An Analysis of the Precursor Measurements of Mars Needed to Reduce the Risk of the First Human Mission to Mars
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The need for martian dust/regolith simulant(s)An important strategy for reducing the risks related to the effects of granular materials on both engineering and biological systems is to establish one or more martian dust/regolith simulants. Widely accepted standard materials make it possible to compare technology performances from different laboratories and to generate empirical rather than theoretical data. For risks associated with MEPAG Goal IV Investigation 1A, we recommend using the simulants to test dust accumulation on various types of materials; dust repellant, removal and cleaning technologies; various types of decontamination procedures; flight hardware designs; reliability, maintainability and waste minimization technologies; and operational procedures. For risks associated with MEPAG Goal IV Investigation 2, we recommend using simulants to perform in-vitro and in-vivo laboratory exposure testing, laboratory animal tests, establishment of respiratory and inhalation limits, and the development of operational procedures, mitigation methods, and exposure levels. Investigation 1A addresses Risk #4 in Table 1 ("Adverse effects of dust on mission surfaces"). Investigation 2 addresses Risk #5 in Table 1 ("Direct dust hazards to crew (toxicity)"). FINDING: PROPOSED INVESTIGATION AND MEASUREMENTS1A. Characterize the particulates that could be transported to hardware and infrastructure through the air (including both natural aeolian dust and particulates that could be raised from the martian regolith by ground operations), and that could affect engineering performance and in situ lifetime. Analytic fidelity sufficient to establish credible engineering simulation labs and/or performance prediction/design codes on Earth is required. Measurements a. A complete analysis, consisting of shape and size distribution, mineralogy, electrical and thermal conductivity, triboelectric and photoemission properties, and chemistry (especially chemistry of relevance to predicting corrosion effects), of samples of soil from a depth as large as might be affected by human surface operations. Note #1: For sites where air-borne dust naturally settles, a bulk regolith sample is sufficient—analysis of a separate sample of dust filtered from the atmosphere is desirable, but not required. Note #2: Obtaining a broad range of measurements on the same sample is considerably more valuable than a few measurements on each of several samples (this naturally lends itself to sample return). Note #3: There is not consensus on adding magnetic properties to this list. b. Polarity and magnitude of charge on individual dust particles suspended in the atmosphere and concentration of free atmospheric ions with positive and negative polarities. Measurement should be taken during the day in calm conditions representative of nominal EVA excursions. Note #4: This is a transient effect, and can only be measured in situ. c. The same measurements as in a) on a sample of air-borne dust collected during a major dust storm. d. Subsets of the complete analysis described in a), and measured at different locations on Mars (see Note #2). For individual measurements, priorities are: i. shape and size distribution and mineralogy ii. electrical iii. chemistry. In addition, Mars regolith simulants are needed for geotechnical modeling. A large amount of work can be done in terrestrial laboratories (and in terrestrial computers) in this area, but the models need to be based on some ground truth—they would rely heavily on simulants for initial development. The relative priority of regolith-related geotechnical risks, and the kind of information that would be needed to address them (possibly including both in situ measurement of regolith in its undisturbed state, and full characterization of a regolith sample) are not yet clear. The one currently available martian simulant (JSC Mars-1) is of uncertain relevance, because we simply don’t have enough information about all of the appropriate properties of the martian regolith. Most obviously, we have incomplete information about the mineralogy, the grain size distribution, or the particle shape of the martian regolith, and as of this writing, we have measurements of its chemistry at only five landing sites. This study assumed that dust properties at one site would be representative of global dust because global dust storms would homogenize the dust. However, heavier particle properties may vary from location to location. Several types of simulated regolith are expected to be needed to support different kinds of tests. For example, when evaluating a technology for adhesion, the simulant may need to represent the electrostatic and magnetic properties of martian dust. For toxic effects, a simulant may represent the corrosive properties of dust that could lead to illness. Measurement 1A.a would provide data to create simulants with abrasivity (shape, size, mineralogy), adhesion (size, electric and thermal conductivity), electrical (electrical conductivity), and corrosion (chemistry) properties representative of martian dust. These simulants can be used in artificial martian environments to test promising technologies for dust removal, cleaning, sealing and corrosion prevention. This investigation was deemed the highest priority because, given the Apollo experience, the probability of occurrence of the risk associated with these properties is very high as is the consequence as described earlier in the risk discussion. Measurement 1A.b would provide data (triboelectric and photoemission properties) to use in simulant development for testing electrical grounding systems. This information is needed to create computer and laboratory simulations to ensure the systems designed to ground electrical systems are adequate. FINDING: PROPOSED INVESTIGATION AND MEASUREMENTS 2. Determine the possible toxic effects of martian dust on humans. Measurements: a. For at least one site, assay for chemicals with known toxic effect on humans. Of particular importance are oxidizing species such as CrVI. (May require MSR). b. Fully characterize soluble ion distributions, reactions that occur upon humidification and released volatiles from a surface sample and sample of regolith from a depth as large as might be affected by human surface operations. c. Analyze the shapes of martian dust grains sufficient to assess their possible impact on human soft tissue (especially eyes and lungs). d. Determine if martian regolith elicits a toxic response in an animal species which is a surrogate for humans.
Measurement 1A.c would provide information to assist engineers in determining if mitigation technologies determined to be effective in martian environments created with simulants based on Measurement 1A.a are also effective during martian environmental dust storms. It would also provide data for computer simulations. The Dust/Soil Focus Team prioritized this measurement as less than Measurement 1A.a because of the possibility that the properties measured in Measurement 1A.c could be modeled in computer or laboratory simulations. However, ground-truthing is important to validate the models, so it remains a high priority. Measurement 1A.d determines if dust and soil are homogenous across the planet. The Dust/Soil Focus Team believes that the upper layers of regolith are homogenous, however, this assumption has not been validated. If the properties of the dust vary widely by region new simulants may need to be developed for missions depending on the destination. Also, if additional risks related to lower layers are identified by the granular materials community in the future, this information would be useful in mitigating those risks. Measurement 2a through 2d address Risk #5. These measurement have equal priority and the data from each would be used develop simulants for toxicological studies as described above. Importance and Reasoning The likelihood of risks associated with investigation 1A are very high. We know from our Apollo missions that dust rendered space suits useless after three days. Abrasion and accumulation of dust on systems led to failure and degradation. We know that abrasion and accumulation of dust would lead to adverse consequences [NASA, 1969a; 1969b; 1971a; 1971b; 1972; 1973]. On the other hand we have some evidence that dust may adversely affect crew health [Lam 2002a; Lam, 2002b; Conners et al., 1994]. However, we do not have as high a level of confidence in the existence of toxicity as we have in the abrasive and adhesive properties of martian dust. Therefore, the likelihood level of risks associated with investigation 1A are deemed higher than those associated with investigation 2. If we do not build our systems to handle dust, a catastrophic failure could happen very quickly, and given the laws of orbital mechanics, there may be no way home. On the other hand, toxic materials inhaled by the crew may lead to chronic illness, probably after the crew returned home, which could be treated on Earth. So, given the time to effect, Investigation 1A is deemed higher priority than Investigation 2. Further, the mitigation of risk associated with measurements obtained by Investigation 1A, would lead to limiting the amount of dust entering the EVA suit and brought into the habitat after EVA activities that would naturally reduce risks associated with Investigation 2. Additionally, sensitive electronics that control life safety systems may require an even cleaner environment than the crew requires. Given the above analysis Investigation 1A is deemed a higher priority than Investigation 2. Timing, Frequency and Location Measurements 1A.a. and 1A.b require one-time sampling and analysis at a single location. Measurement 1A.c also requires one-time sampling and analysis at a single location, however, this measurement must be performed during a major storm. Measurement 1A.d requires sampling and analysis be performed at high and low albedo and a polar region. The Dust/Soil Focus Team assume that the dust is homogenous because global dust storms would distribute the dust across the planet. However, even if the parameters were obtained by measurement rather than models, extrapolating an entire planet from a single measurement would result in low confidence in our knowledge. Moreover, nearly all our data is for the surface, globally distributed wind born layer of dust, not for what may lie beneath. Therefore the Dust/Soil Focus Team recommended investigation 1A.d. Measurements for Investigation 2 require one-time sampling and analysis at a likely landing site for the first human mission. Dust, soil and toxicology measurements should be performed as soon as possible to provide engineers with the time needed to develop engineering techniques to mitigate the risks [National Research Council, 2002]. Possible Research and Technology investments needed to obtain the recommended measurements are:
Radiation encountered on a mission to Mars would present two kinds of exposures to the crew, chronic and (possibly) acute (Risk #12 and Risk #13 in Table 1). The chronic exposure comes the Galactic Cosmic Radiation (GCR), which is a continuous, low-dose source of charged particles. The GCR consists of energetic atomic nuclei of all species from hydrogen to uranium, fully stripped of their electrons. (Ions heavier than iron are rare.) These nuclei are not encountered on Earth because of its thick atmosphere and planetary magnetic field, but in deep space they cannot be entirely avoided under realistic shielding scenarios. Chronic exposure to radiation can produce “late” effects, i.e., effects that are manifested years, perhaps decades, after exposure. The late effects of greatest concern are increased cancer risk and damage to the central nervous system (CNS). Acute radiation exposures are possible when strong Solar Energetic Particle (SEP) events occur. In SEP events, very large fluxes, dominated by protons, are accelerated to energies sufficient to traverse a few grams per square cm of matter. Very high-flux SEP events have been seen, producing doses that might have caused acute effects and possibly death to unshielded or lightly shielded humans outside the geomagnetosphere [Wilson et al., 1998]. A good introduction to and discussion of the issue of radiation exposures on deep-space missions can be found in a 1996 report by the Space Studies Board of the National Research Council [NRC, 1996]. In this report, we expand on the discussion of radiation issues in “Safe on Mars” [NRC, 2002]; the conclusions reached by the MHP Radiation Focus Team are substantially the same as those reached in that report. We will discuss both chronic and acute exposures in more detail below. Since this subject matter is unfamiliar to many readers, we first define some relevant terms. For additional definitions, see the Review of Particle Properties [Eidelman et al., 2004]. Linear Energy Transfer (LET or L) – A charged particle traversing loses energy principally by ionization. Energy lost by the charged particle is transferred to the medium, with some energy escaping in the form of energetic delta-rays. The amount of energy transferred is, in radiation biology, referred to as Linear Energy Transfer (abbreviated LET or sometimes just L), and is given in units of energy/length. Ionization energy loss is described with great accuracy by the Bethe-Bloch equation, which gives a reasonable approximation of LET under most conditions. Dose – Radiation dose is defined as the energy transferred to a medium (usually tissue or water) per unit mass. The standard unit of dose is the Gray, abbreviated Gy, equal to one joule per kilogram. Assuming the medium in question is water, and assuming charged particle equilibrium, the dose at a point can be related to the integral particle fluence (particles per cm2) as a function of LET, (L), as follows: 
where D is in nanoGray,  is in units of MeV/cm and  is in units of g/cm3.
Quality Factor – Different types of radiation vary in their effects on biological systems. To account for this, the current risk assessment methodology employs (for “mixed field” radiation as encountered in spaceflight) a weighting factor called the quality factor, Q which is defined to be solely a function of LET. The current functional form of Q vs. LET is shown in Figure 1; it is determined by the International Commission on Radiation Protection [ICRP, 1990], using a sizable database obtained in decades of research in radiobiology. Q is defined for cancer risk and hereditary effects only, and since it represents the current state of knowledge, it is subject to change. The biological effects of energetic heavy ions such as those in the GCR are not well understood at present, which makes use of the quality factor controversial in some parts of the space radiation research community. Further, Q has been determined using particles of considerably lower energy than are typical for Galactic Cosmic Rays. And if heavy ions also cause damage to, e.g., the central nervous system, the present paradigm based on Q breaks down because CNS damage does not result from exposure to low LET radiation. Thus the entire present paradigm of radiation risk assessment rests on data that are of questionable applicability to long-duration spaceflight. 
Radiation such as x-rays, energetic protons, etc., have LET below 10 keV/m and Q of 1. Between 10 and 100 keV/m, Q rises. Iron ions in the GCR typically have LET in the range from 150-200 keV/m, which is why iron is the leading contributor to dose equivalent in the GCR. Dose equivalent – Dose equivalent, H, is related to cancer risk. The space radiation environment is a “mixed field” of different particle types, energies, and LETs. The mixed-field average quality factor is given by 
and dose equivalent is given by  . The unit of dose equivalent is the Sievert (Sv). An exposure to 1 Gy of radiation with Q = 1 gives a dose equivalent of 1 Sv.
Acute Exposures – Doses above 1 Gy, received in a short time, cause immediate deleterious health effects. Doses above 5 Gy are typically lethal. Historically, such exposures have occurred only in accidents and in the bombings of Hiroshima and Nagasaki. Acute effects during spaceflight could occur if astronauts are exposed to a large SEP event at a time they have little or no shielding. Chronic Exposures – Chronic exposure at a low dose-rate is unavoidable for humans traveling in deep space. Long-term health effects of chronic radiation exposure include increased incidence of cancer, cataracts, and damage to the central nervous system. Mitigation strategies include shielding (which is difficult given mass constraints and the physics of GCR transport) and improved propulsion systems to reduce flight duration. Active shielding approaches have been investigated but do not appear viable. A modest depth of shielding can either reduce or increase the dose equivalent received by crew, depending on the composition of the material used. Long-term Health Effects – Studies of the A-bomb survivors provide the largest database for determining long-term health effects of radiation exposure [NRC, 1990]. The applicability of those data to exposures received in flight is dubious, given the radically different exposure modes. On Earth, exposures large enough to show statistically significant effects in a surviving population have only occurred when a dose of low-LET radiation was received in a short time. In deep space, the situation will be quite different, as crew will receive a high-LET radiation at a very low dose rate. HZE – The high-charge (Z > 2) and high-energy particles in the GCR are sometimes referred to as “HZE” particles. This is a convenient shorthand we will use here. ALARA and Career Limits – The ALARA principle (“as low as reasonably achievable”) guides NASA’s approach to radiation protection. NASA is legally and ethically obliged to keep radiation exposures as low as possible. The radiation astronauts receive in flight is much different from that received by, e.g., nuclear reactor workers; this means that NASA, with input from the research community, must determine its own specific guidelines. Limits are now set to keep an astronaut’s career exposure below the value that is believed to increase his or her lifetime risk of a fatal cancer by less than 3%. Since the lifetime risk of an individual depends on age and gender, the career limit depends on age and gender. Least susceptible are older males; most susceptible are younger females. Career limits have not yet been defined for deep-space missions. Limits for low-Earth orbit range from 0.5 to 4 Sv. Galactic Cosmic Radiation (GCR) – The GCR consists of fully stripped atomic nuclei of all known species. GCR particles originate outside the solar system. About 87% of GCRs are protons, 11% helium nuclei, 1% electrons, and 1% ions heavier than helium. Though not highly abundant, the heavier ions contribute significantly to radiation exposures in deep space. Typical differential flux distributions peak in the energy region of several hundred MeV/nucleon. The flux is on the order of 0.1 to 1 particle cm-2 s-1 sr-1, varying inversely with solar activity. Solar Energetic Particles (SEPs) – SEP fluxes are dominated by protons; occasionally energetic helium ions are also seen, and in very rare events high-energy heavy ions are seen [Tylka and Dietrich, 1999]. For the most part, however, when discussing SEPs, we are referring to protons with typical energies below 100 MeV. Fluxes above 104 particles cm-2 s-1 sr-1 have been seen, e.g., by GOES in the E > 10 MeV proton channel in the Halloween 2003 event. In some events, there can be large fluxes of more energetic (and hence more penetrating) protons as well. The event of Sep. 29, 1989 had an event-integrated flux of > 100 MeV protons in excess of 108 particles cm-2.
To summarize the preceding section, radiation presents two types of risks to crew, chronic and acute. Chronic exposure to the GCR increases the probability of an individual developing a fatal cancer (or other serious illness) over the course of his or her lifetime. Acute exposure to SEPs can have immediate deleterious health effects. Chronic exposures cannot be prevented, and must instead be managed; in stark contrast, acute exposures must be prevented through a combination of proper shielding of spacecraft and habitats, and early warnings.
The current state of knowledge of the relevant physics (flux of particles, effects of shielding materials) is good. In contrast, the knowledge of the relevant radiobiology is far from complete. The radiobiology of HZE particles is the subject of a ground-based effort using the new NASA Space Radiation Laboratory facility at the Brookhaven National Laboratory. The biological problem is extremely difficult, because there are many endpoints (cancers in different organs, damage to the CNS, cataract induction, etc.) and many levels at which one needs to understand both radiation effects and the abilities of living systems to correctly repair themselves. This side of the problem presents far larger uncertainties than the physics side. Of particular interest for MHP is the role that neutrons would play in the exposures crewmembers would receive on the martian surface. The biological effects of neutrons are a subject of ongoing debate, and a revised set of weighting factors (used to convert from fluence to dose equivalent) has recently been proposed by the ICRP. This, and other uncertainties in the biological effects of radiation, are issues that the MHP program cannot address. To accurately calculate the dose equivalent requires knowledge of the types and energies of particles present. In space, it is impractical to measure all the relevant particles that impinge on the crew. Also, radiation protection requires that dose to sites internal to the body – where it is impossible to place detectors – must be estimated. Therefore, predictions of radiation exposures in spaceflight, as well as post-mission assessments, will continue to depend on transport models. MHP should address this essential issue in radiation safety: the validation of radiation transport models that predict the detailed flux of particles on the martian surface. Ideally, a space radiation transport model would reproduce both the incident radiation (GCR and SEP) and all the mechanisms relevant to transport of the incident particles through matter. Not surprisingly, current models fall short of the ideal, but some are still found to be fairly accurate when compared with flight data and/or data obtained at particle accelerators. However, as has been pointed out in many contexts, no matter how perfect one might hope to make a model, there is no substitute for experimental verification. Further, SEP events are presently (and perhaps intrinsically) unpredictable, so that one cannot hope to make a predictive model for that part of the dose. Existing models include the BRYNTRN [Wilson et al., 1989] and HZETRN [Wilson et al., 1991] codes developed originally at NASA-Langley. There are ongoing efforts to bring other well-established high-energy physics codes such as HETC and FLUKA to bear on the problem. Many sources have contributed to the current state of knowledge, including flight data acquired over several decades. As new data that bear on the problem become available, they are incorporated into the transport models. For predicting the GCR flux, including its modulation by the solar cycle, a recent paper [O’Neill and Badhwar, 2004] reports a highly accurate computer code that reproduces the latest ACE/CRIS data. In the study of SEP events, a large database of events exists, with an active community of researchers working to understand (and perhaps predict) the exact mechanisms of particle acceleration. Most relevant to MHP, orbital data on charged particles and neutrons have been acquired by instruments on the 2001 Mars Odyssey spacecraft – MARIE for charged particles, and the Neutron Spectrometer and High-Energy Neutron Detector for neutrons [Saunders et al., 2004]. To assess the radiation environment on the surface of Mars, we must take account of the sources of energetic particles (GCR and SEP events) and the ways in which Mars itself modifies the flux. Table 3 summarizes the main components of the radiation environment on the surface. The GCR flux at the top of the atmosphere is reasonably constant in time (it is modulated over the course of the solar cycle and varies in total particle count by factors of 2-3 from solar min. to solar max.) and does not differ significantly between Earth and Mars. In SEP events, depending on the relative position of Mars and Earth, the fluxes of particles can be entirely different in the two locations. |