landing site defined by the anticipated range of operations of EVAs, including the use of human transport and/or science rovers. The level of resolution required of this imaging will be determined by the capabilities of the equipment to be used on the surface. Presentations to the committee illustrated human transport rover designs using wheels 1 meter in diameter. Vehicles using standard wheels can typically roll over objects one-third the diameter of the wheel being used. This suggests that if human transport and scientific rovers will use 1-meter wheels, the mission planners will need to know the distribution of rocks one-third of a meter and larger in the landing and operation zone. Imaging rocks this size requires a pixel resolution of 10 cm. The committee anticipates that the three-dimensional mapping would be conducted from Martian orbit. Recommendation: NASA should map the three-dimensional terrain morphology of landing operation zones for human missions to characterize their features at sufficient resolution to assure safe landing and human and rover locomotion.

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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface Mobility on the Martian Regolith The regolith is the complex outer layer of rock and soil on the surface of Mars. The potential hazards associated with the regolith include unstable movement or the inability to move in a timely manner across the Martian surface. These risks will be present when humans walk anywhere or when human transport rovers and critical science rovers are in operation on the surface of Mars. Stable and Timely Traverse The greatest threats to the safe movement of humans and critical equipment on the surface of Mars involve the following: Degradation of mobility, Instability and collision (human or machine), Mechanical failure, and Rovers that move too slowly. Each of these hazards is associated with interaction with the Martian regolith and the regolith's ability to support planned rovers and humans. It is necessary to understand these hazards better to permit the design of appropriate systems and machines that will be used on the planet's surface. Degradation of Mobility An emergency could arise if humans or equipment become stranded in dunes or other eolian-deposited dust hazards (loose dust deposits that have a consis-tency of powder snow) because the regolith is unable to support large-mass objects operating on the Martian surface. Rovers could conceivably bog down in loose soil or break through a crusty surface. The power budgets for the movement of rovers and the life support consumables for EVA operations will be based on the designer's understanding of the energy transfer mechanisms involved. Instability and Collision An astronaut might fall while on an EVA, and it is more likely that the astronaut would fall on irregular terrain than on smooth terrain. It is also possible that science and human transport rovers might collide with rocks, causing damage to critical systems or injury to astronaut passengers. By understanding the bulk properties of the regolith, NASA should be able to minimize the risk of falling through appropriate design and operational procedures. The characterization of the Martian regolith will also allow science and human transport rovers to be designed so as to minimize unexpected mobility-related events that result in tip-over or collision. Mechanical Failure The Martian regolith could induce mechanical failure not only by causing catastrophic collision or tip-over where mechanical parts are broken, but also by abrading or gouging surfaces that contact the regolith. This hazard would be a concern if long-range rovers and extended surface operation times are involved. The vast majority of wear on critical systems will result from rock abrasion in the form of gouging from point contacts on rocks and shear action. Wear in the sub-surfaces of materials, resulting from rolling friction, will also be an issue with which NASA must contend. Rock abrasion and wear will impact space suits and rover wheels the most. For long surface stays with multiple EVAs, space suit boots would be worn down by the process of walking over the Martian regolith. Rock abrasion would probably be more apparent on heavy science and human transport rover wheels, where sharp rocks might gouge grooves in the wheels. Rovers That Move Too Slowly A hazard will be introduced if humans or their critical systems cannot move quickly enough on the Martian surface. If there is an emergency caused by an unpredicted solar particle event or a critical system failure, it will be imperative for the astronauts to reach shelter in a timely manner. Critical support rovers need to be able to keep pace with humans walking on the surface. If a critical rover is left behind because it cannot navigate Martian terrain quickly enough, a safety hazard would be introduced and human life might be at risk. NASA needs to understand the mobility and trafficability characteristics of the Martian surface in order to design systems that can quickly navigate the operations area around the landing site.

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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface The Need for Measurements It is necessary to understand the bulk physical properties of the Martian regolith that will interact with the rover vehicles, which in some scenarios are greater than 1,000 kg in mass. In addition to the need to characterize terrain morphology, as mentioned previously, NASA should also characterize the mechanical properties of the Martian regolith at the landing site or comparable terrain and rock properties in the landing and operating area. These properties include the following: Rock distribution and shape, Rock abrasive properties, and Regolith sinkage properties, including shear strength, bulk modulus, yield strength, and internal friction angle. The experiments required to determine these characteristics must be conducted on the surface of Mars. The rocks that would have to be analyzed are simply too large to return to Earth, and most rocks in the operations zone will be too small to view from orbit. Also, the bulk properties could only be measured in an undisturbed environment. Recommendation: To ensure that humans and critical rover systems can land on and traverse the Martian surface in a safe, efficient, and timely manner, NASA should characterize the range of mechanical properties of the Martian regolith at the landing site or comparable terrain. Specifically, in situ experiments should be performed to determine the regolith's aggregate strength, stability, and sinkage properties, including bearing strength, bulk modulus, yield strength, and internal friction angle. Rock Distribution and Shape If high-resolution orbital imaging is not available, the average rock size distribution and shape can be determined by in situ observations on the surface of Mars as those observations become available prior to human missions to Mars. The committee notes that rock size is currently determined during all precursor lander and rover missions. It is reasonable to expect that this activity will continue to be included in all future in situ missions, whether at the proposed landing site for the first mission or on similar terrain. Recommendation: NASA should determine, in advance of human missions to Mars, rock size distribution and shapes in situ, at the landing site or on comparable terrain, in order to predict human and rover trafficability. Rock Abrasive Properties The abrasive properties of rocks on Mars, including hardness and surface roughness (as dictated by rock grain size and shape), are unknown. The committee believes that, even faced with this lack of knowledge, NASA can still design systems by making certain educated assumptions about the rocks on Mars. Specifically, the committee believes that NASA can adequately design systems by assuming that the rocks on Mars have a worst-case hardness similar to basalt and that NASA can test its systems using rocks with a variety of worst-case surface roughnesses. By such testing, NASA will be able to ensure that astronauts operating on Mars can work safely on a variety of rock surfaces. No further in situ experiments to determine the abrasive properties of Martian rocks are required. Finding: By testing space suit and rover equipment on Earth using rocks with the hardness of basalt and a variety of worst-case surface roughnesses, NASA does not need to further characterize the properties of rocks on Mars. Regolith Sinkage Properties To determine the regolith sinkage properties, including shear strength, bulk modulus, yield strength, and friction angle, NASA must conduct an in situ cone penetrometer or equivalent experiment, which would characterize the aggregate properties of the mixture of rock and soil that composes the Martian regolith. This experiment could be conducted at either the landing site or on comparable terrain. It should be geared to understanding how the Martian regolith would react to the presence of large-mass human transport rovers. Cohesion and friction are intrinsic properties of the most basic soil mechanics models. Friction is commonly expressed as an angle in these models and is referred to as “friction angle.” Airborne Dust Intrusion and Adhesion Airborne dust presents a potentially significant hazard to human operations on the surface of Mars. The

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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface ubiquity and pervasiveness of the dust will be a con-stant source of concern. Dust intrusion and accumulation will need to be continuously monitored and will require well-designed filter systems and periodic housecleaning. The possible hazards associated with airborne dust accumulation are the following: Abrading and wear of mechanical systems, Degrading of EVA suit seals, Filter clogging, Decrease in visibility (optics, space suit visors), Changing thermal properties, and Accumulating triboelectric charge (from wheel movement or wind). The first two hazards involve dust intrusion; the next four, dust adhesion. Dust Intrusion The ability of dust to penetrate and damage a system is a function of the dust grain size, hardness, and shape. Once dust particles have penetrated a system, especially one with moving mechanical parts, the grains could wear critical moving parts and seals. The hazard presented by dust intrusion is more crucial for long stays on the Martian surface, where critical systems are exposed to the environment for a much longer time. Dust Adhesion Airborne dust on Mars will accumulate on surfaces by a wind-driven process abetted by electrostatic adhesion, magnetic attraction, or other adhesive properties such as the adhesion resulting from van der Waals (i.e., intermolecular) forces. The Martian wind, while being a cause of dust deposition, should also help limit the amount of dust that will accumulate on an exposed surface. However, if habitat filters are not designed properly, they could clog from the dust and soil that enter the habitat after an EVA return. Dust accumulation on external communication antennas and solar panel arrays, for instance, could create very hazardous situa-tions by disrupting critical communications or decreas-ing the efficiency of power generation systems. Dust on optical systems and space suit helmet visors could seriously hinder operations and data measurements. Thermal radiators coated with microlayers of dust will have different heat exchange properties than originally intended as a result of the thermal radiative characteristics of the dust coating. Finally, the triboelectric charge generated by wheel movement in the Martian soil may cause surface soil and dust to clump onto tires, increasing the amount of dust around driveshaft seals and reducing wheel efficiency. While this list is not intended to be all-inclusive, it should give some indi-cation of the far-reaching effects of Martian dust. The committee does note, however, that the three robot landers that have operated on the surface of Mars to date have all operated successfully in the Martian dust environment by virtue of careful system engineering to prevent many of the effects described above. The Need for Measurements After reviewing NASA's experience with dust on the Moon and Mars, the committee is confident that NASA engineers and scientists will be able to design and build systems to mitigate the hazards posed by airborne dust on Mars. Some systems that would be used on the first human mission can be designed either by employing what is currently known about Mars dust or by assuming a worst case scenario in the design process, as described below. Abrasive Properties of Dust It is known that the average diameter of airborne dust particles is 3.4 microns. However, little is known of the size distribution, hardness, or shape of the grains. By careful analysis of the abrasive properties of airborne dust, engineers and scientists should be able to design systems that account for the potential for Martian dust to cause abrasive wear of moving parts and seals. To that end, it would be desirable to know the grain size distribution, hardness, and shape of Martian airborne dust from the analysis of a sample return of airborne dust. However, a sample return is not required. The present Mars soil simulant that has been developed and characterized by NASA for engineering studies (JSC Mars-1: Martian Regolith Simulant) con-sists of palagonite (glassy volcanic ash altered at low temperature) having a bulk chemical composition similar to that of soils analyzed on Mars. While this simulant is a chemical proxy, it may not represent the physical properties or mineralogy of Martian airborne dust, so it is not adequate for testing mechanical systems for human missions to Mars.

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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface The committee does not recommend that any precursor in situ measurements be taken on Mars to characterize the mechanical and abrasive properties of air-borne dust. Rather, it expects that a simulant having an average particle diameter of approximately 3.4 microns (similar to the average size of airborne dust particles) and a hardness similar to that of basalt, would adequately stress the design of any mechanical and seal systems that will be used during a human mission to Mars. Basalt is a common surface rock on Mars (Mustard et al., 1997; Bandfield et al., 2000). It is thought to be the material from which Martian soils were formed (McSween and Keil, 2000). Alteration minerals& —that is, materials that originated as pure basalt but have been modified in some physical way&—would be softer than the minerals comprising basalt. There is no need for a simulant to be harder than basalt, as spectroscopic searches have not detected minerals with greater hardness (e.g., quartz) (Bandfield et al., 2000). Any mechanical system that withstands testing using such a worst-case basalt simulant will in all likelihood be overdesigned for use on Mars. Finding: The present Mars soil simulant developed by NASA does not adequately simulate physical properties for engineering purposes. Adhesion Properties of Dust It is critical to fully characterize soil and airborne dust adhesion properties in order to design systems that minimize the risk of failure resulting from soil and dust accumulation. Also, a full understanding of Martian dust adhesion will allow NASA to predict and plan for maintenance protocols for critical systems such as air filters, solar panels, EVA suit seals, and communications equipment. The committee recommends that NASA conduct precursor experiments on the surface of Mars to determine the adhesive properties of Martian soil and air-borne dust. The alternative to conducting these experiments on Mars is for NASA to develop an accurate simulant that is based on a sample of airborne dust collected from the Martian atmosphere and soil. Recommendation: NASA should determine the adhesive properties of Martian soil and airborne dust in order to evaluate the effects of dust adhesion on critical systems. This characterization must be conducted in situ by means of experiments to measure airborne dust adhesion. HAZARDS FROM ATMOSPHERIC DYNAMICS The physical dynamics of the Martian atmosphere will offer challenges to habitat and system design not encountered on the Moon or here on Earth. NASA engineers will have to predict the effects of electrostatic discharges and high winds to mitigate the hazards caused by these atmospheric phenomena. Electrostatic Discharge When an electrically neutral particle collides with another neutral particle, electric charge may be ex-changed, so that one particle takes on a positive charge and the other a negative charge of equal magnitude. In Earth's atmosphere, lightning is generated by an accumulation of charges resulting primarily from collisions between ice particles of different size in clouds. As more collisions occur, more charge is built up (Wallace and Hobbs, 1977). The thermodynamics of the changing states of water (solid, liquid, gas) plays the central role in driving the vertical separation of oppositely charged particles within a cloud. The electric potential differences from one region of the cloud to another build up to very large values, producing lightning storms on Earth. On Mars, the water-driven mechanism for charge separation is not present. Charged particles are expected to be produced by collisions in dust storms, but if the particles are well mixed in adjacent volumes containing many particles, the total positive charge would be essentially the same as the total negative charge in each region. Thus, there would be no large electric potential differences between parts of the cloud and between the dust cloud and the Martian surface (Kolecki and Landis, 1966). Although some electrical activity in Martian dust storms and dust devils should be anticipated, its intensity is not expected to be comparable to that of terrestrial lightning storms. The counterpart of lightning bolts on Earth may be small sparks on Mars. The two Viking landers were enveloped in a global dust storm soon after landing, and dust devils have been observed many times on Mars by Viking orbiters and landers, the Mars Pathfinder, and the Mars Global Surveyor. Windblown dust appears to be a common condition on Mars and would cause electrostatic charging of astronauts' space suits during operations

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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface on the surface of Mars, as well as of their equipment and habitat. Despite this phenomenon, there has been no report of electrostatic damage to delicate electronics on any of the surface systems. Furthermore, neither the Viking missions nor the Mars Pathfinder mission experienced any problems due to electrostatic charging. While it would be useful to learn more about the electrical activity in Martian and terrestrial dust storms and vortices, the committee concludes that such increased knowledge is not essential for planning the first human mission to Mars. Probably of greater significance is the charging of such objects even in the absence of airborne dust, due to the motion of boots and wheels relative to the surface material and differential motion of different parts of equipment and material. A glove brushing dust from a space suit may cause more dust to cling to both. The electrical discharge paths between objects at different electric potentials may be through a good con-ductor, resulting in fast charge neutralization, or through the atmosphere or another poorly conducting material, resulting in a much slower charge dissipa-tion. Objects on moist ground on Earth are said to be grounded since the conducting ground has an almost unlimited capacity to accept either positive or negative charge without changing its electric charge potential from what is essentially zero. Such natural discharge or prevention of charging is not expected to occur on Mars, because there is no near-surface liquid water. The lack of a local electrical ground on Mars may be so electrically isolating that astronauts operating on the Martian surface would build up large potential differences relative to the equipment they will be using or the habitat in which they will live. For example, an astronaut on an EVA may experience an arc between the space suit and equipment or habitat. Such a discharge, if not properly isolated, could damage sensitive, unprotected electronic components or the space suit. In particular, the committee is concerned with the level of charging that might occur as a result of the high-velocity movement that is likely to take place when using a human transport rover. The hazards from electrostatic discharge on Mars can range from a simple spark, equivalent to feeling a sting here on Earth after walking on certain types of carpet and reaching for a doorknob, to potentially more potent bursts between astronauts and large equipment or structures on Mars. The principal risk, as the committee sees it, is how these discharges could affect the electronic equipment that is critical for human survival on the planet. Here on Earth, the charge generated from walking across a carpet is usually more than enough to disable and potentially destroy certain electronic components. The Need for Measurements The dry conditions and uncertainty about conduc-tivity, charging, and discharging rates in the Mars environment create uncertainties about electrostatic effects on human operations in the Mars environment. However, even given the potential hazards, the committee believes that the risk to humans from electrostatic charging on the surface of Mars can be managed through standard design practice and operational procedures. NASA should design accordingly and assume that no effective local ground is available on Mars. In one case, the physics of the Martian environment may actually help reduce the risk of electrical discharge to humans or systems. So-called Paschen electrical discharge is likely to mitigate the hazard of differential voltage buildup on humans and systems due to the atmospheric composition and pressure on Mars. Owing primarily to its low pressure, the atmosphere ionizes more easily, which dissipates electrical charges at a lower voltage, minimizing the charge buildup. This means that lower overall electric charge should be present when humans and equipment are working on the surface. For a specific case of parallel plates separated by half a centimeter, on Earth the breakdown voltage is approximately 7,000 volts, as opposed to slightly over 400 volts on Mars. The committee does not advocate any specific engineering design solution. However, for the sake of discussion, there are many potential solutions for the electrostatic discharge risk. For example, a device that allows discharge through a resistive contact to prevent electrical arcing might be used to mitigate the risk of discharge occurring between an astronaut and the habitat when the astronaut returns from an EVA. A combination of technologies might also be considered, such as point-discharge, needlelike devices or even small radiation sources to prevent charge buildup. The committee believes that no further in situ measurements are required to characterize the electrostatic properties of the Martian environment, including those properties associated with dust devils, for an initial human mission to Mars. NASA's experience with the Viking and Pathfinder missions supports this conclusion. It should be noted that if or when highly energetic

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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface (i.e., high-speed) rovers are used, the same might not be true. Such high-speed rovers could conceivably induce very strong charges. While no specific in situ experiments are required at this time, NASA should pay close attention to electrostatic effects on subsequent science rover missions to aid in the design of future fast-moving rovers. The committee believes it would be helpful for NASA to investigate the design considerations and procedures used at the Siple research station in Antarctica, where there is little to no local electrical ground. Again, as an example of potentially innovative design solutions, two crossed-dipole antennas at Siple, each 21.4 kilometers long, occasionally charged up to the order of 20,000 volts when windborne ice particles passed over them. The danger of discharge was removed by connecting the antennas to the station buildings. The buildings prevented a charge from accumulating on the antenna conductors by acting as large capacitors that stored the charge. The electrostatic voltage on the antennas was reduced to near zero, and since ice is not a perfect electrical insulator, the charge on the buildings dispersed gradually. Sharp conducting points, the needlelike devices referred to above, were also used near the buildings to bleed off the electrical charge. High Wind Speeds There has been some concern about the risk to astronauts on the surface of Mars from high-speed wind from either regional or global dust storms or localized dust devils. Global storms are neither reliably seasonal nor predictable. Nor do we know the upper limit on how long these storms may last. Most global storms on Mars start in its southern hemisphere near the beginning of southern summer, when Mars is near perihelion, that is, when it is 17 percent closer to the Sun than at aphelion (Kieffer et al., 1992). The storms occurring at this time usually last several months. The impaired visibility caused by these storms could represent a hazard to astronauts on the surface. However, this phenomenon is well characterized and no further in situ measurements are required. The strongest surface winds observed by in situ measurements on Mars are believed to be 30 to 50 meters per second (67 to 111 miles per hour) based on eolian deposits at the Viking I landing site. From a terrestrial perspective, these wind speeds appear to represent a significant hazard. However, when the lower atmospheric dynamic pressure on Mars, resulting from a less dense atmosphere than on Earth, is accounted for, the Earth-equivalent wind speeds are much less. Dynamic pressure is proportional to the air density times the square of the wind speed, so that the following com-parisons can be made: For the same wind speed, the dynamic pressure on Mars is less than on Earth by the ratio of air densities, or a factor of about 82. For the same dynamic pressure, the wind speed on Mars must be greater than on Earth by the square root of this number, or a factor of about 9. Simply stated, the wind must blow nine times faster on Mars than here on Earth to achieve the equivalent dynamic pressure. In the strongest wind case mentioned above, a 30 to 50 meter per second (67 to 111 mile per hour) wind on Mars is roughly equivalent to a 3.3 to 5.5 meter per second (7.4 to 12 mile per hour) wind on Earth. Another potential hazard associated with wind is abrasion by windblown particles. Suspended dust is so fine that it is unlikely to cause significant abrasion. Although ventifacts (rocks sculpted by windblown particles) were observed at the Mars Pathfinder landing site, these features do not necessarily indicate higher wind speeds. Ventifacts form by saltation&—that is, by sand grains bouncing along the surface. Saltation is unlikely to cause abrasion except at or very near ground level, and the time scale of the abrasion that produced ventifacts is certainly much longer than the duration of human Mars missions. Lack of evidence that any Mars landers have been affected by this process suggests that any abrasion by windblown particles can be mitigated by habitat and equipment design. The Need for Measurements The committee believes that in light of the relatively low dynamic pressures experienced on Mars, no further characterization of wind speed on Mars is required prior to the first human mission. It believes that the surface winds are sufficiently characterized based on Viking and Pathfinder data and atmospheric dynamic models with regard to speed (based on Viking and Pathfinder experience) and dust devils (based on Pathfinder data) to allow system designers to ensure human safety on the planet by means of robust designs. Therefore, no new in situ experiments to validate global storm or dust devil wind speeds are recommended. The committee

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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface acknowledges that many precursor lander missions will include meteorological instrument payloads, so additional wind speed measurements will in all likelihood be gathered, which will further aid designers. Wind may become a factor in certain areas on Mars with large terrain slope changes, such as on the flanks of volcanoes or inside canyons, depending on the time of day. The winds on these slopes may need to be studied more closely if it is determined that humans will be operating in such areas during the first mission to Mars.1 Finding: The risk to humans on the surface of Mars from electrostatic discharge and wind can be managed or mitigated through standard design practice and operational procedures. RADIATION HAZARDS Astronauts are by definition radiation workers. Radiation exposure in space will be a significant and serious hazard during any human expedition to Mars. There are two major sources of natural radiation in deep space: sparse but penetrating galactic cosmic radiation (GCR) and infrequent but very intense solar particle events (SPEs) associated with solar storms. In addition, many of the scenarios discussed for human missions to Mars involve the use of advanced propulsion systems that use nuclear power sources. In this event astronauts will also have to be shielded from this additional radiation source. While on the surface of Mars, the astronauts will be afforded some protection by the planet itself. Instead of having to contend with radiation from all sides, as in space, the astronauts will have only radiation from above, and that amount will be reduced somewhat by the Martian atmosphere. However, absorption and reradiation by the Martian regolith will alter the spectrum of the radiation environment. The radiation dose received by astronauts on the surface of Mars will be a significant fraction of the total radiation exposure for the mission. The radiation environment on Mars is the result of complex processes of radiation absorption and re-emission. The radiation arriving at the surface of Mars from space is a mixture of ions with a wide range of energies. This radiation interacts with the regolith and the atmosphere to create a shower of secondary particles, including recoil nuclei, nuclear fragments, neutrons, electrons, and subatomic mesons. The radiation impinging from all directions on a point at the Martian surface is influenced by the density and composition of the Martian atmosphere and by the composition of the Martian surface as well as the first few meters of the subsurface. Radiation Effects on Humans The effects of radiation exposure on humans can be grouped into two basic categories, those effects that occur very soon after exposure and those effects that are apparent months or years after exposure. Acute effects, those that occur very soon after exposure, can range from headaches, dizziness, or nausea to severe illness or death. Acute effects of radiation exposure can have a serious impact on an astronaut's ability to complete the mission. More details on radiation effects can be found in the NRC reports Radiation Hazards to Crews of Interplanetary Missions, The Human Exploration of Space, and Radiation and the International Space Station and in a report by the National Council on Radiation Protection and Measurements, Radiation Protection Guidance for Activities in Low-Earth Orbit (NRC, 1996, 1997, 2000; NCRP, 2000). The limits established by NASA for exposure to radiation during missions to low Earth orbit are clearly defined (NASA, 1995). The severity of delayed effects depends on dose. For the most part, any long-term effects of radiation exposure will not be apparent until well after a mission has returned to Earth. These delayed effects may include the following: Cancer, Cataracts, Nonmalignant skin damage, Death of nonregenerative cells/tissue (potentially including the central nervous system), Genetic damage, Impact on fertility, and Suppression of immune function. The causes of damage from short-term radiation exposure are fairly well characterized. However, the causes of long-term effects are poorly understood. The uncertainty associated with the biological impacts creates problems when trying to quantify the risk of 1   Ronald Greeley, Arizona State University, e-mail correspon-dence to the committee, August 10, 2001.

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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface radiation exposure on human missions to Mars.2 A study by the National Research Council found that the uncertainty in biological effects caused by radiation range from 200 to 1,500 percent depending on the specific effect. In addition, the uncertainties surrounding the interaction of galactic cosmic radiation, the interstellar radiation not coming from our Sun, with materials on the surface of Mars are believed to be 10 to 15 percent. Finally, the radiation transport model uncertainties are judged to be on the order of 50 percent (NRC, 1996). Developing and Validating Three-Dimensional Radiation Transport Models In examining the radiation hazard, the committee sought to balance the testing capabilities NASA has at its disposal on Earth with experiments that must be conducted in space or on the surface of Mars. There have been no direct measurements of the radiation environment on the surface of Mars. Rather, the radiation environment is estimated using computer codes that model the transport of the deep space radiation through the Martian atmosphere and its interactions with the Martian surface. The committee believes that the models predicting the absorbed radiation dose on the surface of Mars need to be validated before sending humans to Mars. These models will influence the overall design of a Mars mission, including nominal and emergency operation scenarios and habitat design. The committee was primarily concerned with two issues involved in the development and validation of the models: Developing models that realistically portray operational scenarios on Mars and Establishing the most direct method of validating the absorbed radiation dose models. The current models that NASA is using to predict astronaut absorbed radiation dose are not designed for detailed three-dimensional analysis of structures (habitats or vehicles) on the Martian surface. As a result, the radiation transport models need to be improved in concert with the development of any in situ measurement instrument and experiment intended to verify radiation transport models. The models must be applicable to a three-dimensional structure in a complex three-dimensional environment. This theory and model development will allow NASA to gain confidence in their predictions of astronaut radiation doses. Two classes of simulations (Monte Carlo and analytic) are used to model radiation transport. The analytic approach treats the cascade of particles by estimating the average incident particle energy losses as those particles travel along a path and the average buildup of secondary particles. The Monte Carlo models compute the path of representative incident particles and individual secondary particles. Both techniques can be applied to complex shielding geometries. The analytic model computes averages and generates results much faster but employs many approximations. The Monte Carlo model uses fewer simplifications, but because of the large number of particles that must be propagated it is slower and less amenable to systematic trade studies. Both transport models use approximations to characterize the physics of nuclear scattering. These approximations are necessary because of the inherent complexity of the collisions between large nuclei. To minimize the error introduced by these approximations, experiments are conducted at accelerator laboratories that help to simplify the process by examining specific particles and initial energies that impact some types of sample radiation shielding. The results of such tests provide feedback to the model developers, who, in turn, refine their models. However, practical considerations involving the amount of time it takes to conduct individual experiments and the types of ions and energies available with Earth-based testing limit the number of experiments that can be performed. For these reasons it is not feasible to experimentally measure the effects of all the ion species at all energies on all the shielding conditions that will be experienced on the Martian surface. In spite of these limitations, researchers anticipate that with continued development of both models, transport codes will be used to simulate the Martian surface radiation environment. These codes will be integral to the design of the space vehicles, surface vehicles, surface habitats, and other shelters. The dose estimates established by these models will be used to set operational rules for surface expeditions. The rules will account for such items as the maximum amount of time 2   F. Cucinotta and W. Schimmerling, “NASA Strategic Program Plan for Space Radiation Health Research,” briefing to the committee, August 2, 2001.

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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface an astronaut may spend on a surface EVA and the maximum distance from a shelter an astronaut may go. Effect of Localized Hydrogen and Iron The effective radiation dose that an astronaut will absorb is strongly affected by the number of secondary neutrons generated when radiation first impacts a material or substance. When radiated material contains hydrogen, neutrons are more readily absorbed: a desirable effect. However, when material contains heavy nuclei, more secondary neutrons are generated: a deleterious effect. There has been some concern that localized concentrations of hydrogen in subsurface ice or hydrated minerals or iron within iron-rich rocks in the Martian regolith could skew the results of in situ testing for absorbed radiation dose if such testing is restricted to a small, localized area. At the committee 's request, scientists at NASA Langley Research Center ran model simulations testing the effects of hydrogen and iron concentrations on absorbed dose. The simulations were conducted with varying amounts of hydrogen and iron (Tables 3.1 and 3.2). This preliminary analysis indicates that the current understanding of the elemental composition of Martian soil is adequate for radiation transport calculations through bulk Martian regolith. The analysis further suggests that even substantial variations in the amount of localized hydrogen and iron have little effect on the absorbed dose. The Need for Measurements Because of the central role that radiation transport and absorbed dose models will perform in the planning and design of human missions to Mars, it is important that the code predictions be validated to verify that the models are providing predictions representative of the real radiation environment on Mars. The committee recommends that NASA conduct a precursor experiment on the surface of Mars to measure total absorbed radiation dose in a tissue-equivalent material. The measurement may take place at one location. The committee acknowledges that by conducting such an experiment, NASA will only be testing for the validity of the absorbed dose models. This could lead to some ambiguity in the validation test if the experiment design is too simplistic. For example, the model may correctly predict a dose rate that the experiment observes. However, the match between the model and results may be a result of underestimating one component's TABLE 3.1 Compositions of Five Different Martian Regolith Scenarios   Nominal Regolith High Fe/ High H High Fe/ Low H Low Fe/ High H Low Fe/ Low H H 0.00 1.00 0.05 1.00 0.05 O 44.56 31.52 32.03 47.55 48.05 Mg 6.48 4.58 4.66 6.91 6.99 Si 27.16 19.21 19.52 28.98 29.30 Ca 5.21 3.69 3.74 5.56 5.62 Fe 16.59 40.00 40.00 10.00 10.00 NOTE: Values are presented in weight percent. The results are from model simulations conducted at the committee's request by Martha Clowdsley at NASA, “Examination of the Sensitivity of Mars Surface Radiation Exposures to Variations in Regolith Composition of Iron and Hydrogen,” 2001. The nominal regolith element breakdown represents expected regolith composition, while the other four categories bound extreme range possibilities of iron and hydrogen in the Martian soil. TABLE 3.2 Effect of Hydrogen and Iron Content on Absorbed Radiation Dosea Shield Thickness Nominal Regolith High Fe/ High H High Fe/ Low H Low Fe/ High H Low Fe/ Low H 1 g/cm2 aluminum 2219 22.9 22.7 22.7 22.6 22.8 10 g/cm2 aluminum 2219 22.0 21.8 21.7 21.7 21.8 NOTE: Values represent the dose equivalent for blood-forming organs (BFOs). The results are from model simulations conducted at the committee 's request by Martha Clowdsley at NASA, “Examination of the Sensitivity of Mars Surface Radiation Exposures to Variations in Regolith Composition of Iron and Hydrogen,” 2001. a Compositions from Table 3.1. contribution to the dose and overestimating that of another component, so that the two balance each other. Since, as discussed earlier, the neutron flux is very sensitive to the environment, one way to address this problem would be to require that the experiment be set up to distinguish the radiation dose contribution induced by charged particles from that induced by neutrons. In addition to providing more insight into the source of the absorbed dose, the ability to make this distinction will test the value of the models. This infor-

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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface mation will provide some assurance that it is not merely coincidence if the models' predicted doses coincide with the measurement made on Mars, but rather that the models are based on sound data. The full impact of individual variable inputs on model performance will not be resolved with this single experiment. However, if the measurements of total dose and dose contribution from neutrons on Mars agree with model predictions, these variable inputs need not be resolved to conduct the first human mission to Mars. The in situ test should take place at a location that is representative of potential landing sites for human missions to Mars. Most of Mars appears to have generally the same composition, with the exception of the poles and certain locations with hematite deposits. The average composition of the test site need not exactly match that of the planned landing site. The experiment should take place at a location with an altitude similar to that of the human missions to Mars. The depth of the Martian atmosphere will play a significant role in astronaut absorbed radiation dose, so the measurement should be made with an atmospheric thickness similar to that which the human missions will encounter. If practical, it would be beneficial to have the measurements taken at multiple locations separated by tens of meters, such as could be accomplished with a rover. The results of such an experiment could validate the predictions by models that the absorbed dose is relatively insensitive to local variations in the subsurface composition of hydrogen and iron. Finally, the committee recommends that this in situ test be made a priority in the Mars program and conducted as soon as reasonably possible. Radiation risk mitigation strategies will be an integral part of overall mission design and planning. Should the results of the in situ experiment prove that the radiation transport models are flawed, more time will be needed to adjust the models to account for the differences between the models and the measurement. If the difference is substantial enough to have a significant impact on the design and operation of the mission, further in situ tests may be required. Habitats designed to protect astronauts against GCR will also protect them against solar particle events. Operational procedures will be necessary to ensure the astronauts have timely access to these habitats or to other, similarly shielded safe havens such as robust, long-range transports. The committee recognizes that the proposed approach would not be adequate to explicitly validate model predictions of the surface dose during solar particle events. While the GCR flux varies slowly within the inner solar system and at Mars, it can be reliably estimated from measurements near Earth. SPEs, however, can be highly localized, and the flux varies significantly with distance from the Sun and location relative to the source of the solar eruption. The flux of particles at Mars during an SPE cannot be determined by measurements taken far from Mars. To directly validate a model prediction during an SPE would require that the surface measurement be supported by a second measurement of the particle flux above the Martian atmosphere. While it would be valuable during an SPE to directly correlate measurements of dose on the surface if the particle flux is available from an orbiting instrument, the committee believes that a validation of the modeled GCR contribution to dose will add sufficient confidence to the simulations such that they could be used reliably to estimate the surface dose during an SPE. The related (and difficult) task of forecasting SPEs at Mars in a timely fashion is important, but it is outside the purview of this committee. Recommendation: In order to validate the radiation transport codes, thereby ensuring the accuracy of radiation dose predictions, NASA should perform experiments to measure the absorbed dose in a tissue-equivalent material on Mars at a location representative of the expected landing site, including altitude and bulk elemental composition of the surface. The experiments should distinguish the radiation dose contribution induced by charged particles from that induced by neutrons. These experiments should be made a priority in the Mars exploration program. REFERENCES Bandfield, J.L., V.E. Hamilton, and P.R. Christensen. 2000. “A Global View of Martian Surface Compositions from MGS-TES.” Science 287:1626-1630. Bell, J.F., et al. 2000. “Mineralogic and Compositional Properties of Martian Soil and Dust: Results from Mars Pathfinder.” Journal of Geophysical Research 105:1721-1755. Clark, B.C., et al. 1982. “Chemical Composition of Martian Fines.” Journal of Geophysical Research 87:10059-10067. Kieffer, H., et al. 1992. “The Martian Dust Cycle.” Mars. University of Arizona Press, Tucson, pp. 1017-1053.

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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface Kolecki, J., and G. Landis. 1966. “Electrical Discharge on the Martian Surface,” November, available at <http://powerweb.grc.nasa.gov/pvsee/ publications/marslight.html>. Madsen, M.B., et al. 1999. “The Magnetic Properties Experiments on Mars Pathfinder.” Journal of Geophysical Research 104:8761-8789. McSween, H.Y., and K. Keil. 2000. “Mixing Relationships in the Martian Regolith and the Composition of Globally Homogeneous Dust.” Geochimica and Cosmochimica Acta 64:2155-2166. Mustard, J.F., S. Murchie, S. Erard, and J.M. Sunshine. 1997. “In Situ Compositions of Martian Volcanics: Implications for the Mantle .” Journal of Geophysical Research 102:25,605-25,615. National Aeronautics and Space Administration (NASA). 1995. NASA-STD-3000. Man-Systems Integration Standards, Vol. I, Revision B, July. National Council on Radiation Protection and Measurements (NCRP). 2000. Radiation Protection Guidance for Activities in Low-Earth Orbit, Report No. 132. Washington, D.C. National Research Council (NRC). 1996. Radiation Hazards to Crews of Interplanetary Missions. National Academy Press, Washington, D.C. NRC. 1997. The Human Exploration of Space. National Academy Press, Washington, D.C.3 NRC. 2000. Radiation and the International Space Station. National Academy Press, Washington, D.C. Rieder, R., et al. 1997. “The Chemical Composition of Martian Soil and Rocks Returned by the Mobile Alpha Proton X-ray Spectrometer: Preliminary Results from the X-ray Mode.” Science 278:1771-1774. Tomasko, M.G., et al. 1999. “Properties of Dust in the Martian Atmosphere from the Imager on Mars Pathfinder.” Journal of Geophysical Research 104:8987-9008. Wallace, J.M., and P.V. Hobbs. 1977. Atmospheric Science. Academic Press, New York, pp. 202-209. 3   Three NRC reports reprinted in a single volume: Scientific Prerequisites for the Human Exploration of Space (1993); Scientific Opportunities in the Human Exploration of Space (1994); and Science Management in the Human Exploration of Space (1997).