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Development and Testing of in situ Materials for Human Exploration of Mars M-H Y Kim, S A Thibeault, J W Wilson, L C Simonsen, L Heilbronn, K Chang, R L Kiefer, J A Weakley and H G Maahs High Performance Polymers 2000; 12; 13 DOI: 10.1088/0954-0083/12/1/302 The online version of this article can be found at: http://hip.sagepub.com/cgi/content/abstract/12/1/13

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High Perform. Polym. 12 (2000) 13–26. Printed in the UK

PII: S0954-0083(00)10280-6

Development and testing of in situ materials for human exploration of Mars M-H Y Kim†, S A Thibeault‡, J W Wilson‡, L C Simonsen‡, L Heilbronn§, K Changk, R L Kiefer†, J A Weakley† and H G Maahs‡ † College of William and Mary, Williamsburg, VA 23187, USA ‡ NASA Langley Research Center, Hampton, VA 23681, USA § Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA k Christopher Newport University, Newport News, VA 23606, USA E-mail: [email protected] Received 6 October 1999, accepted for publication 13 December 1999 Abstract. Interplanetary space radiation poses a serious health hazard in long-term manned space missions. Natural Martian surface materials are evaluated for their potential use as radiation shields for manned Mars missions. The modified radiation fluences behind various kinds of Martian rocks and regolith are determined by solving the Boltzmann equation using NASA Langley’s HZETRN code along with the 1977 Solar Minimum galactic cosmic ray environmental model. To make structural shielding composite materials from constituents of the Martian atmosphere and from Martian regolith for Martian surface habitats, schemes for synthesizing polyimide from the Martian atmosphere and for processing Martian regolith/polyimide composites are proposed. Theoretical predictions of the shielding properties of these composites are computed to assess their shielding effectiveness. Adding high-performance polymer binders to Martian regolith to enhance the structural properties also enhances the shielding properties of these composites because of the added hydrogenous constituents. Laboratory testing of regolith simulant/polyimide composites is planned in order to validate this prediction and also to measure various structural properties.

1. Introduction NASA has renewed interest in a possible human mission to Mars at the injection opportunity in the year 2014 [1]. The Martian explorations under consideration take 130–180 days in transit each way and about 600 days on the surface of Mars. Space radiation has been identified as one of the major constraints; it is potentially life threatening for such long-term missions. Two sources of radiation that are especially hazardous for such missions are solar particle events (SPEs) and galactic cosmic radiation (GCR). Although SPEs are infrequent in occurrence, one is likely to occur during any Martian mission. Protecting the astronauts from these events, however, is relatively easy: the approach is to monitor for events and then to take shelter at the appropriate time. In contrast, the GCR is continuous. The highly energetic, heavy-ion flux of the GCR can cause deleterious effects over a long period of time. Humans on a mission to Mars will require more protection from the exposure to galactic cosmic radiation than that which has been used heretofore on human space missions [2]. In fact, one of the most important enabling technologies for human exploration of Mars is radiation protection from the GCR. The other environmental hazard on the Martian surface arises from the global dust storms. The wind speeds of these dust storms (measured at the Viking lander site) are 17–30 m s−1 . 13

© 2000 US Government


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A structural habitat is required to protect the astronauts from such dust storms. Therefore, a surface habitat that is an integrated structural and radiation shielding habitat is critical to providing a safe haven for human explorers on Mars. The primary GCR consists mostly of protons and alpha particles with a small, but significant, component of heavier particles. The GCR flux is modulated over the solar cycle according to changes in the interplanetary plasma, and the 1977 Solar Minimum GCR environmental model [3] represents a maximum in intensity of the GCR flux within several astronomical units (AU) of the sun. The transmitted GCR environment at the 1977 Solar Minimum behind various materials is calculated for the complex radiation environmental components as a function of shield composition and thickness. For eventual settlement on Mars, the in situ materials can be used as feedstock to produce habitat and shielding materials. Martian surface material is a convenient candidate to consider for bulk shielding and habitat in order to avoid excessive launch weight requirements from Earth. Natural Martian surface materials, such as Martian meteorites [4] and the Martian regolith model composition based on Viking lander data [5], are evaluated for their potential use as radiation shields for manned Mars missions. The radiation attenuation characteristics behind each material are assessed theoretically for dose equivalent using radiation quality factors [6] and for the response of certain biological systems [7]. The relative shield effectiveness of these materials is studied without including the effect of shielding due to the Martian atmosphere. The second significant source of Martian in situ material is the Martian atmosphere. In this paper, we are proposing to synthesize a high-performance structural polyimide from constituents of the Martian atmosphere in the presence of certain catalysts. This envisioned surface operation has the promise of minimizing the amount of material transported to the Martian surface. Because hydrogenous material is known to be a good shielding material [8], making a structural composite of Martian regolith with a polyimide, which contains hydrogen atoms, is considered. To fabricate model structural shielding composite materials, we are conducting processing studies of microcomposites of regolith simulant and polyimide on Earth. These fabricated targets will be tested in ground-based facilities for the validation of theoretical predictions and for measurements of material properties. The objectives of this work are to predict the theoretical shielding properties of the Martian surface materials and composites for their shielding effectiveness, and to identify synthetic routes that could possibly be employed to produce polyimides utilizing the primary constituent of the Martian atmosphere, carbon dioxide. Studies are under way to fabricate regolith simulant/polyimide targets for ground-based testing. In future studies, the actual processing details will be explored in greater depth and optimized for space applications. 2. Biological response behind Martian surface materials and composites from GCR exposure 2.1. Chemical analyses of Martian meteorites and Martian regolith For the purpose of identifying high-performance local shield materials, Martian meteorites and Martian regolith are considered to be representative of Martian rocks and the soil, respectively. Chemical analyses of 11 Martian meteorites are found from the Mars Meteorite Compendium [4]. These igneous rocks are grouped into five subgroups according to their own distinct chemical signatures. The first group of Basalt includes some Martian meteorites such as QUE94201, Shergotty, Zagami and EETA79001; Martian meteorites named LEW88516 and


In situ materials for human exploration of Mars

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Table 1. Atomic parameters for each group of Martian meteorites. Atomic density (atoms g−1 ) Atomic Atomic Element number, Z weight, A Basalt O Na Mg Al Si P K Ca Ti Mn Fe

8 11 12 13 14 15 19 20 22 25 26

× 1022

16 23 24 27 28 31 39 40 48 55 56

1.60 2.35 × 1020 1.61 × 1021 8.50 × 1020 5.01 × 1021 5.60 × 1019 1.23 × 1019 1.06 × 1021 8.21 × 1019 4.42 × 1019 1.58 × 1021

Lherzolite

Clinopyroxenite Orthopyroxenite Dunite

1.58 × 1022

1.54 × 1022 1.13 × 1020 1.79 × 1021 1.94 × 1020 4.88 × 1021 3.76 × 1018 2.93 × 1019 1.56 × 1021 2.59 × 1019 5.69 × 1019 1.79 × 1021

9.15 × 1019 4.07 × 1021 3.34 × 1020 4.50 × 1021 2.00 × 1019 3.31 × 1018 4.04 × 1020 3.28 × 1019 3.91 × 1019 1.67 × 1021

1.65 × 1022 2.66 × 1019 3.77 × 1021 1.45 × 1020 5.38 × 1021 — 1.95 × 1018 1.98 × 1020 1.53 × 1019 4.04 × 1019 1.46 × 1021

1.51 × 1022 2.51 × 1019 4.81 × 1021 8.23 × 1019 3.87 × 1021 6.41 × 1018 5.31 × 1018 6.52 × 1019 7.60 × 1018 4.51 × 1019 2.29 × 1021

Table 2. Atomic parameters of a representative sampling of Martian regolith.

Element

Atomic number, Z

Atomic weight, A

Atomic density (atoms g−1 )

O Mg Si Ca Fe

8 12 14 20 26

16 24 28 40 56

1.67 × 1022 1.62 × 1021 5.83 × 1021 7.81 × 1020 1.80 × 1021

ALHA77005 are in the second group called Lherzolite; Martian meteorites named Governador Valadares, Nahkla and Lafayette are in the next group called Clinopyroxenite; ALH84001 is in the Orthopyroxenite group; and Chassigny is in the Dunite group. The average weight percentage from chemical analyses is used for the examination of the attenuation characteristics of each petrographic group, which shows their own distinct variation of compositions. Composition data for the Martian meteorite subgroups for up to 11 of the most prevalent elements are given in table 1 as atomic parameters for the transport code. In assessing the radiation protection for future manned exploration and habitation of Mars, the Martian regolith model composition [5] has been used [9]. A representative sampling of Martian regolith composition based on the Viking lander data [5] was reported to have a density of 1.4 g cm−3 and to contain almost exclusively only five elements (in mol%): O (62.5), Si (21.77), Fe (6.73), Mg (6.06) and Ca (2.92). The data pertaining to the Martian regolith model for the five most prevalent elements are given in table 2. 2.2. GCR transport Galactic cosmic rays come from outside the solar system, a region that extends to about 160 AU. Although GCRs probably include every natural element (from hydrogen to uranium), and are fully ionized, not all are important for space radiation protection purposes. The abundances for species heavier than nickel (atomic number, Z > 28) are typically two to four orders of magnitude smaller than for the highly prevalent iron (Z = 26) [10]. Figure 1 shows the annual integral fluence spectra of GCR at 1 AU for various charge groups up to nickel using the 1977 Solar Minimum GCR environmental model [3] which represents a maximum in intensity of


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Figure 1. The annual integral fluence spectra of various charge groups for the 1977 Solar Minimum galactic cosmic rays.

the GCR from the relatively quiet solar cycle 21 (1975–1986). The GCR spectrum is broad with a peak at several hundred MeV amu−1 with appreciable numbers of particles out to 10–50 GeV amu−1 shown in figure 1. The primary mechanism for loss of energy by energetic particles is by means of coulombic interactions with electrons in the target, as indicated by atomic/molecular stopping cross sections (ionization). Additional energy is lost through coulombic interactions and collisions with target nuclei (projectile and target fragmentations). Although nuclear reactions are far less numerous, their effects are magnified because of the large momentum transferred to the nuclear particles and the impacted nucleus itself. Many secondary particles of nuclear reactions are sufficiently energetic to promote similar nuclear reactions and thus cause a build-up of secondary radiation. The propagation and interactions of high-energy ions up to atomic number 28 (Ni) in Martian meteorites and Martian regolith model are simulated by using the transport code HZETRN [11]. This code solves the fundamental Boltzmann transport equation and applies the straight-ahead approximation for nucleon, light ions and high-charge, high-energy (HZE) nuclei and with velocity-conserving fragmentation interactions for HZE nuclei colliding with shield materials. The interactions of the HZE nuclei have been carefully investigated because of their unusually high specific ionization and their enormous energy range [11]. The nuclear reactions of light particles (proton, neutron, 2 H, 3 H, 3 He and 4 He) have been added into this code because of their abundance in primary GCR and their build-up with increasing shield thickness due to longer ranges and greater multiplicities in inelastic events [12]. For the comparison of shield effectiveness of the materials on the Martian surface, the freespace fluences are used without modification of the spectra by the Martian carbon dioxide atmosphere.



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Table 3. Annual dose behind Martian rock groups and Martian regolith (cGy yr −1 ). Thickness (g cm−2 )

Basalt

Lherzolite

Clinopyroxenite

Orthopyroxenite

Dunite

Martian regolith

Free space 1 5 10 30 50

19.44 21.95 22.28 21.97 20.65 19.48

19.44 21.93 22.25 21.94 20.62 19.44

19.44 21.97 22.31 22.02 20.70 19.53

19.44 21.91 22.21 21.89 20.56 19.39

19.44 21.96 22.31 22.01 20.71 19.54

19.44 21.94 22.27 21.96 20.64 19.47

Table 4. Annual dose equivalent behind Martian rock groups and Martian regolith (cSv yr −1 ). Thickness (g cm−2 )

Basalt

Lherzolite

Clinopyroxenite

Orthopyroxenite

Dunite

Martian regolith

Free space 1 5 10 30 50

120.1 132.3 111.5 94.0 64.8 56.5

120.1 132.3 111.4 93.8 64.6 56.2

120.1 132.4 111.8 94.3 65.2 56.8

120.1 132.2 111.1 93.4 64.2 55.9

120.1 132.4 111.8 94.3 65.2 56.8

120.1 132.3 111.5 93.9 64.8 56.5

2.3. Biological risk assessments and their comparison for shield effectiveness from GCR exposure The level of biological injury from the transmitted GCR environment behind a shield material is assessed in terms of two biological models: a conventional dosimetry model [6] and a track structure repair kinetic model [7]. The conventional approach of extrapolating the human radiation risk database to high linear energy transfer (LET) exposures is introduced by the dose equivalent, H , given by H = QD

(1)

where Q is the LET-dependent quality factor defined by the International Commission on Radiobiological Protection (ICRP) [6] to represent trends of measured relative biological effectiveness (RBE) in cell culture, plant and animal experiments, and D is the absorbed dose due to energy deposition at a given location by all particles. The dose and dose equivalent as a function of slab thickness approximate the radiation risk behind the material and are shown in tables 3 and 4. As can be seen in table 4, the attenuation characteristics of the dose equivalent among Martian rock groups and Martian regolith vary by no more than 1% for each thickness. Therefore, Martian regolith is considered to be a reasonably accurate representation of typical Martian surface materials. Using Martian regolith has a great advantage over using Martian rock. For manufacturing structural blocks, the distribution of fragment size affects the production of a well consolidated, void-free composite. Rocks need to be processed first into fragments, while Martian regolith is merely gathered. The different rock fragmentation methods require different amounts of energy, referred to as the specific energy, to fragment a unit volume of rock. To make smaller fragment sizes to minimize voids requires higher specific energy [13]. Using hard rocks requires higher energy and provides no advantage in shielding effectiveness. Therefore, Martian regolith is chosen as the in situ Martian habitat/shielding construction material for further analysis.


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M-H Y Kim et al Table 5. The biological responses behind Martian regolith and aluminium shieldings after one year GCR exposure. Thickness (g cm−2 ) Martian regolith Free space 1 5 10 30 50 Aluminium Free space 1 5 10 30 50

C3H10T1/2 cell death rate

C3H10T1/2 cell transformation rate

Excess Harderian gland tumour prevalence (%)

3.18 × 10−2 3.92 × 10−2 3.28 × 10−2 2.74 × 10−2 1.89 × 10−2 1.65 × 10−2

1.13 × 10−5 1.74 × 10−5 1.65 × 10−5 1.54 × 10−5 1.34 × 10−5 1.29 × 10−5

2.23 3.50 3.28 3.02 2.63 2.56

3.18 × 10−2 3.94 × 10−2 3.33 × 10−2 2.80 × 10−2 1.91 × 10−2 1.65 × 10−2

1.13 × 10−5 1.76 × 10−5 1.70 × 10−5 1.59 × 10−5 1.39 × 10−5 1.33 × 10−5

2.23 3.57 3.37 3.12 2.73 2.63

The second model is a track structure repair kinetic model [6] for several biological systems for which there is a large body of experimental data with various ions and in which repair kinetic studies were made. Using the track structure model [6], the biological responses of the several biological systems including cell death and neoplastic transformation in C3H10T1/2 mouse cells, and Harderian gland tumour induction in mice were calculated. These are shown in table 5 for the Martian regolith (as the in situ Martian construction material) and aluminium (as the standard construction material). The performance of these materials is virtually the same. A modest reduction in cell death rate is found behind both Martian regolith and aluminium at a thickness of 10 g cm−2 or more, while the transformation rate and tumour prevalence are noticeably increased relative to free space, but which decrease as thickness is increased. The reason for this is because each particle type transmitted through the shield and the increased number of particles due to fragmentations result in more biological injury according to its specific biological effect. Because Martian regolith is the material on the Martian surface, and its shielding properties are equivalent to those of aluminium, no advantage would be gained by transporting aluminium from Earth. Although the absolute human risk is not known, the results shown in tables 4 and 5 do provide some indication of the response of living tissue behind Martian regolith. However, the effectiveness of regolith can be enhanced by employing a polymeric binder. The effects of adding a hydrogen-containing polymer (here, polyimide) to the regolith are examined by varying its weight fractions from 10%, 20%, 30%, to 40%. Polyimide was selected for this investigation because of its high hydrogen content as well as its structural properties. The results are shown in figure 2 for the two different biological models for thicknesses up to 50 g cm−2 . Adding polyimide to Martian regolith to bind it into a composite enhances its shielding properties for GCR; the thicker the shield, the better are its shielding characteristics. Figure 2 illustrates that increasing the concentration of lighter atoms is effective for developing shield materials against GCR. A material with a high hydrogen density, here a composite of Martian regolith with 40 wt% polyimide, provides the most effective shielding at all thicknesses because of its greater efficiency in attenuating the heavier GCR ions and fragments that are most destructive to living tissue [14]. In these figures, the responses behind aluminium, Martian regolith, and pure polyimide are shown for comparison.



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Figure 2. Biological response behind various materials after one year GCR exposure: (a) dose equivalent and (b) excess Harderian gland tumour prevalence. (, aluminium; , Martian regolith; N, Martian regolith/LaRC-SI composite of 90/10 wt%; ×, 80/20 wt% composite; , 70/30 wt% composite; , 60/40 wt% composite; and +, LaRC-SI.)

•

In addition to the increased radiation shielding capability, incorporating the polyimide into unconsolidated Martian regolith for manufacturing structural blocks also affords other advantages. Composites provide more durable structures with significantly less material and more versatility in design and utility of structures than Martian regolith. The benefits of a regolith/polyimide composite must be folded into the context of a complete design reference mission scenario which takes into account total mass of shielding, additives, processing equipment, extra vehicular activity (EVA) time, etc, relative to crew exposure risks [15].


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3. Schemes for synthesizing polyimide from the Martian atmosphere 3.1. Background As previously shown in figure 2, making composites of Martian regolith with polyimide is effective for making radiation shielding and habitat against GCR. To make composites, polyimide can be produced from the Martian atmosphere. Many chemical process technologies are available for a Mars mission using indigenous space resource utilization (ISRU) to produce a propellant combination (methane and oxygen) as a means of reducing launch mass. Common hardware and energy sources of nuclear and solar types will be also available at the landing site [1]. The Sabatier reaction, discovered by the French chemist Sabatier in the 19th century, is one of the more discussed processes that could be used on Mars. In this reaction, carbon dioxide, which is 95 vol% of in the Martian atmosphere, is converted into methane (CH4 ) and water by reacting it with imported hydrogen from Earth at elevated temperatures in reaction (2) [16]: CO2 + 4H2 → CH4 + 2H2 O 2H2 O → 2H2 + O2 CO2 + 2H2 → CH4 + O2

(2) (3) (4)

where the water can be used for a crewed mission. In the water electrolysis reaction (3), the water can be electrolyzed to release oxygen for a variety of uses on Mars and the hydrogen can be recovered for recycling back into reaction (2). The sum of these reactions in reaction (4) converts CO2 and H2 into CH4 and O2 for a propellant manufacture for the return of vehicles. In order to burn methane at the optimum mass ratio of 3.5:1 for oxygen and methane, additional O2 is required. This also produces additional methane. In the simplest implementation, it could be vented. This vented methane can be collected for use in synthesizing polyimides. 3.2. One conceptual idea for synthesizing polyimide from Martian atmosphere The rigid, high melting point, thermally stable polyimides can be synthesized from dianhydride and diamine monomers; possible schemes for producing these monomers starting from CH4 are shown in figures 3–5. Benzene can be synthesized from one of the following paths.

Figure 3. Synthesis of benzene.



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Figure 4. Synthesis of monomers, required as precursors.

Methane can be converted to acetylene and hydrogen; the acetylene is cyclized to benzene as shown in equation (5) in figure 3. Methane can also be halogenated via photochemical reactions. The number of carbon atoms can be further increased by employing many known chemical reactions including the Grignard reaction to synthesize n-propyl chloride and propyl magnesium chloride, the starting reagents in equation (6) in figure 3. Such photochemical reactions are appealing because they offer the potential to minimize energy consumption; but, on the other hand, methods to recover the halogens in a useful form would be needed in order


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Figure 5. Process of durene synthesis from mesitylene and pseudocumene produced by transalkylation of toluene, xylene and trimethyl benzenes.

to minimize the consumption of important chemical reactants. Considerations such as these apply not only here, but also for all of the other reaction schemes discussed in this paper, and these factors need thorough consideration. The diamine and dianhydride monomers can be derived from benzene as shown in figure 4. The diamine monomer is derived from benzene through the steps shown in equation (7) and the dianhydride monomer can be derived from benzene as illustrated in equation (8). The overall reaction sequence of durene synthesis is shown in equation (9) in figure 5. Mesitylene, pseudocumene and durene are obtained by trans-alkylation of toluene, xylene and trimethyl benzenes. Methylating agents include formaldehyde, methanol and methylchloride in the presence of a catalyst of the aluminium chloride type. The process of methylation is carried out under rather severe conditions (350–400 ◦ C, 10–20 atm) and requires increased hydrogen consumption. By using a stable catalyst obtained from zeolite, a waste- and sewage-free process has recently been proposed by others. Durene is used for obtaining the pyromellitic dianhydride (PMDA) monomer in equation (8) for heat-resistant polymers. A series of polyimides can be synthesized by condensation reactions of dianhydride monomers with diamine monomers. Various polyimides can be synthesized from modifications of the polymer backbone through monomer selection to increase some of the criteria required for high-performance materials. Such polymers have distinct processing advantages as well as distinct mechanical and physical properties [17–22].



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4. Fabrication of targets for laboratory testing 4.1. Matrix resin and filler for microcomposites Langley Research Center soluble imide (LaRC-SI), which is the reaction product of 3,40 oxydianiline (ODA) with biphenyltetracarboxylic dianhydride (BPDA) and oxydiphthalic anhydride (OPDA) terminated with phthalic anhydride (PA), is considered as a possible binder for the Martian regolith because of its excellent adhesive property and melt processing capability for advanced composites [17–20]. Furthermore, its powder form facilitates safe handling and easy processing under the near-vacuum conditions on the Martian surface. Its atomic hydrogen content is rather high per unit mass (∼30% in atomic number density) making it a potentially good shield material [14]. Low molecular weight LaRC-SI (MW ∼9000 g mol−1 , 5 mol% offset) can be purchased from Imitec Incorporated, as a matrix for microcomposites. Its glass transition temperature is about 250 ◦ C. Simulated regolith, which resembles the surface materials from the Apollo 11 site on the Moon and is comparable with the Martian regolith in terms of shielding properties, is made by crushing, grinding and sieving 1.1 billion-year old basaltic rock from Minnesota. This simulant is purchased from the Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN, as a filler for the microcomposites. 4.2. Processing of microcomposite samples To establish the optimal fabrication procedure, which is intended to be used on the Martian surface to make building blocks, various microcomposites sample strips of a 8.8 cm × 1.9 cm cross section and a mass per unit area of 0.33 g cm−2 are processed. Measured quantities of the dry LaRC-SI powder and regolith simulant powder are combined and thoroughly mixed by hand at room temperature. The varying weight fractions of LaRC-SI powder are 100– 10 wt% in 10 wt% intervals. Then, the mixture at a temperature of 71–77 ◦ C is de-aerated under vacuum. The mixture is then carefully transferred to a tooled mould pretreated with FrekoteTM release agent and Kapton film, and cured according to the following cure profile. It is heated at the rate of 1–3 ◦ C min−1 to 250 ◦ C, which is the softening point (Tg ) of LaRC-SI, and held for 1 h, which is necessary to remove most of the volatile material. Then, it is heated at the same rate to 330 ◦ C under a pressure of 2.07 MPa (300 psi) to obtain the maximum benefit of the melt fluidity. Finally, it is held at 330 ◦ C for 1 h under the same pressure, based upon the crystallization behaviour of LaRC-SI, whose melting endotherm, Tm , is 310–330 ◦ C [21], to ensure the complete melting of crystalline regions of LaRC-SI and the compaction of the mixture. The ingot is ejected from the mould when the temperature has decreased to 150 ◦ C to avoid cracks due to the different expansion coefficients of the mould and the composites. The final composite ingots are well consolidated with no voids until the amount of LaRCSI is decreased below 15 wt%. The measured specific gravities (densities) of composites are 2.5, 2.28 and 2.07 g cm−3 for the 20, 30 and 40 wt% LaRC-S concentrations, respectively. LaRC-SI possesses adequate fluidity for a relatively low molecular weight (9000 g mol−1 ) and the cured material becomes insoluble in a typical solvent (N-methyl pyrrolidinone), yet retains its melt processing capability above Tg [22]. These samples will be analysed for various structural properties. In this short and easy hot-pressing microcomposite processing, the pressure will be a variable for producing different properties of the cured composites with different thickness. Miniature block samples of a 3.2 cm × 3.2 cm cross section and a mass per unit area of 3.32 g cm−2 are processed with the same cure profile as previous samples except for an increased pressure of 2.66 MPa (385 psi). The varying weight fractions of LaRC-SI powder


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M-H Y Kim et al

for these samples are 40, 30, 20 and 10 wt%. The minimum required pressure without major property degradation for making building blocks will be examined. Because the curing cycle is also expected to be a variable for different properties, its effect would be also examined. Various targets of 15 cm × 15 cm cross section with a thickness of 5 g cm−2 will be fabricated according to the optimized curing procedure for heavy-ion beam exposures to validate the current shielding prediction and for various laboratory testing to measure structural properties. The calculated bulk densities of the regolith/polyimide casts are 1.398, 1.396, 1.394 and 1.392 g cm−3 , for the 10, 20, 30 and 40 wt% polyimide concentrations, respectively. 4.3. Ground-based testing The radiation transport properties of the materials described above will be measured in ground-based, heavy-ion accelerators that provide GCR-like beams. In an ongoing NASA space radiation research programme, in collaboration with the Lawrence Berkeley National Laboratory, 1.05 A GeV and 580 A MeV 56 Fe beams produced at the Brookhaven National Laboratory (BNL) Alternating Gradient Synchrotron (AGS) facility are being used to measure the properties of the secondary radiation field behind various materials. The beam energies of 1.05 A GeV and 580 A MeV are near the peak of the solar-modulated GCR 56 Fe energy spectrum and, as such, are a reasonable choice for the simulation of HZE GCR. Other ions will be tested in the near future. Regolith simulant/LaRC-SI composites will be exposed to 56 Fe beams at BNL in order to acquire data that will be used for the validation of their predicted radiation transport properties. These targets will be also tested for other mechanical and thermal properties and neutron absorbing abilities at the NASA Langley Research Center. 5. Discussion and concluding remarks The preceding sections have discussed, in a conceptual manner, selected key issues relating to the protection of humans from the hazards of ionizing radiation while on the Martian surface. Numerous issues remain to be resolved. Key among these is how the several existing biological response models relate to human tissue damage and cancer induction. Without such fundamental information, it is not possible to design, with any degree of confidence [23], radiation shields that provide adequate protection to humans and, at the same time, are not overdesigned and excessively heavy so that launch weight is large and the mission is unnecessarily penalized in terms of launch vehicle size and fuel requirements. A clear illustration of the existing biological uncertainty [24] is seen in figure 2: if a shield were designed on the basis of dose equivalence, one would conclude that significant reduction in biological damage can be obtained by increasing the thicknesses of the aluminium, regolith or polyimide with the polyimide clearly providing the greatest protection. On the other hand, if a shield were designed on the basis of excess Harderian gland tumour prevalence, one would have to conclude that any thickness of these same materials (out to at least 50 g cm −2 ) would not only be ineffective, but their use would actually enhance tumour prevalence. Hence, it is imperative that studies are conducted to resolve these biological uncertainties and that data are developed that are relevant to the particular biological system that is to be protected (namely, humans), and for the proper radiation environment (namely, GCR). From the preceding sections, it is also clear that Martian regolith, although abundantly available on the Martian surface, is not an ideal shielding material. Namely, it does not pack well and its shielding effectiveness is virtually equivalent to that of aluminium—not a particularly good shield material. It is clear that hydrogen-containing materials, such as organic polymers, are considerably more effective, but polymers or their precursors are not available



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on the Martian surface, and efficient and effective schemes need to be developed to transport them (or their precursors) to Mars and process them on the surface. Several conceptual reaction schemes have been proposed. Certainly, there may be others. All of these schemes must be considered in detail regarding individual benefits and drawbacks. Specifically, various reagents and catalysts will have to be transported to the surface, and these consume spacecraft volume and add to the launch weight. Furthermore, processing on the Martian surface poses special problems concerning equipment and power needs. The necessary processing equipment must also be transported to the surface, and an adequate source of power must be available to achieve the temperatures and pressures associated with the various reaction schemes. These must be examined in depth on a case-by-case basis, and there is no simple answer. For example, one reaction scheme may require transporting the smallest amount of reagents and catalysts to the surface, but it could, on the other hand, require the heaviest and most complex equipment and the greatest power consumption. Toxicity, hazard and containment issues are also problems, since these materials will have to be processed by astronauts in an extremely hostile environment. Finally, much of what can be accomplished, and what will eventually be practical, depends on specific mission parameters, namely, mission objectives on the surface, surface mobility requirements, mission duration and other factors. Hence, these trade-offs are not simple and considerable work remains to be done to sort out all of the factors that will impact in reaching the final optimum solution. In spite of the foregoing considerable uncertainties, however, to get a start on the problem, the initial studies that are to be conducted in ground-based facilities are being planned. Regolith simulant/polyimide composite targets are being fabricated and will be tested in order to validate the shielding predictions in this paper. By this approach, it will be possible to verify the transport codes used to predict the transport of radiation through these materials, even though the particular biological implications still remain uncertain. However, by closely cooperating with biologists in their quest to develop adequate biological response and risk models appropriate to humans, it is hoped that this very complex problem can be solved in a timely manner to enable a human expedition to Mars in the early 21st century. In addition to measuring the transport properties of the composite materials, the physical and mechanical properties will also be characterized; this is important information because it is desirable to utilize these materials not only for radiation shielding, but also as structural components. Acknowledgment We would like to acknowledge Dr Gary Hammer at Christopher Newport University for providing the synthetic steps of the monomers. References [1] Hoffman S J and Kaplan D I (eds) 1997 Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Study Team NASA-SP 6107 [2] National Council on Radiation Protection and Measurements 1989 Guidance on radiation received in space activities NCRP Report No 98 [3] Badhwar G D, Cucinotta F A and O’Neill P M 1993 Depth–dose equivalent relationship for cosmic rays at various solar minima Radiat. Res. 134 9–15 [4] Meyer C (compiler) 1996 Mars Meteorite Compendium—1996 NASA JSC No 27672 [5] Smith R E and West G S (compilers) 1983 Space and Planetary Environment Criteria Guidelines for Use in Space Vehicle Development, 1982 Revision vol 1, NASA TM-82478 [6] International Commission for Radiological Protection 1991 1990 Recommendations of the International Commission on Radiobiological Protection (ICRP Publication 60) (Oxford: Pergamon)


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[7] Wilson J W et al 1993 Biological Effects and Physics of Solar and Galactic Cosmic Radiation ed C E Swenberg, G Horneck and E G Stassinopoulos (New York: Plenum) part B, pp 295–338 [8] Kim M Y et al 1994 Performance Study of Galactic Cosmic Ray Shield Materials NASA-TP 3473 [9] Simonsen L C, Nealy J E, Townsend L W and Wilson J W 1990 Radiation Exposure for Manned Mars Surface Missions NASA-TP 2979 [10] Adams J H Jr, Silberberg R and Tsao C H 1981 NRL memo US Navy Report 4506-Pt. I [11] Wilson J W et al 1995 HZETRN: Description of a Free-Space Ion and Nucleon Transport and Shielding Computer Program NASA-TP 3495 [12] Cucinotta F A, Townsend L W, Wilson J W, Shinn J L, Badhwar G D and Dubey R R 1996 Adv. Space Res. 17 77–86 [13] Gertsch L and Gertsch R 1997 Shielding Strategies for Human Space Exploration ed J W Wilson, J Miller, A Konradi and F A Cucinotta NASA-CP 3360, pp 327–64 [14] Schimmerling W et al 1996 Adv. Space Res. 17 31 [15] Simonsen L C, Schimmerling W, Wilson J W and Thibeault S A 1997 Construction Technologies for Lunar Base: Prefabricated Versus In Situ Materials for Lunar Base Space Radiation Shielding ed J W Wilson, J Miller, A Konradi and F A Cucinotta NASA-CP 3360, pp 297–326 [16] Sullivan T D et al 1995 J. Propulsion Power 11 1056 [17] Bryant R G 1994 US Patent Specification LAR 15205-1 [18] Soichi E I et al 1995 SAMPE Int. Symp. 40 11 [19] Bryant R G 1996 Proc. 19th Annual Meeting of the Adhesion Society p 36 [20] Bryant R G and Buchman A 1997 Proc. 20th Annual ‘Anniversary’ Meeting of the Adhesion Society vol 20, p 335 [21] Bryant R G 1994 Polym. Prep. 35 517 [22] Angelovici M M et al 1998 Mater. Lett. 36 254 [23] National Academy of Science 1996 Radiation Hazards to Crews on Interplanetary Missions (NAS Press) [24] Wilson J W et al 1995 Health Phys. 68 50
