This study, commissioned by the National Aeronautics and Space Administration (NASA), examines the role of robotic exploration missions in assessing the risks to the first human missions to Mars. Only those hazards arising from exposure to environmental, chemical, and biological agents on the planet are assessed.
To ensure that it was including all previously identified hazards in its study, the Committee on Precursor Measurements Necessary to Support Human Operations on the Surface of Mars referred to the most recent report from NASA's Mars Exploration Program/ Payload Analysis Group (MEPAG) (Greeley, 2001). The committee concluded that the requirements identified in the present NRC report are indeed the only ones essential for NASA to pursue in order to mitigate potential hazards to the first human missions to Mars.
THE MARS PROGRAM IN CONTEXT
Even though NASA is actively pursuing a Mars exploration program, it is not yet actively pursuing a human mission to Mars, and there is no officially selected reference human exploration mission. Accord-ingly, the committee determined that it might best assist NASA by assuming that a long-stay mission to Mars will take place, as such a mission would levy the more stringent demand for the safety of astronauts while in the Martian environment. The reader should not conclude that this assumption implies an endorsement of the long-stay mission as a baseline mission, nor that the committee concluded that the long-stay mission is, in total, the least hazardous option.
In its review of the Mars robotic program, the committee found that NASA has done an excellent job of designing science rovers capable of operating on the surface of Mars. The committee believes, however, that the engineering knowledge being gained from the science rover experience will not scale up nor will it easily apply to human assistant rovers or larger human transport rovers. Furthermore, the committee notes that current science rover activities do not provide an adequate research base for the development of rovers needed for the human exploration of Mars.
NASA has allocated risk factors and reliability requirements for missions in low Earth orbit and for the International Space Station but has not done so for missions traveling beyond Earth orbit.
Recommendation: Because NASA has not allocated risk factors and reliability requirements for missions beyond Earth orbit, it should establish the risk standards necessary to provide preliminary guidance to Mars mission planners and hardware designers.
The concept of acceptable risk involves ethical, psychological, philosophical, and social considerations. The committee relied instead on standard risk sources. In reviewing the toxicology risk estimates for toxic metals, the committee chose to use an acceptable risk range (ARR) rather than a single risk level. In this report, the ARR for developing cancer as a result of exposure to toxic metals is between 1 in 10,000 and 1 in 100,000. The committee understands certain risks may overshadow others. Regardless of the large differ-
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.
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.
Hazards from Atmospheric Dynamics
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.
The committee believes that in light of the low dynamic atmospheric pressures experienced on Mars, no further characterization of wind speed on Mars is required prior to the first human mission. The surface winds are sufficiently characterized to allow system designers to ensure human safety on the planet by means of robust designs.
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.
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 after its interactions with the Martian surface. Because of the central role of radiation transport and absorbed dose models in the planning and design of human missions to Mars, it is important that the code predictions be validated by means of a precursor experiment on the surface of Mars. 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.
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.
CHEMICAL ENVIRONMENTAL HAZARDS ON MARS
In addition to the hazards from materials on Mars interacting dynamically with humans or critical systems, the committee has also assessed hazards associated with the chemical reactivity of materials on Mars.
Chemical Interaction of Martian Soil and Airborne Dust with Astronauts and Critical Equipment
Some dust and soil will in all probability be brought into the habitat through the airlock by returning astronauts, as was the case during the Apollo missions to the Moon. The committee has concluded that Martian airborne dust could present the same chemical hazards as Martian soil, so soil and dust should be characterized in the same way. In choosing the “worst” toxic chemical hazards to humans, the committee considered inorganic substances separately from organic substances. With respect to inorganic substances, it identified certain toxic metals as the worst threat to humans at the lowest concentrations.
Soil and airborne dust on Mars could contain trace amounts of hazardous chemicals, including compounds of toxic metals, which are known to cause cancer over the long term if inhaled in sufficient quantities. If NASA protects astronauts against the risk of developing cancer in the long term as a result of having been exposed to particulate matter on Mars, NASA will also be protecting astronauts from acute and short-term noncancer effects that could potentially interfere with mission success. While the committee is confident in its knowledge of the possible concentrations of most toxic metals on Mars, the committee believes the uncertainty surrounding the amount of toxic hexavalent chromium (Cr VI) on Mars warrants a precursor measurement. Hexavalent chromium on Earth is very rare in natural materials, but the great abundance of chromium (in unknown form) on the surface of Mars, combined with the high oxidation state of Martian soil,
suggests that hexavalent chromium might be present in small but potentially hazardous amounts.
Recommendation: In order to evaluate if hexavalent chromium on Mars poses a threat to astronaut health, NASA should conduct a precursor in situ measurement to determine if hexavalent chromium is present in Martian soil or airborne dust at more than 150 parts per million (ppm). This measurement may take place anywhere on Mars where well-mixed, uniform airborne dust is present. If such a measurement is not possible, a sample of airborne dust and fine particles of Martian soil must be returned to Earth for evaluation.
The committee believes that NASA can provide filtration systems capable of minimizing the hazards of exposure to toxic elements, including hexavalent chromium, arsenic, and cadmium, that are present at concentrations of less than 150 ppm.
However, if a filtration system cannot be designed to limit the average astronaut respirable particulate inhalation exposure to 1 milligram of particulate matter per cubic meter of air (mg/m3) in the habitat, then a sample of airborne dust, taken from the Martian atmosphere, and soil must be analyzed for toxic metal concentrations. The level of analytical precision required for this measurement will be dictated by the filtration capability of the astronauts' habitat.
It should be very clear to the reader that, in the view of the committee, the 1 mg/m3 specification is the maximum acceptable respirable particle average concentration to which astronauts should be exposed. This concentration level will protect astronauts from exposure to toxic metals, which&—of all inorganic chemi-cals&—the committee considers to pose the greatest health risk to astronauts. Filtering at or below the recommended 1 mg/m3 average with a 1.5 mg/m3 peak concentration should be readily achievable for NASA. Indeed, to minimize risks from exposure, the committee strongly believes that filtering should be implemented below 1 mg/m3, to as low a concentration as is reasonably achievable in the Martian habitat.
It is essential that NASA implement proper humidification in conjunction with the filtration system as part of habitat atmosphere conditioning to mitigate the threat of strong oxidants in Martian soil and airborne dust. The committee concluded that even if strong oxidants are present, there will be negligible risk associated with oxidation on the Martian surface if the proper humidification systems are in place and the particulate level is maintained at 1 mg/m3 or less.
However, even with the filtering systems in the habitat as discussed above, the filtration level may not be stringent enough to protect astronaut health and critical mechanical equipment from dust and soil that are extremely acidic. There are high concentrations of sulfur and chlorine in Martian soil, which implies the possibility of acidity in both the soil and airborne dust (Clark et al., 1982; Wanke et al., 2001). When inhaled by astronauts, acidic soil and dust could degrade their lung tissue and, if humidified and allowed to penetrate control units inside the habitat, could corrode sensitive critical equipment, such as control circuits.
Recommendation: In order to evaluate the potential corrosive effects of Martian soil and airborne dust on humans and critical systems in a humidified environment, NASA should measure the pH and buffer capacity of soil and airborne dust either via an in situ experiment or on Earth with returned samples of soil and airborne dust collected from the Martian atmosphere.
If NASA decides not to implement the necessary engineering controls or for other science-related reasons chooses to measure the oxidation properties of Martian airborne dust and soil, then the measurement should be performed on the surface of Mars rather than via a sample return.
Certain organic compounds can be highly toxic to humans, even if those compounds are not associated with a life-form, and the threat should be evaluated in planning the first human mission to Mars. Any hazard from organic compounds would most likely come from handling subsurface samples that might contain organic compounds. The committee concludes that if organic carbon is present at a concentration of more than 150 ppm in soil to which astronauts might be exposed, a possible threat exists. Filtration systems that reduce astronaut exposure to organic carbon to concentrations less than 150 ppm would mitigate this threat. If experiments determine that organic carbon is present in concentrations greater than 150 ppm, the subsurface soil should be considered a toxic hazard until proven otherwise. The need to assess the potential threat posed by a hazardous life-form consisting of organic carbon requires a more stringent measurement of organic carbon concentration.
Toxicity of the Martian Atmospheric Gases
The Martian atmosphere, when mixed in small amounts with the habitat atmosphere, does not pose a toxic risk for astronauts, and no further characterization is required before the first human mission takes place. The primary hazardous components are easily removed by standard cabin-atmosphere conditioning systems.
POTENTIAL HAZARDS OF THE BIOLOGICAL ENVIRONMENT ON MARS
The committee was charged with addressing issues of biological risks on Mars from two perspectives: (1) ensuring the safety of astronauts operating on the surface of Mars and (2) ensuring the safety of Earth 's biosphere with respect to potential back-contamination from returning human missions.
The probability that life-forms exist on the surface of Mars (that is, the area exposed to ultraviolet radiation and its photochemical products) is very small. However, as a previous NRC study (NRC, 1997) notes, there is a possibility that such life-forms exist there “in the occasional oasis,” most likely where liquid water is present, and, furthermore, that “uncertainties with regard to the possibility of extant Martian life can be reduced through a program of research and exploration.”
This charge to the committee results in a dilemma. How can NASA use human ingenuity and creativity on Mars to search for life when that life (if it exists) may pose a threat to astronaut health and safety (and therefore the success of a human mission) as well as to Earth 's biosphere?
Ensuring the Safety of Astronauts
The committee believes it is highly unlikely that infectious organisms are present on Mars. Neverthe-less, once an astronaut has been directly exposed to such life, it would be very difficult, to the point of being impractical, to determine conclusively that the astronaut would not pose a contamination threat to Earth life. In such an event, NASA might be faced with requiring quarantine and surveillance of returning astronauts until it is determined that a threat no longer exists.
Ensuring the Safety of Earth's Biosphere
While the threat to Earth's ecosystem from the release of Martian biological agents is very low, “the risk of potentially harmful effects is not zero” and cannot be ignored (NRC, 1997). NASA should assume that if life exists on Mars, it could be hazardous to Earth's biosphere until proven otherwise. As such, NASA should ensure proper quarantine or decontamination of equipment that may have been exposed to a Martian life-form.
To protect Earth from contamination by Martian life-forms aboard a returning human mission and astronauts while they are on the surface of Mars, the committee recommends that NASA employ the concept of zones of minimal biologic risk (ZMBRs) for astronaut exploration. These zones, operational areas on the surface of Mars, would be determined, to the maximum extent practicable, to be devoid of life or to contain only life-forms that would not be hazardous to humans or Earth's biosphere.
The committee recognizes that the requirement to establish and operate in a ZMBR, while intrinsic to the study charter to manage risk to astronauts, may be in conflict with one of the primary goals of the exploration of Mars: to find extraterrestrial life.
To establish a ZMRB, NASA should first attempt to determine whether or not life exists (1) at the physical locations where astronauts will be operating and (2) in the Martian material to which astronauts will be exposed. The establishment of a ZMBR might initially be based on an in situ testing protocol conducted prior to the first human visit. Once a landing site is established as a ZMBR, the astronauts can land and freely operate within it.
While some have suggested that non-carbon-based life might be present on Mars, this committee agrees with assumptions made by previous NRC committees that should hazardous life exist on Mars it would be carbon-based and thus would contain organic compounds (NRC, 2002a, 2002b). A search for life should therefore include a search for organic carbon. The detection of organic carbon might indicate the presence of life-forms.
If a sample of Martian soil and airborne dust is returned to fulfill this requirement, the returned sample should be considered hazardous and NASA should follow quarantine procedures as outlined in previous
NRC studies (NRC, 2002b). The committee also urges NASA to set an operational value for the life detection threshold limit through a separate advisory process drawing on a broad range of relevant expertise.
Recommendation: The committee recommends that NASA establish zones of minimal biologic risk (ZMBRs) with respect to the possible presence of Martian life during human missions to Mars. In order to do so, NASA should conduct a precursor in situ experiment at a location as reasonably close to the human mission landing sites as possible to determine if organic carbon is present. The measurement should be on materials from the surface and down to a depth to which astronauts may be exposed. If no organic carbon is detected at or above the life detection threshold, the landing site may be considered a ZMBR. If no measurement technique can be used to determine if organic carbon is present above the life detection threshold, or if organic carbon is detected above that threshold, a sample should be returned to Earth for characterization prior to sending humans to Mars.
There has been some concern that if a sample return is required, the planning for the first human mission to Mars may be delayed until a sample can be obtained. The committee believes that, even should a sample be required because organic carbon has been found, a baseline mission plan for a mission to Mars and even hardware development may still proceed under the assumption that a sample return will not find anything significant enough with regard to Martian biology to invalidate the baseline mission plan.
Return Vehicle Contamination
To prevent contamination of Earth by Martian material, great care must be exercised to ensure the containment of all material returned from Mars to Earth. There must be a sterile, intermediate transfer conducted in space that ensures Earth's environment will not be exposed to any Martian material, including dust or soil deposits on the outside surface of the return vehicle. The protocols for such a sterile transfer will be complex and, if the transfer is unsuccessful, may require that the return vehicle be discarded in space and never returned to Earth. Ultimately, however, only contained materials should be transported back to Earth, unless sterilized first (NRC, 1997).
Clark, B.C., A.K. Baird, R.J. Weldon, D.M. Tsuasaki, L. Schnabel, and M.P. Candelaria. 1982. “Chemical Composition of Martian Fines.” Journal of Geophysical Research 87:10,059-10,067.
Greeley, R., ed. 2001. Scientific Goals, Objectives, Investigations, and Priorities, Mars Exploration Program/Payload Analysis Group (MEPAG), March 2. Also known as Jet Propulsion Laboratory (JPL) Publication 01-7 (2001). JPL, Pasadena, Calif.
National Research Council (NRC). 1997. Mars Sample Return: Issues and Recommendations. National Academy Press, Washington, D.C.
NRC. 2002a. Assessment of Mars Science and Mission Priorities. National Academy Press, Washington, D.C.
NRC. 2002b. The Quarantine and Certification of Martian Samples. National Academy Press, Washington, D.C.
Wanke H., J. Bruckner, G. Dreibus, R. Rieder, and I. Ryabchikov. 2001. “Chemical Composition of Rocks and Soils at the Pathfinder Site.” Space Studies Review 96: 317-330.