An Analysis of the Precursor Measurements of Mars Needed to Reduce the Risk of the First Human Mission to Mars
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Transport model validation remains the highest priority for radiation investigation, which is the same conclusion reached by the “Safe on Mars” panel. Below, we list a small number of highly-relevant ground-based experiments that could contribute in this area prior to the first MHP mission. Ultimately, though, a direct comparison of charged particle and neutron spectra obtained on the surface of Mars to those predicted by transport models is needed. This would lead either to improvements of the models, or to a consensus that the models are already adequate according to some pre-determined criteria for their accuracy. As helpful as ground-based experiments may prove to be, they are no substitute for data obtained in situ.As discussed above, the physics part of the radiation problem is fairly well understood, and we can categorize and make some quantitative estimates of the radiation health risks to astronauts on a Mars mission. Improved understanding of HZE radiobiology would reduce the uncertainties on those estimates. However, there is an additional risk that is not always described as such: the possibility that, due to large uncertainties on the physics side, concerns about radiation would force mission planners to include shielding mass that might turn out to be unnecessary were the problem better-understood. The penalty for launching excess mass, and perhaps landing it on Mars, is very high; in the absence of greater certainty about the risks, radiation shielding mass could be a major cost driver of a human mission. It is therefore beneficial from a programmatic perspective to obtain the needed knowledge early in the MHP program so that accurate calculations of shielding requirements can feed forward into the designs of spacecraft and habitats. Definition of Model Validation We have repeatedly referred to “validation” of transport models as the main goal of measuring the radiation on the surface of Mars. However, a universally agreed-upon definition of validation is elusive. To that end, the Radiation Focus Team suggests that NASA should sponsor a limited, focused workshop on the martian surface radiation environment, providing direction for a coordinated effort to optimize instrument requirements in advance of placing a radiation instrument on a MHP Lander/Rover. Modeling of radiation at the martian surface requires an understanding and integration of the sources, nuclear interactions, and the properties of Mars itself – the atmosphere, surface, and subsurface, including temporal and spatial variations. Experiment design should be optimized through a collaboration of NASA’s Radiation Health Officer, radiation transport modelers, experimentalists, and experts in Mars’ atmosphere, surface and subsurface. An early workshop could set the stage for optimal use of MHP opportunities to perform the needed validation of transport codes.
The dose from albedo neutrons on the surface is the least understood of the various contributions to radiation exposure on a Mars mission. Transport model predictions of this contribution should be viewed with some skepticism, since neutron production is thought to be a weak point of the models. Data from Odyssey can be extrapolated to give an estimate, but results are model dependent since one must account for the transport of neutrons from the surface through the atmosphere. Therefore, a surface neutron measurement with a detector that has a well-understood response is the highest priority radiation measurement for MHP. Since hydrogen is an excellent moderator of neutrons, the surface dose from albedo neutrons will be highly dependent on the location(s) chosen on the surface and the water ice content of the regolith in that location. It would be of considerable interest to do two measurements, one in icy terrain presumably near the poles, and one in a more equatorial region away from the large ice concentrations. We note that a low-mass (~ 1 kg) radiation instrument has been selected as part of the 2009 Mars Science Laboratory. The instrument is designed to measure both energetic charged particles and neutrons. The MHP SSG Radiation Focus Team – which has no overlapping personnel with the instrument team – strongly supports the inclusion of this detector as a much-needed first step in the model validation process. Because the mass of the instrument is so low, it would not provide all of the data that are needed for model validation, and more detailed measurements would remain necessary even after the successful conclusion of MSL. Nonetheless, any “ground truth” that can be obtained would be extremely valuable, particularly in the area of neutron flux. Measurements on the Martian Surface and In Orbit Measurements of both neutrons and charged particles on the surface of Mars are intended to provide the data needed for transport model validation. The consensus of the Radiation Focus Team is that the weak point in the current models is in the calculation of neutron dose equivalent on the martian surface. The neutron component is present at all times due to the GCR, and is enhanced, possibly by orders of magnitude, during large SEP events. In typical SEP events, neutrons are the main source of dose on the surface, since virtually all of the charged particles stop in the atmosphere. Validation of transport codes would be advanced by landing a well-characterized neutron detector system that is capable of measuring over a wide energy range, with the further capability to distinguish between albedo neutrons coming up from the regolith and neutrons traveling downward from the atmosphere. In conjunction with this system, it would also be helpful to have a modest charged-particle spectrometer on the surface, and a similar spectrometer in orbit.155 MeV to penetrate the Martian atmosphere. I would say that the orbital instrument needs to measure protons from 5 to at least 200 MeV while the surface charged particle telescope does not need to go so high in energy. --> Duration, Frequency, Location Because measurement of the particle flux on the surface during a large SEP event is a high priority, and because the exact timing of events is unpredictable, it would be most useful to have a detector system in place that can operate for several years during solar maximum. It would be most advantageous to have this in place at or shortly after the time of the next solar maximum, which occurs in 2011. It would also be advantageous for the charged-particle detector to operate for several years during solar maximum. Assuming, as seems likely, that it would be quite compact, it would have a small geometry factor and it would be necessary to integrate the data over many weeks to get good statistics on the high-LET end of the spectrum. Since the presence of large amounts of water ice in the regolith has a large effect on the effective dose from albedo neutrons, it would be useful to have at least two detectors, one in icy terrain and the other in dry terrain. This would provide an additional test of the ability of the model codes to predict the effective dose from neutrons under different conditions. To simplify the analysis of the data and to minimize systematic errors, the Radiation Focus Team recommends that two copies of the same detector system be landed in very different locations. Use of Existing Data Obtained in Mars Orbit The Radiation Focus Team suggests that neutron data from Odyssey’s Neutron Spectrometer (NS) and HEND be carefully analyzed to determine the neutron doses at various latitudes. (It is probably sufficient to integrate over longitude.) This task requires that the data from both instruments be normalized to absolute fluxes, rather than relative fluxes which have been used in analyses to date. First results have been reported by Mitrafanov et al. [2004]. A similar analysis should be performed on Odyssey NS data. These results would be extremely useful, but would not be as reliable as measurements made on the surface, since the neutron fluxes in orbit differ from those on the surface due to transport through the atmosphere. Ground-Based Precursor Investigations It is possible to advance the state of knowledge with one or more ground-based experiments in advance of the first MHP mission. One piece of the problem can certainly be addressed with experiments on Earth – or, more precisely, above Earth. Because the martian atmosphere presents a column depth of about 22 g cm-2 of CO2 to incident GCR particles, it is possible to nearly recreate these conditions by flying a balloon to at 85,000 ft above Antarctica. Missions of this type have been successfully flown by NASA for many years. Although the composition of Earth’s atmosphere is somewhat different than Mars’, the differences in transport are small. (From the perspective of nuclear physics, carbon, nitrogen, and oxygen nuclei are all similar targets.) A balloon-borne experiment to perform this measurement should carry high-resolution charged particle detectors and neutron detectors capable of measuring high-energy neutrons. The Deep Space Test Bed (http://sd.msfc.nasa.gov/cosmicray/DSTB/DSTB.htm) that is being prepared by NASA’s Space Radiation Shielding Project could carry out such an experiment. A second part of the problem can, conceivably, be addressed by Earth-based experiments. It is possible, at facilities such as the NASA Space Radiation Laboratory, to accelerate protons and heavy ions to energies typical of the GCR. If one could produce a good approximation of martian regolith, it could be put in beams of GCR-like particles and the resulting albedo particles could be measured. This would provide a stringent test of the models under well-controlled circumstances, but it hinges on the availability of a realistic simulation of martian regolith. Technology Development GCR particles and SEPs have been studied in flight experiments for decades. State-of-the-art detectors such as ACE/CRIS [Stone et al., 1998] and the High Energy Telescope and Low Energy Telsecope energetic particle spectrometers that would fly on the STEREO mission have capabilities that meet or exceed the requirements for charged-particle measurements on the surface of Mars and in orbit. Little or no new technology is needed for the charge particle measurements relevant to MHP. However, one essential capability for measurement of the Mars surface radiation environment has not yet been demonstrated in flight: the ability to measure a neutron spectrum over a wide energy range, with at least crude directional capability (distinguishing the downward-going neutrons produced in an atmosphere from upward-going albedo neutrons produced in regolith). These capabilities exist in accelerator-based particle detectors on Earth, but packaging such an instrument for flight may present some technical difficulties. Since neutrons can only be detected by the recoil protons they produce, a detector gains efficiency by adding hydrogenous mass. The challenge would be to build a compact, low-mass system with reasonable efficiency.
The strong consensus of the Radiation Focus Team is that despite the relatively good state of knowledge, measurements are needed both on the surface of Mars and in orbit in order to validate the radiation transport models. The models are expected to be accurate but have known weaknesses that can best be probed by comparing their predictions to data obtained in situ, both above and below the atmosphere. We suggest revising some of the language in the MEPAG document that describes the radiation investigation. First, it is inaccurate to say that “Soil and dust from the Martian surface offer(s) a readily available source of shielding material” – it is not the case that dust can be used as a shield. Further, shielding properties of the martian regolith against GCR particles may be far less than optimal; it may instead be better to use regolith blended with a hydrogenous compound for the construction of shielding. Second, though knowledge of the UV spectrum is highly desirable for a number of reasons, exposure to UV is not a problem comparable in scope to the risks presented by exposure to GCR particles, SEPs, and the secondary particles they produce. Crew would be protected from UV radiation by habitat walls, spacesuits, and visors. FINDING: PROPOSED INVESTIGATION AND MEASUREMENTS 5. Characterize in detail the ionizing radiation environment at the martian surface, distinguishing contributions from the energetic charged particles that penetrate the atmosphere, secondary neutrons produced in the atmosphere, and secondary charged particles and neutrons produced in the regolith. Measurements: a. Measurement of charged particles with directionality. Identify particles by species and energy, from protons to iron nuclei in the energy range 20-1000 MeV/nuc. b. Measurement of neutrons with directionality. Energy range from 1 keV (or lower) to 100 MeV (or higher). c. Simultaneous with surface measurements, a detector should be placed in orbit to measure energy spectra in Solar Energetic Particle events.
3.5.1 Introduction Martian terrain is a hazard to any landed mission, including human missions. First, flight to the martian surface may result in loss of a vehicle and crew if there is an incomplete knowledge of the terrain (Risk #15 and Risk #18 in Table1). Further, terrain location and morphology knowledge directly affects the approach and landing philosophy, including whether the human pilot would be expected to conduct the actual landing, as in Apollo, or whether the landing would be entirely automatic. Second, the ability to rove significant distances would be affected strongly by vehicle design, one concern of which would be tire/wheel design and the clearance needed between a vehicle underside and the local fine scale terrain (Risk #11 and Risk #18 in Table 1). Third, operations, exploration, and science investigations of a human mission may be limited in geographic extent if risks related to martian terrain are not minimized with precursor measurements (risk to science return). For example, the distance the Apollo astronauts were allowed to travel on their rovers (for science) was restricted to the distance they could walk for safe return to the lunar module in the event that the rover became stuck in the lunar terrain. As an example of these mobility hazards, the Mars Exploration Rover Opportunity became unexpectedly mired in soft dune material on April 26, 2005, and was not able to get free until June 4 (see Figure 4). During this period, wheel rotation commands were sent that would have moved the rover a total of approximately 200m, but actual movement was only about 1m--average wheel slippage was 99.5%. For a human mission, this could have severe consequences. 
Figure 4. Image taken on Sol 482 from the front hazard camera onboard the Mars Exploration Rover Opportunity. The rover was mired in soft dune material which severely limited the rover’s mobility. For the purpose of this analysis, we assume that a landing site certification process would continue to be in place for all landed missions (including the first human mission), and that this certification process would be an essential part of the mission planning and development process. Landing site certification is not something that can productively be assigned to a long-lead robotic precursor program (for one thing, the landing site is not likely to be specified until much closer to the time of the mission). For the purpose of the MHP analysis, therefore, we have focused on the precursor measurements needed in addition to the site certification process. 3.5.2 Some thoughts about landing site certification Although the site certification process has not yet been defined, the MHP SSG identified several aspects that should be considered by the future team that establishes it. It is clearly beyond the scope of the MHP SSG to determine the maximum acceptable risk related to landing site hazards, but the issues below would need to be considered. Issue #1. Establish terrain knowledge of “lander-scale” terrain obstacles for each landing site (horizontal location and vertical height) to an accuracy consistent with the design of landing systems and mission rules
Issue #2. Establish rock abundance at potential landing sites to a scale consistent with lander footpad design; and if timing permits, feed these data into the lander design process. This relates to the risk of coming to rest in a tilted attitude due to a rock lodged under a footpad (preventing a safe egress, or a stable habitat or proper attitude for future departure from the martian surface).
FINDING: PROPOSED INVESTIGATION AND MEASUREMENTS 6. Determine traction/cohesion in martian soil/regolith (with emphasis on trafficability hazards, such as dust pockets and dunes) throughout planned landing sites; where possible, feed findings into surface asset design requirements. Measurements: a. Determine vertical variation in in-situ soil density within the upper 30 cm for rocky areas, on dust dunes, and in dust pockets to within 0.1 g cm-3. b. Determine variation in in-situ internal angle of friction of soil for dust dunes and dust pockets to within 1 degree. c. Determine soil cohesion for rocky areas, dust dunes and in dust pockets to within 0.1 kN m-2.
For basic design of mobility systems, the following measurements are needed (not just at the dust pockets and dunes, but also on the consolidated soil surfaces where we may do most of the driving): (i) rolling resistance (the torque that soil applies to a rolling wheel while driving), (ii) traction test (torque required to spin a wheel while the rover is held stationary), and (iii) shape/size of the resulting wheel ruts while driving normally. Note #1: These three things will probably be measured routinely on all Mars rovers. Note #2: ISRU excavation could require hauling larger loads (soil/ice payload) than what we have ever hauled in the past. Therefore they will need these data to properly design wheels and chases (e.g., rover and large structure mobility systems), avoiding energy-wasteful designs or risk of getting bogged down. 3.5.3 Terrain/trafficability Risks The scope considered for terrain risks includes meter to sub-meter scale elements that would affect manned and robotic rover travel, as well as are needed for large vehicle foot pad design, such as rocks of various sizes and the presence of features such as dunes and local dust accumulations. A critical mission operations question that needs to be addressed, particularly for the first landing, is the degree of landing risk that would be acceptable, as balanced against the scientific interest in a particular landing site. It is a generally held corollary that the most interesting landings sites from a science perspective are in the most difficult landing sites to access. Early definition of risk levels would help define acceptable landing sites, and would provide a better definition of the hazard that the large and small scale terrain features would pose to future human exploration. Surface operations risks include (Risks #11, #15, and #18 in Table 1):
3.5.4 Proposed Investigations and Measurements Thermal inertia measurements must be sufficient to identify dust accumulations that would cause either roving or landing hazards. The TES instrument was able to achieve 3 km/pixel on Mars Global Surveyor. Definition of a particular landing hazard is, in part, related to the landing ellipse for a given landing craft, which does not exist yet for a Mars lander associated with a human mission. However, we can estimate that having a knowledge of dust accumulations that are on the order of 1 km2 would be sufficient to do adequate landing site and rover traverse determination. Therefore, we would expect to require an order of magnitude better resolution than TES. Penetrometry and trenching experiments have been done on previous spacecraft as far back as Surveyor and the Viking Landers, so this should not require a significant increase in technology development. Imaging needs should be met by present and future orbital spacecraft (e.g., Mars Express, 2005 Mars Reconnaissance Orbiter).
3.6.1 Introduction ISRU as a general term refers to all in situ resources that may be of value, including building materials, water, oxygen for breathing, natural shielding materials, and other things. Of these many possibilities, the one that has by far the greatest potential to affect the design of human missions to the martian surface is the availability of water resources. In order for the potential savings to be realized, accurate information needs to be obtained relatively early, so that the knowledge can be incorporated into the mission design. A number of design reference missions (DRMs) have been developed for the human exploration of Mars (e.g. Drake, 1998; Hoffman and Kaplan, 1997). In support of the Vision for Space Exploration, multiple architecture concepts continue to be evaluated to determine what capabilities and technologies need to be developed to enable the human exploration of Mars. These kinds of studies make it clear that there is a significant challenge to launch and land the heavy vehicles required for human exploration. Two of the largest masses needed on Mars are propellants for ascent from the martian surface, and consumables for life support. If it were possible to reduce the mass brought to Mars by either obtaining or manufacturing these from local resources, it would make a dramatic difference in the mass budget of the mission. Because of the potential significant mass, cost, and risk reduction benefits associated with incorporating ISRU into mission plans, close examination of resource availability along with mission surface durations, pre-positioning of critical assets, etc. is required. The precursor measurement program needs to focus on crustal water resources for two reasons: (1) Water is by far the most valuable of the martian resources, because of its potential use (via ISRU) in offsetting huge mass requirements for both life support systems and ascent vehicle propellant, and (2) Knowledge of crustal water resources (location, concentration, chemistry, and mineralogical form) is far more speculative than for other resources. By comparison, our ability to assess the feasibility of making beneficial use of the atmosphere is not limited by uncertainty in whether the atmosphere is (or is not) present at a given landing site. 3.6.1 Description of risks Even though within the past the last few years, data obtained from Mars Odyssey spacecraft suggests that water is widely available in the top ~1 meter of the martian surface, the individual data pixels represent the average conditions over a very large region (on the order of 100 km x 100 km), and the actual conditions at any individual site within that could be very different than the average. Thus, there is a risk that resources assumed to be present at a human landing site are not present in the quantity, quality, or location assumed. These risks are easily addressable by precursor measurements, and are described as Risk #1 in Table 1. Thus, the MHP SSG concluded that the primary risks associated with planning for ISRU that can be addressed by means of precursor measurements are those connected to distribution and accessibility of crustal water deposits (Risk #1 in Table 1). Moreover, since ISRU may be enabling for human Mars missions, these measurements have the highest priority. 3.6.1 Proposed Investigations and Measurements In order to realize the potential value of such deposits to human missions to the martian surface, a well-conceived and executed exploration program for martian water deposits is needed. This exploration program will need to be analogous to resource exploration programs on Earth, and consist of a reconnaissance phase followed by a site-specific deposit definition phase. This exploration program will have the effect of progressively increasing knowledge certainty of one or more martian water resource deposits in much the same way that on Earth resource exploration goes through the sequence of potential, possible reserves, probable reserves, and proven reserves. The required exploration plan will need to start with formulation of a series of testable geological models for water-bearing deposits that could be accessed by a credible production system. These hypothetical water deposits should be organized by production system: shallow bulk mining methods, well-based methods, and other TBD production systems.
It is important to note that the overall objective (for the purpose of retiring this risk) is not to define all of the water deposits on the planet, but to establish to within a reasonable degree of confidence that at least one deposit of water, with minimum acceptable characteristics (concentration, quality, quantity, depth, state) and in a location consistent with both mission landing safety considerations and with the mission’s objectives, exists. As of this writing, there are four general classes of water deposit on Mars that may have the potential to meet this need: Perceived to have higher priority
Perceived to have lower priority
In addition to the precursor measurement program, engineering assessments and possible precursor missions to validate and quantify the water extraction process based on the resource measurement assessment would need to be undertaken. FINDING: PROPOSED INVESTIGATION AND MEASUREMENTS 1D. Characterize potential sources of water to support ISRU for eventual human missions. At this time it is not known where human exploration of Mars may occur. However, if ISRU is determined to be required for reasons of mission affordability and/or safety, then the following measurements for water with respect to ISRU become necessary (these options cannot be prioritized without applying constraints from mission system engineering, ISRU process engineering, and geological potential): Measurement Options:
At this time there is no definitive list of criteria for selecting the sites on Mars where human exploration would occur. Should water-based ISRU be incorporated into human mission plans, the availability of usable quantities of water to a high degree of confidence would be one of the criteria used in selecting potential mission landing sites. Regardless of all the selection criteria, exploration to locate accessible Mars water resources is needed.
There are a number of risk issues in the general area of soil/regolith mechanical properties, and these are collectively grouped as Risk #7 in Table 1. During the Apollo and Viking programs considerable effort was expended to study the cratering of the regolith when a rocket launches or lands on it [Romine, et al, 1973; Alexander, et al, 1966; Roberts 1964; Clark and Land 1963; Scott and Ko; Ko 1970; Foreman 1967; Clark 1970; Hutton, et al, 1980]. That research ensured the success of those programs but also demonstrated that cratering will be a serious challenge for other mission scenarios.  The most violent phase of a cratering event is when the static overpressure of the rocket exhaust exceeds the bearing capacity
of the soil or sufficiently fluidizes it to excavate vertically beneath the surface. This bearing capacity failure (BCF) produces a small and highly concave cup in the surface. The shape of the cup then redirects the supersonic jet – along with entrained debris – upward toward the spacecraft. This has been observed in terrestrial experiments but never quantified. The blast from such an event will be qualitatively different than the cratering that occurred in the Apollo and Viking programs, because BCF had been successfully avoided in all those missions. In fact, the Viking program undertook a significant research and development effort and redesigned the spacecraft specifically for the purpose of avoiding BCF [Romine, et al, 1973]. (See Figure 5.) However, it will be unavoidable in the Martian environment with the large landers necessary for human exploration. Furthermore, cratering (with or without BCF) will be a significant issue when there is an attempt to land multiple mission-critical spacecraft within short distances of one another, whether on the Moon or Mars. It is possible that the first spacecraft to land may be damaged by the spray from the second spacecraft’s landing. The co-landing of critical hardware has never been done before, but we do have some relevant experience because the Apollo 12 Lunar Module (LM) landed 155 meters away from the deactivated Surveyor 3 spacecraft. Portions of the Surveyor were then returned by the astronauts to Earth for analysis [Cour-Palais 1972; Jaffe 1972]. It was found that the surface of the Surveyor 3 had been sandblasted by a high-speed shower of sand and dust particles during the LM’s landing. The sandblasting cast very sharp, permanent “shadows” onto the spacecraft which very accurately pointed away from the point on the regolith directly beneath the LM. Judging by the sharpness of the shadows and the lack of curvature allowable for the particles to fit the trajectory, the particles must have been moving in excess of 100 m/s. Furthermore, every cavity and opening in the spacecraft was filled with grit by the high-speed spray. Co-landed spacecraft must be designed to withstand the blast and contamination of the cratering. 
To manage the soil/plume interactions, we must have some basic knowledge of the Martian surface and near subsurface to a depth that may be reasonably excavated by the plume. In addition to the plume/soil interaction problem, there are other reasons to measure the geotechnical characteristics of the Martian soil. Apollo astronauts in the early missions experienced difficulty obtaining core samples of the lunar soil due to the soil's unexpected mechanical properties. As the program progressed, geotechnical analysis of the soil improved the core sampling tool design, thereby enabling the scientific goals of the missions. This demonstrated the importance of considering the soil for its engineering properties, and not just as an object of scientific investigation. In the new vision to explore the Moon and Mars, engineering with the soil will take on a vastly greater role as a fundamental resource in ISRU. This necessitates excavation, beneficiation and processing of the regolith materials in order to obtain the consumables that will enable the overall mission. Difficulties working with the soil, such as were experienced in the early Apollo program, would have a much greater detrimental impact upon a program that relies on ISRU, and hence the mechanics of Martian regolith must be thoroughly investigated early in the design of the program. Martian geotechnical properties have been measured by five lander spacecraft within the top several centimeters of the surface using scoops and wheels to trench the surface, and using wheels for traction tests [e.g., Matijevic 1997]. The mechanical properties of the deeper subsurface, however, have never been measured. Furthermore, remote sensing and other data have implied the existence of water ice in the regolith, but its effects on regolith mechanics have never been experimentally investigated. To enable the engineering design of hardware to interact with soil, measurements are usually made in situ to determine the soil's fundamental physical properties or more often its behavior measured against mechanical indices (standard geotechnical tests). Terrestrially, those tests are often the basis of design protocols that develop hardware or structures compatible with the characteristics of a particular soil. These protocols are based upon a wealth of terrestrial experience, and unfortunately that experience does not exist for the Martian environment and soil. Hence, it is not clear that the standard mechanical indexes could lead directly to developing reliable and efficient hardware for a safe and cost effective program when extrapolated to that environment. Furthermore, it is difficult to specify which such indexing tests would be most pertinent to ISRU resource extraction and construction activities, since those activities are still largely undefined at the present time. At the present, it is necessary to rely on a selection of the typical indexing tests, as well as on the more fundamental physical properties tests, and then supplement those measurements with a program of experimentation (in situ and with simulants), modeling (to help extrapolate what is known into what is still unknown), and finally in situ system demonstration validating the final design, where necessary.
Martian near-surface materials have the potential to constitute a hazard for a number of mission critical activities, including:
Some adverse consequences that may result from interaction with these hazards:
Risk Statement: If soil cohesion, shear strength, density, compaction, volatile content and specific energy of boring or chipping are not properly bounded in the vertical column above and within the source of water ice, then an excavator, boring unit, beneficiator, or resource extractor may not produce ice-rich soil at a rate sufficient for scheduled human arrival or not at all. In a contingency it may not be able to resupply water to the crew after their arrival. Failure may result in loss of science or loss of crew. Also, crew equipment for manual access to the subsurface may not be optimally designed if the subsurface is more or less cohesive than expected. Context: If surface soil lacks sufficient cohesion, collapsing material into a bore hole may make it impossible to extract desired subsurface materials. If shear strength, density, or compaction are greater than expected, the device(s) may lack power or energy to produce sufficient quantities or at a sufficient rate. If specific energy is greater than expected, processing may be inefficient or impossible.
Risk Statement: If the flow properties of excavated geomaterials (due to their cohesion, density, compaction, volatile content, electrostatic properties and/or particle sizes/shapes) and their relative significance in the Martian environment (low and seasonally variable atmospheric pressure which affects Darcian versus Knudsen flow regimes, reduced surface gravity, and low humidity) are not properly bounded for materials extracted from the vertical column within the area of ISRU excavation, then an ISRU processing unit may become jammed and fail to process resources at a rate sufficient for scheduled human arrival or not at all. In a contingency it may not be able to resupply water to the crew after their arrival. Context: Terrestrial experiments show that granular materials that would otherwise flow freely in a terrestrial environments will behave like a cohesive powder when flowing in reduced gravity due to the relatively greater significance of Van der Waals, electrostatic, and other cohesive forces. Terrestrial experiments have also shown that granular materials display qualitatively different convective heaping characteristics when the pore pressure is below 10 Torr [Behringer, et al, 2002]. The mechanical properties of the excavated soil due to its volatiles, cementing, or other physical characteristics may make ISRU processing hardware inoperable or unreliable if not properly anticipated. The up-scaling design process used for terrestrial processing plants will not be possible for Martian ISRU processes due to inaccessibility of the Martian environment, including gravity. Hence, there will be a greater reliance on modeling during the design process of ISRU hardware. This necessitates a sufficient knowledge of the regolith flow properties to adequately model them.
Risk Statement: If heat and mechanical load of surface assets drive volatiles from the subsurface over a period of several years (during ISRU build-up period), the regolith may weaken or differentially settle beneath the surface assets. This may result in a loss of upright posture which could impede ISRU processing flows, result in the asset tipping, make entries or access point to be misaligned or inaccessible, and mechanically strain the interfaces of connected assets resulting in a loss of seal integrity. Scientific experiments may also be disturbed by differential settlement. Context: There is a general uncertainty about the Martian subsurface which potentially could allow for a weakening of the subsurface and differential settlement over the duration of a mission (including ISRU processing time prior to crew arrival). This may be especially true if the volatile content is high in the near subsurface. Thermal loads provided by the operational hardware such as nuclear fission reactor or heated habitation module may over the course of several years drive sufficient volatiles from the subsurface to result in a settling of an inch or more. This may be exacerbated for heavier elements such as ISRU processing or storage units when they are loaded with geomaterials or consumables. Worst case could be a sudden loss of competence beneath one or more spacecraft footpads.
Risk Statement: Excessive material ejected in vertical direction may affect spacecraft landing by spoofing its landing sensors, imparting differential momentum to the bottom of the spacecraft, or damaging critical components by direct impact of rocks or pebbles. Falling debris may blanket radiators, solar cells or critical instruments. Context: Substantial prior experience exists only through Apollo and Viking programs, both of which were predicted to produce plume effects significantly different than will occur for a large-scale human-tended mission on Mars. Terrestrial tests and analysis have demonstrated the Mars bearing capacity fails when static overpressure of rocket exhaust plume exceeds about 20 kPa. Viking spacecraft was redesigned to just stay beneath this limit. The method used for Viking will not scale for human tended spacecraft and as a result bearing capacity failure will probably be unavoidable. For supersonic flow in an atmosphere, a ground jet normally proceeds laterally and radially away from the gas stagnation region beneath the spacecraft. However, the onset of bearing capacity failure or local fluidization of the soil beneath the plume will produce a cavity that results in a vertical deflection of the ground jet. Terrestrial tests have shown that this will result in a geyser of ejected material traveling vertically due to the high concavity of the excavated hole into which the plume is pointed.
Risk Statement: Excessive material ejected in lateral direction may damage surrounding ISRU or scientific or other assets. Context: Terrestrial (and lunar) experience shows that prior to (or in the absence of ) a vertical eruption of ejecta, the material is initially ejected in the lateral direction at high velocity. After vertical eruption, there may be a widening of the crater that results in a re-broadening of the ejection cone. This may rain material down upon surrounding hardware. Due to the low density of the Martian atmosphere, terminal velocity of the material is very high. Surrounding hardware may be damage by rocks, or its critical sensors damaged by sand or dust, or its solar panels or radiators blanketed by debris. FINDING: PROPOSED INVESTIGATION AND MEASUREMENTS Contribution to Investigation #1D. For approaches that involve acquiring and mechanically processing natural near-surface water-bearing material, characterize its geotechnical properties. Measurements: 1. Determine the following mechanical and physical properties of regolith (soil and/or ice/soil mixtures) representing the resource deposit type of interest to a depth at least as great as the maximum proposed depth of access:
Contribution to Investigation #1A. Characterize at least one regolith deposit with a fidelity sufficient to establish credible engineering simulation labs and/or software codes on Earth to solve engineering problems related to differential settlement of the regolith, and plume/soil interactions. Measurements: 1. For one site on Mars, measure the following properties of the regolith as a function of depth to 1 meter:
2. Repeat the above measurements at a second site in different geologic terrane: Note #1. Because there is a large engineering lead-time required to solve the geotechnical problems, these data must be obtained early in the precursor program. Note #2. These measurements should be made in a competent soil deposit as opposed to loose drift material (cohesionless sand dunes), as landing is expected to attempt to avoid the looser material. Also, if mission planners select high latitude polar deposits for a human landing site, geotechnical data will be required from a representative location of those deposits. Contribution to Investigation #6. Measurements: 1. No measurements in addition to those already described in Section 3.5:
Risk Statement: Blowback of soil after engine cutoff may contaminate engine preventing a safe restart or may contaminate other critical spacecraft mechanisms and sensors. Context: Experiments have shown that the soil beneath a rocket plume is highly fluidized by the impinging high-pressure gas. At engine cutoff the soil rebounds (is carried upward by escaping gas from the soil) resulting in a large, central core eruption. Since this is aimed directly at the rocket engine, material may enter the bell or the combustion chamber, which would make it dangerous to restart the engine. A hot spot due to a small amount of contamination in the engine may result in engine failure. Rebounding soil may also contaminate sensors or mechanisms.
Risk Statement: If excavation of the soil produces a crater of significant size, this may place the lander’s legs onto the craters’ sloped surfaces, resulting in spacecraft instability. Removal of loose overlying material may also reveal uneven bedrock features just beneath the surface, and the lander would be forced to settle onto an unexpectedly uneven surface. Excessive weakening of the subsurface material through thermal and pressure loading and/or fluidization by forced diffusion into the pores may also result in the landed spacecraft being unstable. Context: No study has ever been done to scale the cratering phenomena for a spacecraft landing in a planetary atmosphere where the spacecraft is the size of those currently being considered for human tended missions to Mars. An atmosphere partially collimates the jet, making the cratering much worse than experienced in Apollo. The larger Martian surface gravity also increases the effect on Mars relative to the Moon. Simple scaling estimations predict that large scale cratering may be possible, although further laboratory work is needed to determine whether the problem is severe. Experiments have shown that the presence of a boundary condition beneath the near limited region, and this results in significant fluidization of the overlying soil until engines have subsurface (e.g., bedrock or an impermeable cryosphere) constrains the diffusing gases into a been shut off. Early engine shutoff and freefall (as was done in Viking) to minimize soil interaction with the plume may be dangerous for a large lander.
To reduce these risks to an acceptable level, much progress can be made using investigations in terrestrial laboratories using martian simulants and in computers using models. However, in order for this experimental/theoretical work to be credible, the work MUST have a foundation of real data regarding the conditions on Mars. Terrestrial laboratory investigations with simulants are needed to characterize the mechanical properties of ice-rich soil, the effectiveness of ISRU techniques (excavation, processing, construction, etc.), volatilization of subsurface ices by surface heat loads, predicted differential settlement that may occur due to such volatilization, and plume/soil interactions plus mitigation techniques. Development of soil geotechnical models is also needed to extrapolate such terrestrial tests to Martian environmental conditions (gravity and atmospheric/pore pressure). Along with these terrestrial investigations, in situ measurements are needed to determine the subsurface boring properties concomitant with the location of subsurface water characterization (see Sec.3.6), and particle size distribution and properties analysis at a sampling of depths beneath the surface to inform physical modeling of the soil. The severity of the soil/rocket plume interaction has not been sufficiently investigated in terrestrial experiments and so it is unknown whether any measurements are needed at the specified landing site. At the present it is assumed that they will not be needed and so measurements will be made at any one soil deposit on Mars (not cohesionless sand dunes), and/or at any one area of polar deposits if the intended landing site will be on polar deposits. These measurements are needed to reasonably model and predict the cratering effects to aid the design of mitigation technologies. One such measurement requires specific explanation: the gas permeability of the soil or soil/ice mixture. To some degree, the fluidization dominates the extent of the cratering, and the fluidization depends on the gas permeability over a certain range determined by the scaling of the cratering physics. Permeability that is in the very low range should not be a concern because it will not admit sufficient gas over the time scale of a rocket landing to fluidize the soil. Nor is permeability in the very high range, which indicates the material is well vented and hence will not fluidize. The measurement is needed in the range where soil fluidization will be effected, and this is inferred to be in the 1 to 300 Darcy range by comparison with typical soils fluidized by impinging gas jets during terrestrial experiments.
Since risk is a combination of probability and consequence, all risks are not equal. If we were to compile a list of all of the risks faced by the first human mission to Mars, and sort it by risk magnitude, there would be a very long tail of lower priority risk issues. The purpose of this report is place some emphasis on the high priority risk issues that can be mitigated by means of a precursor program—it is outside our scope to provide full analysis for issues that have been judged to be low priority either because of a low perceived overall risk, or because the risk cannot be reduced using precursor data. Further analysis is deferred to successor study teams. Some of these issues are listed at the bottom of Table 1, including: Risk #16 Comm. losses & nav alterations caused by atmospheric and topographic conditions. Risk #17 Lander attitude is inadequate for egress or TAO. Risk #19 Risk from previously-ignored radiation sources. Risk #20 Seasonal condensation causes electrical failures. However, if data of relevance to these issues can be acquired without excessive incremental impact, it would clearly be beneficial to the overall program.
As dictated by the MHP SSG charter, this study focuses only on robotic precursor missions needed to achieve success on the first landed human mission to Mars (see Section 1.2). The analysis presented in this study does not consider robotic precursors that would be necessary for subsequent human missions and/or the development of a sustained human presence on Mars. However, the U.S. Vision for Space Exploration calls for “sustainable human and robotic missions to Mars and beyond” [NASA, 2004] and therefore multiple human missions to Mars are required to fulfill this Vision for Exploration. While the risks outlined in the Report, are, of course, also relevant to subsequent human missions or sustained human presence at Mars, additional risk factors are inherent in a series of human missions when compared to the first human mission to Mars. Many of these added risks are related to the extended exposure of humans and machines to the martian environment, as well as to the anticipated diversity of sites which could introduce variable risks, such as the potential increased biologic risks associated with possible contact with liquid water. These additive risks require increased understanding of environment and its impact on safety as well as the drive need for increased overall system reliability and lifetime for extended missions. Further, long-term human missions on Mars would necessarily require increased autonomy and increased reliance on the available resource at Mars to sustain human activity there. Therefore, additional robotic missions would be necessary to support a series of human missions and help mitigate these added risks. Finally, as the missions become more complicated we need to ensure that crew time for cleaning and maintenance is minimized to ensure that time is still left for performing the mission objectives. So, as missions mature, we need to test mission systems in situ before we need them to ensure they are reliable and easily maintainable. An assessment and prioritization of the risks associated with multiple human landings on Mars is beyond the scope of this study. A representative sample list of such risks associated with human mission scenarios supporting a long-term sustained presence on Mars that could be mitigated by robotic precursor missions is presented below in no priority order. This list is not intended to be an exhaustive statement that captures all the increased risks associated with longer-term human activity at Mars, rather it is offered as example topics which require additional detailed studies which could be used to frame requirements for future precursor missions needed to mitigate the anticipated increased risk factors. · Dust. Dust poses a significant risk to the human mission in terms of adhesion and accumulation, inhalation and ingestion, dust storms and biology (see Section 3.3). Due to increased durations for each human mission, dust would become an even greater problem. ISRU and other technologies may become more sophisticated and the increased mission durations would require more rigorous dust management techniques. More sophisticated dust control technologies should be developed and tested in situ. · ISRU. A long-term human occupation of Mars would require the use of in situ resources for life support, energy systems, and/or propellant production. In situ validation of new technologies and resource extraction may be required. · Habitats. Robust surface structures would be required for long-term human occupation, scientific studies, subsystem operations, storage, etc. The development and validation of construction techniques using local materials as well as in situ repair and fabrication (autonomous or human-assisted) may be required. In situ testing of subsystems (closed life support, power, thermal, communications, etc.) may also be needed. · Biological Studies. The adaptation of plant, animal, and microbial species over multiple generations should be investigated to understand the biological response at various levels to the martian environment. Also, the development and testing of greenhouses would provide a systems level test of a plant growth module and would test the response of biological life support components to the martian regolith and dust, radiation environment, and partial gravity. · Life Support. Several options exist in terms of life support systems. Bioregenerative life support systems can produce food and recycle air & water in a closed loop system. Physical/chemical systems are another option for providing life support which are commonly used in current spacecraft. ISRU is a third option which could be used to produce usable air and water for life support purposes. The development of a sustainable life support system imposes different requirements than for a one-time human mission. These life support options must be evaluated for a long-term human mission and technologies validated in situ. · Surface Power Generation. A cost-effective, long life surface power source is required to support a long-term human presence on Mars. Several options exist including nuclear, solar, isotopic, electrochemical, and chemical sources. Again, the development of a sustainable power source imposes different requirements than for a one-time human mission (particularly, the long-surface stay missions, as compared to short-stay surface missions). These options must be evaluated for a long-term human mission and technologies validated in situ. · Planetary Protection. The MHP SSG has identified back contamination as the highest risk associated with Planetary Protection (see Section 3.1). Martian lifeforms could be hazards to terrestrial biota and/or hazardous to astronaut crews during transit to Earth. Depending on the mission architecture, multiple human missions to and/or from Mars may provide additional opportunities for possible martian and terrestrial biota interactions which could increase the risk of adverse biologic interactions. · Biohazards. A sustained human presence would inevitably lead to human interaction with environments not previously contacted during the first landed mission. For example, a deep drilling operation or an expedition to the polar cap would introduce possibilities for forward and/or back contamination that would not be encountered on a simple expedition to the equatorial regions. Proposed MEPAG revisions require sample return and analyses in terrestrial laboratories for any site on Mars to be visited by humans. This requirement may need to be re-evaluated for applicability to the scenario of a sustained human occupation involving frequent investigations of new regions on Mars. · Communications and Navigation. Technology demonstrations may be required for enhanced communication and navigation capabilities for a long-term human mission. A sustained presence on Mars would likely result in longer-range sorties from the primary landing site and/or multiple human bases which necessitates accurate navigational capabilities as well as communication among EVA crews, Habitat crews, and Earth-based teams. Technology development and in situ validation may be required. · Long-Term Exposure. Materials and equipment on the martian surface would be exposed to the martian environment for longer durations to support a sustained human presence when compared to the first human mission to Mars. An exposure facility on the martian surface (similar to the Long Duration Exposure Facility (LDEF)) may be needed to test the effects of the ambient martian environment (radiation, dust, thermal, etc.) on various materials. · Increased EDL accuracy. As human exploration expands, there would likely be a build up of capability (posts, outposts, expedition bases, research laboratories, etc.) at selected sites of scientific interest, or where needed resources may be available. Therefore increased overall EDL systems performance to accurately land at these sites would be essential.
In addition to identifying the needed measurements, and their required accuracy and precision, the MHP SSG considered which measurements need to be made once for the entire planet, which need to be made at multiple locations, which need to collect data over an extended period of time, and which need to be made specifically at the human landing site. These relationships are summarized in Table 5. Table 5. Summary of Location Considerations for High-Priority Human Precursor Investigations.
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