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Estimates of Carrington-class solar particle event radiation exposures as a function of altitude in the atmosphere of Mars☆
Acta Astronautica 89 (2013) 189–194
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Estimates of Carrington-class solar particle event radiation exposures as a function of altitude in the atmosphere of Mars$ L.W. Townsend n, J.A. Anderson, A.M. Adamczyk, C.M. Werneth The University of Tennessee, Knoxville, TN 37996-2300, USA
a r t i c l e i n f o
abstract
Article history: Received 11 October 2012 Received in revised form 20 March 2013 Accepted 10 April 2013 Available online 18 April 2013
Radiation exposure estimates for crew members on the surface of Mars may vary widely because of the large variations in terrain altitude. The maximum altitude difference between the highest (top of Olympus Mons) and the lowest (bottom of the Hellas impact basin) points on Mars is about 32 km. In this work estimates of radiation exposures as a function of altitude, from the Hellas impact basin to Olympus Mons, are made for a solar particle event proton radiation environment comparable to the Carrington event of 1859. We assume that the proton energy distribution for this Carrington-type event is similar to that of the Band Function fit of the February 1956 event. In this work we use the HZETRN 2010 radiation transport code, originally developed at NASA Langley Research Center, and the Computerized Anatomical Male and Female human geometry models to estimate exposures for aluminum shield areal densities similar to those provided by a spacesuit, surface lander, and permanent habitat as a function of altitude in the Mars atmosphere. Comparisons of the predicted organ exposures with current NASA Permissible Exposure Limits (PELs) are made. & 2013 IAA. Published by Elsevier Ltd. All rights reserved.
Keywords: Space radiation exposures Mars atmosphere shielding
1. Introduction Human space exploration in the future may include human crews traveling to Mars for extended stays. A significant concern in carrying out such a mission is the possibility that the crew may be exposed to a large solar particle event (SPE)—one of such magnitude that crew survival or completion of the mission is at risk. Exposures of the crews to these potentially harmful events are possible during the transits to and from Mars as well as during surface operations on Mars. The highest exposures from these events are likely to occur during the transits since the only shielding available is intrinsic to
$ This paper was presented during 62nd IAC in CapeTown. Corresponding author. Tel.: +18659747569; fax: +18659740668. E-mail addresses: [email protected] (L.W. Townsend), [email protected] (J.A. Anderson), [email protected] (A.M. Adamczyk), [email protected] (C.M. Werneth). n
the spacecraft itself. On the surface of Mars, shielding is provided by the planet's bulk, by the overlying CO2 atmosphere, and by the crew's space suits, surface landers, or habitats. Hence, exposures will likely be lower than in deep space [1,2]. Our recent studies [2–4] focused on estimating radiation exposures on the Martian surface near the mean datum using both low-density and high-density atmospheric models, for a variety of galactic cosmic ray (GCR) and solar particle event scenarios. GCR exposures were the focus of one study [2]. In that study, exposures of female crew members were estimated for the most recent solar minimum environment, possibly the most intense of the space age. Females were studied because their smaller stature, and therefore smaller body self-shielding, results in higher exposures than males for the same environment. The results of that study showed that crews on the Mars surface at the mean surface elevation are unlikely to receive exposures from GCR particles that exceed NASA
0094-5765/$ - see front matter & 2013 IAA. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actaastro.2013.04.010
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Permissible Exposure Limits (PELs) [5]. Recent studies have also been carried out by other groups [6–8] for both GCR and SPE exposures. SPE exposure calculations involving recent events, presented in Ref. [6], are significantly lower than those presented here because of their smaller fluences and softer energy spectra. The GCR estimates on the surface, from Ref. [8], are similar to those reported in Ref. [2]. In our second study [3] female crew member exposures on the Martian surface were estimated using Weibull distribution parameterizations of the proton spectra for the August 1972, September 1989 and October 1989 events. That study found that exposures from those events, which are among the largest that have occurred in the past four decades, would not have exceeded the NASA PELs for operations at the mean surface elevation of the planet. However, ice core data from Earth's polar regions [9] and excessive 14C production data from tree rings [10] indicates that much larger events may have occurred over the past ∼1200 years. Due to the possibility of larger events than those observed during the recent spaceflight era, additional analyses [11–13] of the SPE exposures on the surface of Mars were performed using Weibull proton distributions for the September 1989 and November 1960 spectra and Band Function proton distributions for the February 1956, November 1960 and September 1989 spectra. All spectra were normalized to the greater than 30 MeV omnidirectional proton fluence level of 18.8 109 cm−2 estimated from analyses of ice core data [7] of the September 1859 Carrington event, possibly the largest event within the past 500 years. While there is some question as to the validity of the proton fluence estimates from these ice core analyses, we note that they are comparable to the fluence estimate of ∼4.5 1010 cm−2 protons with energies greater than 30 MeV obtained from analyses of 14C production data from tree rings [10]. It is also noteworthy that arguments are made in Ref. [10] that the AD775 tree ring data analyses suggest a spectrum for that event as hard as the February 1956 SPE. Hence, the assumed spectrum for this study appears to be reasonable as a candidate worst-case event. Results of earlier studies of exposures to crews and electronics from possible Carrington-type events in deep space indicated that the doses could be very large and potentially lethal to human crews unless significant shielding was provided [11–13]. In the most recent study [4], organ doses and effective doses for male and female astronauts on the surface of Mars and at an elevation of 8 km above the mean surface elevation were carried out. The highest organ doses, which exceeded the PELs even for the assumed habitat shielding of 40 g cm−2 of aluminum, were obtained for the February 1956 energy spectrum and normalized to the greater than 30 MeV proton fluence level for the possible Carrington event. In the current work, we have extended these results to map the effective dose and organ doses for this event as a function of altitude on Mars, ranging from the height of Olympus Mons (assumed to be 25 km) to the depth of the Hellas impact basin (7 km below the mean surface elevation). We again assume aluminum shielding comparable to that
provided by a spacesuit, surface lander, and surface habitat. The next section outlines the assumed scenarios. This is followed by a description of the computational methods used to obtain the effective doses and organ doses of the male and female crew members. Next, the results of the exposure calculations are presented. Finally, the work is summarized and concluding remarks presented.
2. Surface scenarios As was done in our recent work [4], we assume that the crew member (male or female) is located at ground level on the surface of Mars in the center of an aluminum hemispherical structure. This simple geometry was selected in order to quickly obtain some knowledge of the effectiveness of nominal shield thicknesses comparable to those of more realistic geometries. Also, as was assumed for our previous Mars surface studies [2–4], three areal densities for the aluminum hemisphere are used corresponding to a spacesuit (0.3 g cm−2), surface lander (5 g cm−2), and a surface habitat (40 g cm−2). These hemispheres are placed at various elevations ranging from 25 km above the mean surface elevation to −7 km below it. The Mars atmosphere is assumed to be composed of pure CO2. Unlike some earlier studies of Mars surface doses, where both low-density (16 g cm−2) and highdensity (22 g cm−2) models [12] were used, this work uses the NASA Mars Atmosphere Model based upon data from the 1996 Mars Global Surveyor mission [14]. Since the incoming SPE radiation is isotropic, the atmosphere path lengths traveled by the incident particles are longer for those particles arriving at angles greater than zero, with respect to the local zenith. This is accounted for by averaging the exposures in one degree increments for arrival angles between the zenith and the horizon. As the arrival angle approaches the horizon, the areal densities increase dramatically, especially for altitudes deep in the atmosphere. At atmosphere areal densities exceeding 300 g cm−2, contributions to the organ exposures are trivial and can be ignored. The effects of the surface curvature are included in the atmosphere path lengths. Table 1 displays areal densities above the local zenith versus altitude relative to the mean surface elevation obtained from the model in Ref. [14]. Table 1 Mars atmosphere areal density (g cm−2) at the local zenith as a function of altitude (km) relative to the mean surface elevation. Temperatures at each elevation are also listed. Altitude above mean elevation (km)
Temperature (1C)
Areal density (g cm−2)
25 20 15 10 7 0 −4 −7
−79 −68 −57 −46 −38 −31 −27 −24
2.2 3.3 4.8 7.2 9.2 16.7 23.5 30.5
L.W. Townsend et al. / Acta Astronautica 89 (2013) 189–194
3. Computational methods Since the Band function utilizes high energy data and an actual fit at high energies obtained from ground level enhancements (GLEs) measured by neutron monitors on Earth's surface, it should yield more reliable dose estimates than other commonly used parameterizations [15,16]. The Band function parameterization of the February 1956 event proton distribution [16] is given by 8 > ΛR−γ 1 exp − RR0 ; > > > > < for R≤ðγ 2 −γ 1 ÞR0 Φð 4 RÞ ¼ > ΛR−γ 2 ½ðγ −γ ÞR ðγ 2 −γ 1 Þ expðγ −γ Þ; > > 2 1 0 2 1 > > : for R≥ðγ −γ ÞR 2
1
191
effective dose and Linear Energy Transfer (LET) distributions. For this work we calculate effective dose and organ doses for the three aluminum shield configurations as a function of altitude. Organ doses (D) are in units of centiGray (cGy) where 1cGy¼1 rad and 100cGy¼Gy¼1 J kg−1. Organ dose equivalents (H), which are the product of dose with a quality factor, Q (H¼QD), are in centiSievert (cSv) where 1 cSv ¼1 rem and 100 cSv¼1 Sv¼1 J kg−1. The units of effective dose (E) are also cSv, where the effective dose is obtained using ð2Þ
E ¼ ∑ wT H T ð1Þ
0
where Φ(4R) is the proton fluence, Λ¼1.78 1010 protons cm−2 is the normalization constant re-normalized to the Carrington event fluence, R is the particle rigidity (momentum per unit charge) in units of GV (gigavolts), R0 ¼0.321 GV is the characteristic rigidity, and γ1 ¼0.584 and γ2 ¼5.04 are spectral indices. For clarity and comparison purposes, the assumed spectrum is plotted in Fig. 1 along with the Band function fit to the February 1956 event. This incident SPE proton spectrum is transported in succession through the Mars atmosphere (up to 300 g cm−2 CO2), the appropriate hemispherical aluminum shielding thickness (0.3, 5 or 40 g cm−2), and then through the body self-shielding for the CAM (Computerized Anatomical Man) and CAF (Computerized Anatomical Female) human geometry models using the On Line Tool for the Assessment of Radiation In Space (OLTARIS) developed at NASA Langley Research Center [17]. OLTARIS is web-based [18] and uses the latest version of the NASA HZETRN (High Z and Energy TRaNsport) space radiation transport code (HZETRN 2010) [17]. The HZETRN code transports the incident charged ions (protons in this case) and their nuclear reaction secondary particles (protons, neutrons, deuterons, tritons, 3He and alphas) generated by nuclear collisions. The code includes stopping powers to account for energy loss due to excitation and ionization of the medium, resulting from collisions with the orbital electrons of the atoms and molecules in the target media. The code outputs, which are selectable by the user, include particle fluences, dose, dose equivalent,
T
where HT is the organ dose equivalent for the organ or tissue denoted by T (e.g., skin, bone marrow, etc.) and the tissue weighting factors wT are the proportionate detriment of the organ when the whole body is irradiated. These are taken from Table 5.1 of Ref. [19]. 4. Results 4.1. Organ doses Table 2 lists limits for short-term or non-cancer effects as given in the NASA Permissible Exposure Limits (PELs) [5]. One of the major concerns with SPEs is preventing short term effects, such as acute radiation syndrome effects. Hence, the organ doses will be compared with the 30 days limits. These limits are expressed in units of centiGray-Equivalent, which are obtained from the absorbed dose (D) using D ðcGy−EqÞ ¼ D ðcGyÞ RBE
ð3Þ
where the RBE (Relative Biological Effectiveness) is a multiplicative factor that accounts for the ability of some types of radiations to produce more biological damage than others for the same absorbed dose. For SPE protons an RBE value of 1.5 is used [20]. Organ doses for the skin, eye lens, blood forming organs (BFO) represented by bone marrow, the central nervous system (brain), and heart are presented in Table 3. The dose values in the table are in units of cGy-equivalent, obtained by applying the multiplicative proton RBE factor Table 2 Permissible exposure limits for short-term or career non-cancer effects [5].
Fig. 1. Solar proton integral fluences for the actual February 1956 solar particle event and renormalized to the Carrington event fluence level.
Organ
30 Day limit (cGy-Eq)
1 Year limit (cGy-Eq)
Career (cGy-Eq)
Lensn Skin BFO Heartnn CNSnnn CNSnnn (Z≥10)
100 150 25 25 50 –
200 300 50 50 100 10
400 400 NA 100 150 25
n Lens limits are intended to prevent early ( o 5 yr) severe cataracts (e.g., from a solar particle event). nn Heart doses calculated as average over heart muscle and adjacent arteries. nnn CNS (central nervous system) limits should be calculated at the hippocampus.
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Table 3 Calculated organ doses for female (CAF) and male (CAM) astronauts in units of cGy-equivalent as a function of altitude above (positive) or below (negative) the mean surface elevation on Mars. Entries in bold type exceed the 30 d Permissible Exposure Limit guidelines. All doses are rounded to the nearest integer values. Elevation (km)
Skin dose (cGy-Eq)
Eye dose (cGy-Eq)
BFO dose (cGy-Eq)
Heart dose (cGy-Eq)
CNS dose (cGy-Eq)
CAM
CAF
CAM
CAF
CAM
CAF
CAM
CAF
CAM
CAF
568 418 311 222 179 100 69 51
457 360 281 208 170 98 68 51
464 365 284 210 172 99 69 52
283 237 196 153 129 79 57 43
293 245 202 157 132 81 58 44
244 209 176 140 120 75 55 42
251 215 180 144 122 75 56 42
304 256 212 166 140 85 61 46
319 267 220 172 144 88 63 47
360 293 235 178 147 87 61 46
326 271 221 171 143 86 61 46
331 275 224 173 145 87 62 46
227 195 164 132 113 71 52 40
234 200 169 135 115 73 53 40
204 177 151 122 105 69 50 38
209 181 155 125 108 69 51 39
246 212 178 143 122 77 56 42
256 219 184 147 125 78 57 43
105 95 86 71 63 43 33 27
105 94 84 71 63 44 34 27
106 95 84 72 64 44 34 28
88 80 71 61 54 39 30 24
90 81 72 62 55 39 31 25
84 76 68 58 52 37 29 24
86 78 69 59 53 38 30 24
94 85 76 65 58 41 32 26
97 87 78 66 59 42 33 26
0.3 g cm−2 Aluminum shield 25 568 20 417 15 310 10 221 7 178 0 100 −4 69 −7 51 5 g cm−2 Aluminum shield 25 359 20 292 15 234 10 177 7 147 0 86 −4 61 −7 45 −2 40 g cm Aluminum shield 25 105 20 94 15 83 10 70 7 62 0 43 −4 33 −7 26
of 1.5, as described above, to the calculated organ doses. Comparing these organ doses to the 30 d limits in Table 1, we note for the thinnest aluminum shielding (0.3 g cm−2), comparable to a space suit, that all organ limits are exceeded at an altitude above 7 km for both male and female crew members. Also, the dose limit for the lens of the eye is nearly exceeded at the mean surface elevation. Dose limits for the BFO and heart are exceeded at all elevations, for both genders, including the depths of the Hellas impact basin, the lowest point on the surface. Dose limits for the CNS are exceeded for both genders at altitudes several km below the mean surface elevation, but would not be exceeded in the deepest regions of the Hellas impact basin. Depending upon the altitude of the crew, acute radiation syndrome effects could include severe effects such as emesis, diarrhea, hemorrhaging, epilation, and possible death. Clearly a space suit is extremely unlikely to provide adequate protection anywhere on the surface of Mars from a “worst case event” such as the one proposed herein. Organ doses behind 5 g cm−2 of aluminum shielding, comparable to that provided by a surface lander, are lower than those inside a space suit. But they still exceed the 30 d limits in many cases. All organ exposures exceed their limits at altitudes above 10 km. Lens of the eye doses are exceeded for altitudes somewhat lower than 7 km. Skin doses are barely below the limit at an altitude of 7 km. BFO and heart doses again exceed their limits for all altitudes, including the depths of the Hellas basin. Again, radiation
effects would be altitude dependent, but could include emesis, diarrhea, hemorrhaging, epilation and death. These results suggest that a surface lander may not provide adequate protection during an event of this magnitude anywhere on the surface of Mars. The situation for crews shielded by a habitat of 40 g cm−2 of aluminum shielding is somewhat improved. Organ doses are reduced to about one-third of their values from those behind a surface lander. Skin doses are below the limits for all altitudes, including on top of Olympus Mons. Eye doses are below the limits except on Olympus Mons (25 km altitude). Doses to the BFO, heart and CNS exceed the 30 d limits for all altitudes above 7 km. For the BFO and heart, limits are exceeded for all altitudes above 4 km below the mean surface elevation. Thus, it appears that the only exposed locations on the surface of Mars which may provide adequate protection against an event of this magnitude are those inside a substantially shielded habitat at depths greater than 4 km below the mean surface elevation, or possibly inside a lava tube or cave surrounded by thick walls. Finally, we note that surface operations on Olympus Mons, the tallest known mountain in the solar system, will leave crews exposed to dangerous, even life threatening levels of radiation from an event of this magnitude. The total surface area covered by Olympus Mons is comparable in size to the area of the state of Arizona in the United States. Hence, seeking shelter at lower elevations may not be an option should an SPE of this magnitude occur.
L.W. Townsend et al. / Acta Astronautica 89 (2013) 189–194
4.2. Effective doses For a NASA astronaut, the career radiation exposure limits are established so as to not exceed a 3 percent risk of exposure induced death (REID), at a 95 percent confidence level, from a fatal cancer. Career PELs of effective dose (in units of cSv) for both male and female crewmembers as a Table 4 NASA Career Permissible Exposure Limits (PELs) for astronauts for a one year mission [5]. Age (years)
Effective dose (cSv)
25 30 35 40 45 50 55
Female
Male
37 47 55 62 75 92 112
52 62 72 80 95 115 147
Table 5 Calculated effective doses for female (CAF) and male (CAM) astronauts in units of cSv as a function of elevation above (positive elevations) or below (negative elevations) the mean surface elevation on Mars. All doses are rounded to the nearest integer values. Human Model
Elevation (km) 25
20
Effective dose (cSv) 0.3 g cm−2 Aluminum shield CAM 294 247 CAF 289 244 5 g cm−2 Aluminum shield CAM 235 204 CAF 234 204 −2 40 g cm Aluminum shield CAM 105 96 CAF 105 97
15
10
7
0
−4
−7
206 204
164 163
140 140
91 92
69 69
55 55
175 174
143 143
124 125
84 84
65 65
52 52
88 88
77 78
70 71
54 54
44 44
37 37
193
function of age at first exposure [5] are shown in Table 4. Note that the limits for females are lower than for males at all ages. Table 5 presents calculated effective doses for both genders for the assumed SPE spectrum, as calculated using OLTARIS. The effective doses are displayed for both males and females, for each aluminum shielding configuration, as a function of altitude from the mean surface elevation Comparing Tables 4 and 5 it is apparent that effective dose limits are exceeded for males and females at some age of first exposure for nearly all of the elevation and aluminum shielding combinations. To simplify the discussion, Table 6 presents the age ranges for each gender for which the effective dose limits are exceeded for each of these elevation and aluminum shielding combinations. If the limits are exceeded for all ages at first exposure, the table entry is “all”. If the limits are not exceeded for any of the ages at first exposure, the table entry is “none”. As displayed in Table 6, it is clear that the effective dose limits are exceeded for both genders of all ages (at first exposure) for elevations above 15 km if shielded only by space suits or surface landers. At lower elevations, older males and females do not exceed their effective dose limits, and the ages at first exposures where the limits are exceeded decrease as the elevation is lowered. For habitats, represented by 40 g cm−2 aluminum shielding, effective doses received by younger crew members of both genders continue to exceed the limits, except for the youngest and then only at elevations below the mean surface elevation. In summary, it appears that radiation exposures from this hypothetical worst case event, as presented in this work, would result in significant radiation injury for both male and female crew members of all ages, even if protected by an aluminum surface habitat with an areal density of 40 g cm−2. This is true even at the most extreme depths in the Hellas Impact Basin, the deepest crater on the surface of Mars. It appears that reducing potential exposures from this extreme event to tolerable levels will necessitate other measures, such as locating the habitat in a narrow valley, canyon or cave in a region well below the mean surface elevation, or burying it.
Table 6 Age at first exposure (years) for which the effective dose limits are exceeded for both males and females as a function of elevation (km) relative to the mean surface elevation (0 km) on the surface of Mars. The summit of Olympus Mons is at 25 km elevation. The depths of the Hellas Impact Basin are at an elevation of −7 km. Human model
Elevation (km) 25
Age at first exposure (years) 0.3 g cm−2 Aluminum shield CAM All CAF All −2 5 g cm Aluminum shield CAM All CAF All 40 g cm−2 Aluminum shield CAM 25–45 CAF 25–50
20
15
10
7
0
−4
−7
All All
All All
All All
25–50 All
25–40 25–50
25–30 25–40
25 25–35
All All
All All
25–50 All
25–50 All
25–40 25–45
25–30 25–40
25 25–30
25–45 25–50
25–40 25–45
25–35 25–45
25–30 25–40
25 25–30
None 25
None 25
194
L.W. Townsend et al. / Acta Astronautica 89 (2013) 189–194
5. Conclusions Estimates of radiation exposures, from a possible Carrington-type solar energetic particle event with spectral distribution comparable to the February 1956 SPE for male and female astronauts on the surface of Mars, have been presented and compared with NASA permissible exposure limits. The calculations use Band function parameterizations of the incident February 1956 proton spectrum, normalized to the greater than 30 MeV proton fluence of the Carrington event of 1859, to provide the input into the space radiation transport code HZETRN 2010, developed at NASA Langley Research Center. The incident SPE spectrum is transported through the CO2 atmosphere of Mars, for a spread of elevations from the summit of Olympus Mons, the highest known mountain in the solar system, to the depths of the Hellas Impact Basin, the deepest crater on the surface of Mars. The SPE protons are then transported through various hemispherical configurations of aluminum shielding representative of a space suit, surface landing craft, and a permanent habitat. Organ doses and effective doses for both male and female astronauts, located at the highest dose point within the hemisphere, are calculated for all combinations of elevation and hemispherical shielding configurations, using the Computerized Anatomical Man CAM and Female CAF human geometry models to represent body self-shielding distributions for the various organs. All calculations were performed using the On Line Tool for the Assessment of Radiation in Space at NASA Langley Research Center. The resulting organ doses and effective doses are generally found to substantially exceed NASA permissible exposure limits. Acute radiation syndrome responses are possible, including emesis, diarrhea, hemorrhaging, epilation and possible loss of the crew. Of particular concern for crew safety are surface operations on Olympus Mons, or at other elevation significantly above the mean surface elevation during this hypothetical, worst case solar energetic particle event. References [1] L.W. Townsend, F.A. Cucinotta, J.W. Wilson, Interplanetary crew exposure estimates for galactic cosmic rays, Radiat. Res. 129 (1992) 48–52. [2] L.W. Townsend, M. PourArsalan, M.I. Hall, Estimates of radiation exposures on Mars for female crews in hemispherical habitats, in: Presented at the 2010 IEEE Aerospace Conference, Big Sky, MT, March 6–13, 2010 [CD-ROM].
[3] L.W. Townsend, M. PourArsalan, M.I. Hall, Estimates of SPE radiation exposures on Mars for female astronauts in hemispherical habitats, in: Presented at the 40th International Conference on Environmental Systems (ICES), Barcelona, Spain, July 11–15, 2010. Paper No. AIAA-2010-6186. [4] L.W. Townsend, M.P. PourArsalan, M.I. Hall, J.A. Anderson, S. Bhatt, N. P. DeLauder, A.M. Adamczyk, Estimates of Carrington-Class solar particle event radiation exposures on Mars, Acta Astronaut. http: //dx.doi.org/10.1016/j.actaastro.2011.05.020. [5] NASA Space Flight Human System Standard Volume 1: Crew Health, NASA-STD-3001, vol. 1, 2007. [6] S. McKenna-Lawlor, P. Goncalves, A. Keating, B. Morgado, D. Heynderickx, P. Nieminen, G. Santin, P. Truscott, F. Lei, B. Foing, J. Balaz, Characterization of the particle radiuation environment at three potential landing sites on Mars using ESA's MEREM models, Icarus 218 (2012) 723–734, http://dx.doi.org/10.1016/j.icarus.2011.04 .004. [7] G. De Angelis, F.F. Badavi, S.R. Blattnig, M.S. Clowdsley, J.E. Nealy, G. D. Qualls, R.C. Singleterry, R.K. Tripathi, J.W. Wilson, Modeling of the Martian environment for radiation analysis, Nucl. Phys. B (Proc. Suppl.) 166 (2007) 184–202. [8] M.Y. Kim, G. De Angelis, F.A. Cucinotta, Probabilistic assessment of radiation risk for astronauts in space missions, Acta Astronaut. 68 (2011) 747–759, http://dx.doi.org/10.1016/j.actaastro.2010.08.035. [9] K.G. McCracken, G.A.M. Dreschhoff, E.J. Zeller, D.F. Smart, M.A. Shea, Solar cosmic ray events for the period 1561–1994. 1. Identification in polar ice, 1561–1950, J. Geophys. Res. 106 (2001) 21585–21598. [10] I.G. Usoskin, B. Kromer, F. Ludlow, J. Beer, M. Friedrich, G. A. Kovaltsov, S.K. Solanki, L. Wacker, The AD775 cosmic event revisited: the Sun is to blame, Astron. Astrophys. 552 (2013) L3, http://dx.doi.org/10.1051/0004-6361/201321080. [11] D.L. Stephens Jr., L.W. Townsend, J.L. Hoff, Interplanetary crew dose estimates for worst case solar particle events based on the historical data for the Carrington flare of 1859, Acta Astronaut. 56 (2005) 969–974. [12] L.W. Townsend, E.N. Zapp, D.L. Stephens Jr., J.L. Hoff, Carrington flare of 1859 as a prototypical worst case solar energetic particle event, IEEE Trans. Nucl. Sci. 50 (2003) 2307–2309. [13] L.W. Townsend, D.L. Stephens Jr., J.L. Hoff, E.N. Zapp, H.M. Moussa, T. M. Miller, C.E. Campbell, The Carrington Event: possible doses to crews in space from a comparable event, Adv. Space Res. 38 (2006) 226–231. [14] 〈http://www.grc.nasa.gov/WWW/K-12/airplane/atmosmrm.html〉. [15] D. Band, J. Matteson, L. Ford, B. Schaefer, D. Palmer, B. Teegarden, T. Cline, M. Briggs, W. Paciesas, G. Pendleton, G. Fishman, C. Kouveliotou, C. Meegan, R. Wilson, P. Lestrade, BATSE observations of gamma-ray burst spectra.1. spectral diversity, Astrophys. J. 413 (1993) 281–292. [16] A.J. Tylka, W. Dietrich, W. Atwell, Band Function Representations of Solar Proton Spectra in Ground-Level Events. Paper F24-0004-10, in COSPAR XXXVIII Scientific Assembly, Bremen, Germany, July 18–25, 2010. [17] R.C. SingleterryJr., S.R. Blattnig, M.S. Clowdsley, G.D. Qualls, et al., OLTARIS:On-line tool for the assessment of radiation in space, NASA/ TP-2010-216722, July2010. [18] 〈https://oltaris.larc.nasa.gov/〉. [19] NCRP, Limitation of Exposure to Ionizing Radiation, National Council on Radiation Protection and Measurements, Report 116, 1993. [20] NCRP, Radiation Protection Guidance for Activities in Low-Earth Orbit, National Council on Radiation Protection and Measurements, Report 132, 2000.
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Estimates of Carrington-class solar particle event radiation exposures as a function of altitude in the atmosphere of Mars$ L.W. Townsend n, J.A. Anderson, A.M. Adamczyk, C.M. Werneth The University of Tennessee, Knoxville, TN 37996-2300, USA
a r t i c l e i n f o
abstract
Article history: Received 11 October 2012 Received in revised form 20 March 2013 Accepted 10 April 2013 Available online 18 April 2013
Radiation exposure estimates for crew members on the surface of Mars may vary widely because of the large variations in terrain altitude. The maximum altitude difference between the highest (top of Olympus Mons) and the lowest (bottom of the Hellas impact basin) points on Mars is about 32 km. In this work estimates of radiation exposures as a function of altitude, from the Hellas impact basin to Olympus Mons, are made for a solar particle event proton radiation environment comparable to the Carrington event of 1859. We assume that the proton energy distribution for this Carrington-type event is similar to that of the Band Function fit of the February 1956 event. In this work we use the HZETRN 2010 radiation transport code, originally developed at NASA Langley Research Center, and the Computerized Anatomical Male and Female human geometry models to estimate exposures for aluminum shield areal densities similar to those provided by a spacesuit, surface lander, and permanent habitat as a function of altitude in the Mars atmosphere. Comparisons of the predicted organ exposures with current NASA Permissible Exposure Limits (PELs) are made. & 2013 IAA. Published by Elsevier Ltd. All rights reserved.
Keywords: Space radiation exposures Mars atmosphere shielding
1. Introduction Human space exploration in the future may include human crews traveling to Mars for extended stays. A significant concern in carrying out such a mission is the possibility that the crew may be exposed to a large solar particle event (SPE)—one of such magnitude that crew survival or completion of the mission is at risk. Exposures of the crews to these potentially harmful events are possible during the transits to and from Mars as well as during surface operations on Mars. The highest exposures from these events are likely to occur during the transits since the only shielding available is intrinsic to
$ This paper was presented during 62nd IAC in CapeTown. Corresponding author. Tel.: +18659747569; fax: +18659740668. E-mail addresses: [email protected] (L.W. Townsend), [email protected] (J.A. Anderson), [email protected] (A.M. Adamczyk), [email protected] (C.M. Werneth). n
the spacecraft itself. On the surface of Mars, shielding is provided by the planet's bulk, by the overlying CO2 atmosphere, and by the crew's space suits, surface landers, or habitats. Hence, exposures will likely be lower than in deep space [1,2]. Our recent studies [2–4] focused on estimating radiation exposures on the Martian surface near the mean datum using both low-density and high-density atmospheric models, for a variety of galactic cosmic ray (GCR) and solar particle event scenarios. GCR exposures were the focus of one study [2]. In that study, exposures of female crew members were estimated for the most recent solar minimum environment, possibly the most intense of the space age. Females were studied because their smaller stature, and therefore smaller body self-shielding, results in higher exposures than males for the same environment. The results of that study showed that crews on the Mars surface at the mean surface elevation are unlikely to receive exposures from GCR particles that exceed NASA
0094-5765/$ - see front matter & 2013 IAA. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actaastro.2013.04.010
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Permissible Exposure Limits (PELs) [5]. Recent studies have also been carried out by other groups [6–8] for both GCR and SPE exposures. SPE exposure calculations involving recent events, presented in Ref. [6], are significantly lower than those presented here because of their smaller fluences and softer energy spectra. The GCR estimates on the surface, from Ref. [8], are similar to those reported in Ref. [2]. In our second study [3] female crew member exposures on the Martian surface were estimated using Weibull distribution parameterizations of the proton spectra for the August 1972, September 1989 and October 1989 events. That study found that exposures from those events, which are among the largest that have occurred in the past four decades, would not have exceeded the NASA PELs for operations at the mean surface elevation of the planet. However, ice core data from Earth's polar regions [9] and excessive 14C production data from tree rings [10] indicates that much larger events may have occurred over the past ∼1200 years. Due to the possibility of larger events than those observed during the recent spaceflight era, additional analyses [11–13] of the SPE exposures on the surface of Mars were performed using Weibull proton distributions for the September 1989 and November 1960 spectra and Band Function proton distributions for the February 1956, November 1960 and September 1989 spectra. All spectra were normalized to the greater than 30 MeV omnidirectional proton fluence level of 18.8 109 cm−2 estimated from analyses of ice core data [7] of the September 1859 Carrington event, possibly the largest event within the past 500 years. While there is some question as to the validity of the proton fluence estimates from these ice core analyses, we note that they are comparable to the fluence estimate of ∼4.5 1010 cm−2 protons with energies greater than 30 MeV obtained from analyses of 14C production data from tree rings [10]. It is also noteworthy that arguments are made in Ref. [10] that the AD775 tree ring data analyses suggest a spectrum for that event as hard as the February 1956 SPE. Hence, the assumed spectrum for this study appears to be reasonable as a candidate worst-case event. Results of earlier studies of exposures to crews and electronics from possible Carrington-type events in deep space indicated that the doses could be very large and potentially lethal to human crews unless significant shielding was provided [11–13]. In the most recent study [4], organ doses and effective doses for male and female astronauts on the surface of Mars and at an elevation of 8 km above the mean surface elevation were carried out. The highest organ doses, which exceeded the PELs even for the assumed habitat shielding of 40 g cm−2 of aluminum, were obtained for the February 1956 energy spectrum and normalized to the greater than 30 MeV proton fluence level for the possible Carrington event. In the current work, we have extended these results to map the effective dose and organ doses for this event as a function of altitude on Mars, ranging from the height of Olympus Mons (assumed to be 25 km) to the depth of the Hellas impact basin (7 km below the mean surface elevation). We again assume aluminum shielding comparable to that
provided by a spacesuit, surface lander, and surface habitat. The next section outlines the assumed scenarios. This is followed by a description of the computational methods used to obtain the effective doses and organ doses of the male and female crew members. Next, the results of the exposure calculations are presented. Finally, the work is summarized and concluding remarks presented.
2. Surface scenarios As was done in our recent work [4], we assume that the crew member (male or female) is located at ground level on the surface of Mars in the center of an aluminum hemispherical structure. This simple geometry was selected in order to quickly obtain some knowledge of the effectiveness of nominal shield thicknesses comparable to those of more realistic geometries. Also, as was assumed for our previous Mars surface studies [2–4], three areal densities for the aluminum hemisphere are used corresponding to a spacesuit (0.3 g cm−2), surface lander (5 g cm−2), and a surface habitat (40 g cm−2). These hemispheres are placed at various elevations ranging from 25 km above the mean surface elevation to −7 km below it. The Mars atmosphere is assumed to be composed of pure CO2. Unlike some earlier studies of Mars surface doses, where both low-density (16 g cm−2) and highdensity (22 g cm−2) models [12] were used, this work uses the NASA Mars Atmosphere Model based upon data from the 1996 Mars Global Surveyor mission [14]. Since the incoming SPE radiation is isotropic, the atmosphere path lengths traveled by the incident particles are longer for those particles arriving at angles greater than zero, with respect to the local zenith. This is accounted for by averaging the exposures in one degree increments for arrival angles between the zenith and the horizon. As the arrival angle approaches the horizon, the areal densities increase dramatically, especially for altitudes deep in the atmosphere. At atmosphere areal densities exceeding 300 g cm−2, contributions to the organ exposures are trivial and can be ignored. The effects of the surface curvature are included in the atmosphere path lengths. Table 1 displays areal densities above the local zenith versus altitude relative to the mean surface elevation obtained from the model in Ref. [14]. Table 1 Mars atmosphere areal density (g cm−2) at the local zenith as a function of altitude (km) relative to the mean surface elevation. Temperatures at each elevation are also listed. Altitude above mean elevation (km)
Temperature (1C)
Areal density (g cm−2)
25 20 15 10 7 0 −4 −7
−79 −68 −57 −46 −38 −31 −27 −24
2.2 3.3 4.8 7.2 9.2 16.7 23.5 30.5
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3. Computational methods Since the Band function utilizes high energy data and an actual fit at high energies obtained from ground level enhancements (GLEs) measured by neutron monitors on Earth's surface, it should yield more reliable dose estimates than other commonly used parameterizations [15,16]. The Band function parameterization of the February 1956 event proton distribution [16] is given by 8 > ΛR−γ 1 exp − RR0 ; > > > > < for R≤ðγ 2 −γ 1 ÞR0 Φð 4 RÞ ¼ > ΛR−γ 2 ½ðγ −γ ÞR ðγ 2 −γ 1 Þ expðγ −γ Þ; > > 2 1 0 2 1 > > : for R≥ðγ −γ ÞR 2
1
191
effective dose and Linear Energy Transfer (LET) distributions. For this work we calculate effective dose and organ doses for the three aluminum shield configurations as a function of altitude. Organ doses (D) are in units of centiGray (cGy) where 1cGy¼1 rad and 100cGy¼Gy¼1 J kg−1. Organ dose equivalents (H), which are the product of dose with a quality factor, Q (H¼QD), are in centiSievert (cSv) where 1 cSv ¼1 rem and 100 cSv¼1 Sv¼1 J kg−1. The units of effective dose (E) are also cSv, where the effective dose is obtained using ð2Þ
E ¼ ∑ wT H T ð1Þ
0
where Φ(4R) is the proton fluence, Λ¼1.78 1010 protons cm−2 is the normalization constant re-normalized to the Carrington event fluence, R is the particle rigidity (momentum per unit charge) in units of GV (gigavolts), R0 ¼0.321 GV is the characteristic rigidity, and γ1 ¼0.584 and γ2 ¼5.04 are spectral indices. For clarity and comparison purposes, the assumed spectrum is plotted in Fig. 1 along with the Band function fit to the February 1956 event. This incident SPE proton spectrum is transported in succession through the Mars atmosphere (up to 300 g cm−2 CO2), the appropriate hemispherical aluminum shielding thickness (0.3, 5 or 40 g cm−2), and then through the body self-shielding for the CAM (Computerized Anatomical Man) and CAF (Computerized Anatomical Female) human geometry models using the On Line Tool for the Assessment of Radiation In Space (OLTARIS) developed at NASA Langley Research Center [17]. OLTARIS is web-based [18] and uses the latest version of the NASA HZETRN (High Z and Energy TRaNsport) space radiation transport code (HZETRN 2010) [17]. The HZETRN code transports the incident charged ions (protons in this case) and their nuclear reaction secondary particles (protons, neutrons, deuterons, tritons, 3He and alphas) generated by nuclear collisions. The code includes stopping powers to account for energy loss due to excitation and ionization of the medium, resulting from collisions with the orbital electrons of the atoms and molecules in the target media. The code outputs, which are selectable by the user, include particle fluences, dose, dose equivalent,
T
where HT is the organ dose equivalent for the organ or tissue denoted by T (e.g., skin, bone marrow, etc.) and the tissue weighting factors wT are the proportionate detriment of the organ when the whole body is irradiated. These are taken from Table 5.1 of Ref. [19]. 4. Results 4.1. Organ doses Table 2 lists limits for short-term or non-cancer effects as given in the NASA Permissible Exposure Limits (PELs) [5]. One of the major concerns with SPEs is preventing short term effects, such as acute radiation syndrome effects. Hence, the organ doses will be compared with the 30 days limits. These limits are expressed in units of centiGray-Equivalent, which are obtained from the absorbed dose (D) using D ðcGy−EqÞ ¼ D ðcGyÞ RBE
ð3Þ
where the RBE (Relative Biological Effectiveness) is a multiplicative factor that accounts for the ability of some types of radiations to produce more biological damage than others for the same absorbed dose. For SPE protons an RBE value of 1.5 is used [20]. Organ doses for the skin, eye lens, blood forming organs (BFO) represented by bone marrow, the central nervous system (brain), and heart are presented in Table 3. The dose values in the table are in units of cGy-equivalent, obtained by applying the multiplicative proton RBE factor Table 2 Permissible exposure limits for short-term or career non-cancer effects [5].
Fig. 1. Solar proton integral fluences for the actual February 1956 solar particle event and renormalized to the Carrington event fluence level.
Organ
30 Day limit (cGy-Eq)
1 Year limit (cGy-Eq)
Career (cGy-Eq)
Lensn Skin BFO Heartnn CNSnnn CNSnnn (Z≥10)
100 150 25 25 50 –
200 300 50 50 100 10
400 400 NA 100 150 25
n Lens limits are intended to prevent early ( o 5 yr) severe cataracts (e.g., from a solar particle event). nn Heart doses calculated as average over heart muscle and adjacent arteries. nnn CNS (central nervous system) limits should be calculated at the hippocampus.
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Table 3 Calculated organ doses for female (CAF) and male (CAM) astronauts in units of cGy-equivalent as a function of altitude above (positive) or below (negative) the mean surface elevation on Mars. Entries in bold type exceed the 30 d Permissible Exposure Limit guidelines. All doses are rounded to the nearest integer values. Elevation (km)
Skin dose (cGy-Eq)
Eye dose (cGy-Eq)
BFO dose (cGy-Eq)
Heart dose (cGy-Eq)
CNS dose (cGy-Eq)
CAM
CAF
CAM
CAF
CAM
CAF
CAM
CAF
CAM
CAF
568 418 311 222 179 100 69 51
457 360 281 208 170 98 68 51
464 365 284 210 172 99 69 52
283 237 196 153 129 79 57 43
293 245 202 157 132 81 58 44
244 209 176 140 120 75 55 42
251 215 180 144 122 75 56 42
304 256 212 166 140 85 61 46
319 267 220 172 144 88 63 47
360 293 235 178 147 87 61 46
326 271 221 171 143 86 61 46
331 275 224 173 145 87 62 46
227 195 164 132 113 71 52 40
234 200 169 135 115 73 53 40
204 177 151 122 105 69 50 38
209 181 155 125 108 69 51 39
246 212 178 143 122 77 56 42
256 219 184 147 125 78 57 43
105 95 86 71 63 43 33 27
105 94 84 71 63 44 34 27
106 95 84 72 64 44 34 28
88 80 71 61 54 39 30 24
90 81 72 62 55 39 31 25
84 76 68 58 52 37 29 24
86 78 69 59 53 38 30 24
94 85 76 65 58 41 32 26
97 87 78 66 59 42 33 26
0.3 g cm−2 Aluminum shield 25 568 20 417 15 310 10 221 7 178 0 100 −4 69 −7 51 5 g cm−2 Aluminum shield 25 359 20 292 15 234 10 177 7 147 0 86 −4 61 −7 45 −2 40 g cm Aluminum shield 25 105 20 94 15 83 10 70 7 62 0 43 −4 33 −7 26
of 1.5, as described above, to the calculated organ doses. Comparing these organ doses to the 30 d limits in Table 1, we note for the thinnest aluminum shielding (0.3 g cm−2), comparable to a space suit, that all organ limits are exceeded at an altitude above 7 km for both male and female crew members. Also, the dose limit for the lens of the eye is nearly exceeded at the mean surface elevation. Dose limits for the BFO and heart are exceeded at all elevations, for both genders, including the depths of the Hellas impact basin, the lowest point on the surface. Dose limits for the CNS are exceeded for both genders at altitudes several km below the mean surface elevation, but would not be exceeded in the deepest regions of the Hellas impact basin. Depending upon the altitude of the crew, acute radiation syndrome effects could include severe effects such as emesis, diarrhea, hemorrhaging, epilation, and possible death. Clearly a space suit is extremely unlikely to provide adequate protection anywhere on the surface of Mars from a “worst case event” such as the one proposed herein. Organ doses behind 5 g cm−2 of aluminum shielding, comparable to that provided by a surface lander, are lower than those inside a space suit. But they still exceed the 30 d limits in many cases. All organ exposures exceed their limits at altitudes above 10 km. Lens of the eye doses are exceeded for altitudes somewhat lower than 7 km. Skin doses are barely below the limit at an altitude of 7 km. BFO and heart doses again exceed their limits for all altitudes, including the depths of the Hellas basin. Again, radiation
effects would be altitude dependent, but could include emesis, diarrhea, hemorrhaging, epilation and death. These results suggest that a surface lander may not provide adequate protection during an event of this magnitude anywhere on the surface of Mars. The situation for crews shielded by a habitat of 40 g cm−2 of aluminum shielding is somewhat improved. Organ doses are reduced to about one-third of their values from those behind a surface lander. Skin doses are below the limits for all altitudes, including on top of Olympus Mons. Eye doses are below the limits except on Olympus Mons (25 km altitude). Doses to the BFO, heart and CNS exceed the 30 d limits for all altitudes above 7 km. For the BFO and heart, limits are exceeded for all altitudes above 4 km below the mean surface elevation. Thus, it appears that the only exposed locations on the surface of Mars which may provide adequate protection against an event of this magnitude are those inside a substantially shielded habitat at depths greater than 4 km below the mean surface elevation, or possibly inside a lava tube or cave surrounded by thick walls. Finally, we note that surface operations on Olympus Mons, the tallest known mountain in the solar system, will leave crews exposed to dangerous, even life threatening levels of radiation from an event of this magnitude. The total surface area covered by Olympus Mons is comparable in size to the area of the state of Arizona in the United States. Hence, seeking shelter at lower elevations may not be an option should an SPE of this magnitude occur.
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4.2. Effective doses For a NASA astronaut, the career radiation exposure limits are established so as to not exceed a 3 percent risk of exposure induced death (REID), at a 95 percent confidence level, from a fatal cancer. Career PELs of effective dose (in units of cSv) for both male and female crewmembers as a Table 4 NASA Career Permissible Exposure Limits (PELs) for astronauts for a one year mission [5]. Age (years)
Effective dose (cSv)
25 30 35 40 45 50 55
Female
Male
37 47 55 62 75 92 112
52 62 72 80 95 115 147
Table 5 Calculated effective doses for female (CAF) and male (CAM) astronauts in units of cSv as a function of elevation above (positive elevations) or below (negative elevations) the mean surface elevation on Mars. All doses are rounded to the nearest integer values. Human Model
Elevation (km) 25
20
Effective dose (cSv) 0.3 g cm−2 Aluminum shield CAM 294 247 CAF 289 244 5 g cm−2 Aluminum shield CAM 235 204 CAF 234 204 −2 40 g cm Aluminum shield CAM 105 96 CAF 105 97
15
10
7
0
−4
−7
206 204
164 163
140 140
91 92
69 69
55 55
175 174
143 143
124 125
84 84
65 65
52 52
88 88
77 78
70 71
54 54
44 44
37 37
193
function of age at first exposure [5] are shown in Table 4. Note that the limits for females are lower than for males at all ages. Table 5 presents calculated effective doses for both genders for the assumed SPE spectrum, as calculated using OLTARIS. The effective doses are displayed for both males and females, for each aluminum shielding configuration, as a function of altitude from the mean surface elevation Comparing Tables 4 and 5 it is apparent that effective dose limits are exceeded for males and females at some age of first exposure for nearly all of the elevation and aluminum shielding combinations. To simplify the discussion, Table 6 presents the age ranges for each gender for which the effective dose limits are exceeded for each of these elevation and aluminum shielding combinations. If the limits are exceeded for all ages at first exposure, the table entry is “all”. If the limits are not exceeded for any of the ages at first exposure, the table entry is “none”. As displayed in Table 6, it is clear that the effective dose limits are exceeded for both genders of all ages (at first exposure) for elevations above 15 km if shielded only by space suits or surface landers. At lower elevations, older males and females do not exceed their effective dose limits, and the ages at first exposures where the limits are exceeded decrease as the elevation is lowered. For habitats, represented by 40 g cm−2 aluminum shielding, effective doses received by younger crew members of both genders continue to exceed the limits, except for the youngest and then only at elevations below the mean surface elevation. In summary, it appears that radiation exposures from this hypothetical worst case event, as presented in this work, would result in significant radiation injury for both male and female crew members of all ages, even if protected by an aluminum surface habitat with an areal density of 40 g cm−2. This is true even at the most extreme depths in the Hellas Impact Basin, the deepest crater on the surface of Mars. It appears that reducing potential exposures from this extreme event to tolerable levels will necessitate other measures, such as locating the habitat in a narrow valley, canyon or cave in a region well below the mean surface elevation, or burying it.
Table 6 Age at first exposure (years) for which the effective dose limits are exceeded for both males and females as a function of elevation (km) relative to the mean surface elevation (0 km) on the surface of Mars. The summit of Olympus Mons is at 25 km elevation. The depths of the Hellas Impact Basin are at an elevation of −7 km. Human model
Elevation (km) 25
Age at first exposure (years) 0.3 g cm−2 Aluminum shield CAM All CAF All −2 5 g cm Aluminum shield CAM All CAF All 40 g cm−2 Aluminum shield CAM 25–45 CAF 25–50
20
15
10
7
0
−4
−7
All All
All All
All All
25–50 All
25–40 25–50
25–30 25–40
25 25–35
All All
All All
25–50 All
25–50 All
25–40 25–45
25–30 25–40
25 25–30
25–45 25–50
25–40 25–45
25–35 25–45
25–30 25–40
25 25–30
None 25
None 25
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5. Conclusions Estimates of radiation exposures, from a possible Carrington-type solar energetic particle event with spectral distribution comparable to the February 1956 SPE for male and female astronauts on the surface of Mars, have been presented and compared with NASA permissible exposure limits. The calculations use Band function parameterizations of the incident February 1956 proton spectrum, normalized to the greater than 30 MeV proton fluence of the Carrington event of 1859, to provide the input into the space radiation transport code HZETRN 2010, developed at NASA Langley Research Center. The incident SPE spectrum is transported through the CO2 atmosphere of Mars, for a spread of elevations from the summit of Olympus Mons, the highest known mountain in the solar system, to the depths of the Hellas Impact Basin, the deepest crater on the surface of Mars. The SPE protons are then transported through various hemispherical configurations of aluminum shielding representative of a space suit, surface landing craft, and a permanent habitat. Organ doses and effective doses for both male and female astronauts, located at the highest dose point within the hemisphere, are calculated for all combinations of elevation and hemispherical shielding configurations, using the Computerized Anatomical Man CAM and Female CAF human geometry models to represent body self-shielding distributions for the various organs. All calculations were performed using the On Line Tool for the Assessment of Radiation in Space at NASA Langley Research Center. The resulting organ doses and effective doses are generally found to substantially exceed NASA permissible exposure limits. Acute radiation syndrome responses are possible, including emesis, diarrhea, hemorrhaging, epilation and possible loss of the crew. Of particular concern for crew safety are surface operations on Olympus Mons, or at other elevation significantly above the mean surface elevation during this hypothetical, worst case solar energetic particle event. References [1] L.W. Townsend, F.A. Cucinotta, J.W. Wilson, Interplanetary crew exposure estimates for galactic cosmic rays, Radiat. Res. 129 (1992) 48–52. [2] L.W. Townsend, M. PourArsalan, M.I. Hall, Estimates of radiation exposures on Mars for female crews in hemispherical habitats, in: Presented at the 2010 IEEE Aerospace Conference, Big Sky, MT, March 6–13, 2010 [CD-ROM].
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