ence between the risk of getting fatal cancer from radiation and the cancer risk from exposure to toxic metals, it is prudent to reduce risk in all areas that are amenable to such reductions. It is important to reduce risks in areas that are reasonably achievable, as there can be synergistic effects of combined hazards.

PHYSICAL ENVIRONMENTAL HAZARDS ON MARS

The committee categorized the hazards on Mars by their sources, or causes. It specifically defined the 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.

Geologic Hazards

To ensure safe landing and operations on the surface of Mars, it is necessary for the landing site and the topography of the anticipated surface operation zone to be fully characterized with high-resolution stereoscopic imaging. The operation zone is the area around the landing site defined by the anticipated range of operations of extravehicular activities (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.

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.

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.

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.

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. For this reason, no further in situ experiments to determine the abrasive properties of Martian rocks are required.

Airborne dust presents a potentially significant hazard to human operations on the surface of Mars. Dust intrusion and accumulation will need to be continuously monitored and will require well-designed filter systems and periodic housecleaning. 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.

The present Mars soil simulant that has been developed and characterized by NASA for engineering (JSC Mars-1: Martian Regolith Simulant) is not adequate for testing mechanical systems for human missions to Mars. However, the committee does not recommend that any precursor in situ measurements be taken on Mars to characterize the mechanical and abrasive properties of airborne dust. Rather, it expects that an appropriate simulant would adequately stress the design of any mechanical and seal systems that will be used during a human mission to Mars. It is critical, however, to fully characterize the adhesive properties of airborne dust in order to design systems that minimize the risk of failure resulting from dust accumulation.

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.



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