The Mars Program in Context
Sending astronauts to the Red Planet, having them land, conduct a mission on the surface, and then return safely to Earth will be an enormous undertaking. The mission will be broken into a transit phase, that portion of the journey that takes place in deep space when astronauts are traveling from Earth to Mars, and the surface phase, when astronauts are resident on the surface of the planet. The time it takes to complete any mission to Mars depends on the relative positions of Earth and Mars in their orbits. This means that there are specific launch windows, preferred instances when Mars and Earth are ideally positioned in their respec-tive orbits, for a human mission to take place. The duration of the mission also depends on the type of propulsion used. Depending on the alignment of Earth with Mars and the amount of propulsive energy available, two basic types of missions can take place, a long-stay mission and a short-stay mission. The names refer qualitatively to the amount of time astronauts spend on the Martian surface.
A long-stay mission would require that astronauts spend 16 to 20 months in orbit around Mars or on the surface, with total mission duration being 21/2 to 3 years. On a short-stay mission, astronauts would be able to remain in orbit around Mars or on the surface for only 30 to 45 days before they would have to embark on the return journey to Earth. If they stayed longer, Earth and Mars would move out of optimum alignment and the return to Earth would require an excessive amount of propellant.
There are many safety and mission elements that must be considered in deciding whether to spend 30 to 45 days (short stay) or 16 to 20 months (long stay) in orbit around Mars or on the surface. Some of these include the following:
Transit time. The short-stay mission would require a longer round-trip transit time in space. A short-stay mission scenario would have astronauts traveling in space for 11 to 21 months, versus 10 to 14 months for a long-stay mission. Unless provisions can be made to counter the microgravity environment (by means of exercise protocols or by inducing artificial gravity) and harsh radiation conditions in space, the potential negative effects on health of the longer transit time (short-stay mission) may be prohibitive. In fact, the cumulative effects of radiation on the astronauts during the shorter transits involved in the long-stay mission might be more benign than those for the short-stay mission because of the shielding provided by the planetary mass and atmosphere while on the Martian surface (Cucinotta et al., 2001). However, the effects on astronaut health of a long duration stay in the low-gravity Martian environment are unknown.
Closest approach to the Sun. The short-stay mission would result in transits that bring the spacecraft closer to the Sun (inside the orbit of Venus, 0.72 AU) than would the long-stay mission (1.0 AU). The closer approach might increase the severity of the effects of solar particle events.
Crew exposure to the Martian surface environment. This would obviously be minimized by a short stay.
Susceptibility of critical hardware to failure. The
crew would be dependent on hardware used in surface operations for a longer period of time on a long-stay mission.
Once the astronauts are on the Martian surface, there are a variety of operational scenarios that could be conducted by NASA. The simplest would be that astronauts land and never leave a stationary habitat. The most complex scenario could include astronauts using large, pressurized rovers to travel long distances from a base habitat to conduct extravehicular activities (EVAs). The committee anticipates that a long-stay mission would probably involve the following:
The use of unpressurized rovers similar to the lunar rover from the Apollo program;
Walking EVAs of several kilometers (round-trip) from the base camp; and
Pressurized rovers for transporting the astronauts greater distances. These could allow walking EVAs from the rover to take place, extending human presence even farther from the base camp.
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.
SCOPE OF THIS REPORT
As dictated by the statement of task (Appendix A), this report examines only those hazards to which astronauts will be exposed while on the surface of Mars. For instance, the committee did not address the need for so-called pinpoint landing on Mars, nor did it look at what technologies must be developed to accomplish pinpoint landing. However, the committee does address the terrain issues associated with setting down on the Martian surface.
Also, in accordance with the statement of task the committee considered only indigenous risks on Mars&— that is, those hazards presented by the Martian environment itself, not risks based on engineering design. For instance, the committee did not examine the reliability of habitat control systems or the likelihood of their failure, but it did consider the effects acidic airborne dust or soil might have on such control circuits.
This report does not examine the hazard of forward contamination, that is, transporting Earth life to Mars from a contaminated spacecraft. There are risks associated with forward contamination of Mars by life from Earth, including the possibility of generating false positive tests in life-detection experiments (NRC, 1992a, 2002). This could certainly be a critical issue when astronauts on the surface of Mars are looking for life. A false positive result could inadvertently require a long-term astronaut quarantine. While this is a topic for continued study and debate, it is beyond the scope of this committee 's charge.
Similarly, this report does not address technologies associated with in situ resource utilization (ISRU) or deep drilling. ISRU is the use of indigenous materials to produce consumables (e.g., breathable oxygen, propellant), thus reducing the tonnage of materials that must be transported to Mars. As such, ISRU does not deal directly with the “management of environmental, chemical, and biological risks, ” as set forth in the statement of task. Drilling systems might be used on human missions to Mars to explore the subsurface of Mars for scientific purposes. However, these systems are not critical to human survival. In fact, by using a drill, astronauts might become exposed to other indigenous Martian hazards. The hazards of subsurface probing by astronauts are discussed in detail in this report.
Other potential hazards the committee did not address involve the effects on astronaut health of a long-duration mission to Mars. A recent Institute of Medicine report states that the three most important health issues that have been identified for long-duration missions are radiation, loss of bone mineral density, and behavioral adaptation (IOM, 2001). The committee acknowledges that these issues are important, as is the need to ensure a benign social environment during a multiyear voyage, but again such considerations fall outside the scope of this report.
ROVER TECHNOLOGIES AND ROBOTICS
Even though there is no baseline mission defined for human missions to Mars, it is likely that rovers of some form will be used to perform functions critical to the safety of the astronauts. For example, human assistant rovers may carry life support equipment, while others robots, such as slow-moving scientific rovers, will likely perform mission-critical functions.
For these reasons, the committee explicitly considered the technologies used in current and planned Martian scientific rovers and whether such rovers can be scaled up to support critical tasks for human exploration (see Box 2.1).
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 Mars Pathfinder rover is a success story that testifies to the skill of NASA scientists and engineers. And, the Mars exploration rovers now in development offer some significant advances in rover capabilities.
The committee believes, however, that the engineering knowledge being gained from the science rovers will not scale up nor will it easily apply to human assistant rovers or larger pressurized or unpressurized 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.
Typical science rovers generally move very slowly and for short distances. This is understandable, because once on the surface the rovers do not need to go very far to conduct scientific research in most cases. However, the science rovers are not necessarily good platforms for testing future rover technology. The Mars exploration rover, scheduled to launch in 2003, will be approximately 180 kg (~400 lb) in mass and is expected to have a speed of 1 cm/sec (~0.4 in./sec). The total distance the rover is expected to travel in one Martian day is 100 meters. The NASA Smart Lander with rover capabilities, scheduled to launch in 2009, is expected to be approximately 1,000 kg (2,200 lb) in mass, with a speed of anywhere from 2 cm/sec (~0.8 in./sec) to 10 cm/sec (4 in./sec).
On the human missions to Mars, rovers will need capabilities far beyond what is currently planned. Human assistant rovers would have to be able to keep pace with an astronaut walking on the surface of Mars, to operate for a long time, to have an extended range, and to navigate rough terrain quickly. These needs would be especially important for a long-stay mission, where there might be many hundreds of astronaut EVAs that would require a robotic assistant to traverse hundreds of kilometers over the course of the mission. Such human assistant rovers would require kilowatts of continuous electric power during the EVA. Compact sources of power at that capacity do not exist today.
BOX 2.1 The Anatomy of a Critical System
Critical systems are those upon which a crew's lives depend. NASA has specific requirements for such critical systems (NASA, 1998):
All critical systems essential for crew safety shall be designed to be two-fault tolerant. When this is not practical, systems shall be designed such that no single failure shall cause loss of the crew. For the purposes of this requirement, maintenance can be considered as the third leg of redundancy so long as missions operations and logistics resupply permit it.
For long duration missions . . . fault tolerance is not sufficient. For these missions, multiple failures are expected, and the response must include maintenance and system reconfiguration to restore the failed functions. In the case of Beyond Earth Orbit vehicles, it is unlikely that resupply vehicles can supplement the resources aboard the vehicle unless that capability was planned for in advance via pre-positioned spares. Therefore, safe operation of the vehicle requires that sufficient reliability be achieved through a combination of reliable hardware design, installed redundancy, and logistics capability to support maintenance.
It is important to differentiate systems critical for human safety from those critical for mission success. The latter are for the most part subjected to less stringent, though still demanding, standards. For instance, any robot operating inside the habitat in a labor-saving capacity, such as EVA suit cleaning, is not considered critical. Not all robots that operate outside of a habitat (i.e., in contact with the hostile Martian environment) are critical. Specifically, a human assistant rover that carries oxygen and must keep up with a human walking on Mars is a critical system. A human transport rover (pressurized or not) is also a critical system, since its performance is required to ensure astronaut safety.
However, a science rover that astronauts use to search for life in a remote area before conducting an EVA to that location is not critical. Presumably if such a robot failed, an EVA to that location would simply not take place, so astronaut safety would not be a concern. Nor is a machine that drills to look for subsurface life a critical system, in that if it were to fail, humans would not be directly at risk. The reader should note that this report deals with issues critical to the safety of the crew, not mission success.
Human transport rovers have the same performance issues as human assistant rovers, but on a larger scale. For a long-stay mission, human transport rovers would
need to be able to operate for up to 20 months and traverse thousands of kilometers over the life of the mission. Tens of kilowatts of power might be required to accomplish these goals, and it would most likely not be practical to use solar photovoltaic systems given the relatively large size of those systems.
Even looking ahead to the 2009 rover from the NASA Smart Lander at the optimistic forecast of 10 cm/sec, NASA would have to increase the speed of such a rover by a factor of 13 to reach a human walking speed of 4.8 km/hr (3 mph). The same rover would have to increase its speed by a factor of 44 to reach the 16 km/hr (10 mph) mark that a human transport rover would probably require.
The mass of the 2009 rover from the NASA Smart Lander is comparable to that of the human assistant rovers that may be required on the human missions to Mars, but the dynamic nature of the vehicles is entirely different.
In the committee's judgment, the design and testing of human transport rovers that can traverse long distances and that have long lifetimes and adequate power supplies may be a pacing item in the mounting of the first human mission to Mars.
Finding: NASA's current focus on small, slow robotic rovers with short lifetimes and modest power supplies does not provide an adequate research base for the development of the rovers needed for the human exploration of Mars.
ESTABLISHING RISK STANDARDS
NASA has established detailed requirements and standards for ensuring the safety and reliability of space shuttle and International Space Station operations. There is an ongoing effort to establish a set of requirements for “human rating” the next generation of human-occupied spacecraft that will permit humans to operate more efficiently in Earth orbit and to explore beyond Earth orbit. A human-rated system risk rating is one that “incorporates those designs, features, and operational procedures needed to accommodate human participants, allowing NASA to safely conduct human operations, including safe recovery of astronauts from any credible emergency situation” (NASA, 1998).
As stated in the above-mentioned NASA human missions requirements document, “many of the detailed requirements required to implement these . . . missions are very different. Even with the very high level of the requirements [contained herein] there are some that cannot be applied consistently across [all] missions.” The document further states that “the program shall be designed so that the cumulative probability of safe crew return over the life of the [space flight] program ex-ceeds 0.99” (NASA, 1998).
Using this guideline, a program that involves only 10 flights can absorb a greater risk per flight than one that involves 100 flights. NASA has yet to publish risk allocation guidelines for missions beyond Earth orbit. This report suggests the use of risk factors based on those established by federal agencies or suggested by other studies. For example, in the absence of NASA standards for the risk associated with exposure to toxic metals on Mars, the committee suggests the establishment of a risk factor based on a study by the NRC Committee on Toxicology (COT) and on Environmental Protection Agency (EPA) risk estimates. Given the level of inherent risk associated with space exploration missions, NASA may well choose to establish risk factors different from those proposed in this report, especially if risk estimates change in the future.
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, which are used extensively in Chapter 4, 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.
Historically, for the general population, EPA has considered a 1 in 100,000 risk of getting cancer acceptable. The committee based part of the ARR selection on the fact that the EPA usually (with no overriding regulations or site-specific requirements) takes cleanup action on a site that has a risk of greater than 1 in 10,000 of causing cancer in humans, while the EPA never takes action at a risk of 1 in 1,000,000 (Travis et al., 1987). Therefore the committee chose the lowest reasonable level of acceptable risk for the ARR at 1 in 100,000 of astronauts getting cancer during their lifetime as a re-
sult of exposure to toxic metals in Martian soil or airborne dust.
The 1 in 10,000 maximum risk in the committee's ARR is based on studies by the NRC's Committee on Toxicology (see Box 2.2). It is notable that the Committee on Toxicology adopted a less stringent standard than EPA for carcinogens, namely, 1 in 10,000 (NRC 1992b, 1994, 1996, 1997, 2000). NASA, EPA, and this committee are in general agreement on the concentration limits for hazardous chemicals producing acceptable risks for astronauts. The reader is referred to Box 2.2 for further discussion of the committee's consideration of hazardous chemical exposure limits.
BOX 2.2 Exposure Limits for Chemical Hazards
The use of toxic chemical exposure limits serves to protect individuals from excessive health risk due to exposure to harmful chemicals. Exposure limits or recommendations have been established by a variety of groups; among the most prominent are EPA, the Occupational Safety and Health Administration (OSHA), the National Institute for Occupational Safety and Health (NIOSH), and the American Conference of Governmental Industrial Hygienists. There is a fundamental difference in the limits established by EPA and those of other groups, in that EPA regulations are designed to protect the whole public, including sensitive groups such as the very young and the very old as well as people with respiratory diseases and other illnesses. In contrast, occupational limits are meant to protect healthy adults in the workplace, usually for an exposure duration of an 8-hour workshift, with a substantial recovery period at home. For these reasons, occupational standards allow for considerably higher exposure concentrations than EPA risk estimates, as shown in Table 2.1.
For the present study, the committee based its consideration of potential chemical exposure hazards on the conservative risk estimates established by EPA for long-term cancer risk at the 1 in 100,000 level and the NRC's Committee on Toxicology (COT) at the 1 in 10,000 level. The EPA risk estimates are published in EPA's Integrated Risk Information System database.
It is clear that NASA has for some time recognized the need to consider exposure of astronauts to potentially hazardous chemicals. It commissioned the COT to recommend maximum concentrations of spacecraft contaminants three decades ago (NRC, 1972). More recently, COT published four reports setting spacecraft maximum allowable concentrations (SMACs) for a total of 40 individual contaminants based on guidelines it developed for SMACs in 1992 (NRC, 1992b, 1994, 1996, 1997, 2000). COT has expended considerable effort in developing SMACs, basing its recommendations on an extensive review of toxicity data with application of safety factors. Specifically, SMACs for carcinogenic chemicals are established using toxic element concentrations that produce an estimated 1 in 10,000 increased lifetime risk of a neoplasm, following U.S. Department of Defense practice. The detailed rationale for each recommendation is presented in the related NRC report. In general, these SMACs for 1 in 10,000 risk are comparable to EPA limits estimated for the 1 in 100,000 risk level.
It is notable that the SMACs have focused on the International Space Station. As such, the 40 airborne chemicals considered to date pertain to operation in a closed-loop environment for a maximum of 180 days, which is a shorter period than many proposed missions to Mars. The 40 contaminants include primarily chemicals outgassing from man-made materials within the space station itself. The contaminant list does not include dust or soil infiltrating from outside the habitat, which are a concern for planetary exploration. When reviewing the committee 's findings and performing its own assessment of the risks and acceptable exposure levels of potential chemical hazards, NASA must consider the unique scenarios for operation on Mars.
TABLE 2.1 Exposure Limits for Some Respirable Chemical Hazards (micrograms per cubic meter)
The committee chose not to use Occupational Safety and Health Administration (OSHA) exposure limits since those limits are based on working periods only (8 hours per day, 5 days per week). Once the astronaut living area on Mars is contaminated with soil or airborne dust, the astronauts may be exposed to low levels of Martian airborne particulates on a continuing basis for up to 1.5 to 2 years. Therefore using the risk estimates discussed above for continuous (24-hour) exposure represents a conservative approach for health protection.
The committee understands certain risks may over-shadow others. For instance, as discussed, the committee assumes the allowable risk for astronauts getting cancer (not necessarily fatal cancer) as a result of exposure to toxic trace elements is the range between 1 in 10,000 and 1 in 100,000. For low Earth orbit, NASA has established the limit of 3 percent excess risk of fatal cancer from radiation exposure, or 1 in 33.
Regardless of the large difference between the risk of fatal cancer from radiation and the risk of getting cancer from toxic metal exposure, 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. For instance, radiation exposure may weaken the human immune system and make a person more susceptible to other hazards. Balancing risks from various hazards will be necessary to allow NASA to make informed decisions regarding risk.
Cucinotta, F., W. Schimmerling, J. Wilson, L. Peterson, G. Badhwar, P. Saganti, and J. Dicello. 2001. Space Radiation Cancer Risk Projections for Exploration Missions: Uncertainty Reduction and Mitigation. JSC 29295, January. Johnson Space Center, Houston, Tex.
Institute of Medicine (IOM). 2001. Safe Passage: Astronaut Care for Exploration Missions. National Academy Press, Washington, D.C.
National Aeronautics and Space Administration (NASA). 1998. Human Rating Requirements, JSC 29354. Johnson Space Center, Houston, Tex.
National Research Council (NRC). 1972. Atmospheric Contaminants in Manned Spacecraft. National Academy Press, Washington, D.C.
NRC. 1992a. Biological Contamination of Mars: Issues and Recommendations. National Academy Press, Washington, D.C.
NRC. 1992b. Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants. National Academy Press, Washington, D.C.
NRC. 1994. Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Vol. 1. National Academy Press, Washington, D.C.
NRC. 1996. Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Vol. 2. National Academy Press, Washington, D.C.
NRC. 1997. Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Vol. 3. National Academy Press, Washington, D.C.
NRC. 2000. Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Vol. 4. National Academy Press, Washington, D.C.
NRC. 2002. Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques. National Academy Press, Washington, D.C., in press.
Travis, C.C., S.A. Richter, E.A.C. Crouch, R. Wilson, and E.D. Klema. 1987. “Cancer Risk Management.” Environmental Science and Technology 21(5):415.
U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health (NIOSH). 1997. Pocket Guide to Chemical Hazards. Also available at <http://www.cdc.gov/niosh>.