1This study, commissioned by the National Aero-nautics and Space Administration (NASA), examinesthe role of robotic exploration missions in assessingthe risks to the first human missions to Mars. Onlythose hazards arising from exposure to environmental,chemical, and biological agents on the planet areassessed.To ensure that it was including all previously identi-fied hazards in its study, the Committee on PrecursorMeasurements Necessary to Support Human Opera-tions on the Surface of Mars referred to the most recentreport from NASA’s Mars Exploration Program/Payload Analysis Group (MEPAG) (Greeley, 2001).The committee concluded that the requirements identi-fied in the present NRC report are indeed the only onesessential for NASA to pursue in order to mitigatepotential hazards to the first human missions to Mars.THE MARS PROGRAM IN CONTEXTEven though NASA is actively pursuing a Marsexploration program, it is not yet actively pursuing ahuman mission to Mars, and there is no officiallyselected reference human exploration mission. Accord-ingly, the committee determined that it might best assistNASA by assuming that a long-stay mission to Marswill take place, as such a mission would levy the morestringent demand for the safety of astronauts while inthe Martian environment. The reader should not con-clude that this assumption implies an endorsement ofthe long-stay mission as a baseline mission, nor thatthe committee concluded that the long-stay mission is,in total, the least hazardous option.Executive SummaryIn its review of the Mars robotic program, the com-mittee found that NASA has done an excellent job ofdesigning science rovers capable of operating on thesurface of Mars. The committee believes, however, thatthe engineering knowledge being gained from thescience rover experience will not scale up nor will iteasily apply to human assistant rovers or larger humantransport rovers. Furthermore, the committee notes thatcurrent science rover activities do not provide an ade-quate research base for the development of roversneeded for the human exploration of Mars.NASA has allocated risk factors and reliabilityrequirements for missions in low Earth orbit and forthe International Space Station but has not done so formissions traveling beyond Earth orbit.Recommendation: Because NASA has not allocatedrisk factors and reliability requirements formissions beyond Earth orbit, it should establish therisk standards necessary to provide preliminaryguidance to Mars mission planners and hardwaredesigners.The concept of acceptable risk involves ethical,psychological, philosophical, and social consider-ations. The committee relied instead on standard risksources. In reviewing the toxicology risk estimates fortoxic metals, the committee chose to use an acceptablerisk range (ARR) rather than a single risk level. In thisreport, the ARR for developing cancer as a result ofexposure to toxic metals is between 1 in 10,000 and 1in 100,000. The committee understands certain risksmay overshadow others. Regardless of the large differ-2 SAFE ON MARSence between the risk of getting fatal cancer from radia-tion and the cancer risk from exposure to toxic metals,it is prudent to reduce risk in all areas that are amenableto such reductions. It is important to reduce risks inareas that are reasonably achievable, as there can besynergistic effects of combined hazards.PHYSICAL ENVIRONMENTALHAZARDS ON MARSThe committee categorized the hazards on Mars bytheir sources, or causes. It specifically defined thephysical hazards on Mars separately from the chemicaland biological hazards, because physical hazards canthreaten crew safety by physically interacting withhumans or critical equipment, resulting, for example,in impact, abrasion, tip-over (due to an unstableMartian surface), or irradiation.Geologic HazardsTo ensure safe landing and operations on the surfaceof Mars, it is necessary for the landing site and thetopography of the anticipated surface operation zone tobe fully characterized with high-resolution stereoscopicimaging. The operation zone is the area around thelanding site defined by the anticipated range of opera-tions of extravehicular activities (EVAs), including theuse of human transport and/or science rovers. The levelof resolution required of this imaging will be deter-mined by the capabilities of the equipment to be usedon the surface.Recommendation: NASA should map the three-dimensional terrain morphology of landing opera-tion zones for human missions to characterize theirfeatures at sufficient resolution to assure safe land-ing and human and rover locomotion.Recommendation: To ensure that humans andcritical rover systems can land on and traverse theMartian surface in a safe, efficient, and timelymanner, NASA should characterize the range ofmechanical properties of the Martian regolith at thelanding site or comparable terrain. Specifically, insitu experiments should be performed to determinethe regolith’s aggregate strength, stability, andsinkage properties, including bearing strength, bulkmodulus, yield strength, and internal friction angle.Recommendation: NASA should determine, inadvance of human missions to Mars, rock size dis-tribution and shapes in situ, at the landing site or oncomparable terrain, in order to predict human androver trafficability.The abrasive properties of rocks on Mars, includinghardness and surface roughness (as dictated by rockgrain size and shape), are unknown. The committeebelieves that, even faced with this lack of knowledge,NASA can still design systems by making certain edu-cated assumptions about the rocks on Mars. For thisreason, no further in situ experiments to determine theabrasive properties of Martian rocks are required.Airborne dust presents a potentially significanthazard to human operations on the surface of Mars.Dust intrusion and accumulation will need to be con-tinuously monitored and will require well-designedfilter systems and periodic housecleaning. Afterreviewing NASA’s experience with dust on the Moonand Mars, the committee is confident that NASA engi-neers and scientists will be able to design and buildsystems to mitigate the hazards posed by airborne duston Mars. Some systems that would be used on the firsthuman mission can be designed either by employingwhat is currently known about Mars dust or by assum-ing a worst-case scenario in the design process.The present Mars soil simulant that has been devel-oped and characterized by NASA for engineering