Crew Habitable Element Space Radiation Shielding for Exploration Missions

December 1992

LESC-30455

Foreword

This document was produced by the Lockheed Engineering and Sciences Company, Flight Crew Support Department for the NASA Johnson Space Center, Flight Crew Support Division, Human Factors Project Office. Questions and comments concerning the document should be directed to Paul D. Campbell, Lockheed, (713) 483-9948.

Contents

Section Number and Title

Abstract

1.0 Introduction
1.1 Background
1.2 Purpose
1.3 Scope
1.4 Approach

2.0 Model Development
2.1 Reference Information
2.2 Model Inputs
2.3 Model Data and Algorithms
2.4 Model Outputs

3.0 Lunar and Mars Mission Analyses
3.1 Lunar Mission Analysis
3.2 Mars 600 Day Surface Stay Mission Analysis
3.3 Mars 150 Day Surface Stay Mission Analysis

4.0 Conclusions
4.1 Programmatic
4.2 Mission
4.3 Future Work

5.0 References

6.0 Bibliography

Figures

Number and Title

2.1-1 Crew Radiation Management Analysis Process
2.4-1 CHEERS Model User Interface
2.4-2 Variation of Estimated Habitable Element Shield Mass with Dose Equivalent from GCR at Solar Maximum
2.4-3 Variation of Estimated Habitable Element Shield Mass with Dose Equivalent from GCR at Solar Minimum
2.4-4 Variation of Estimated Habitable Element Shield Mass with Dose Equivalent from a Large SPE
2.4-5 Variation of Estimated SPE Storm Shelter Shield Mass with Dose Equivalent from a large SPE
3.0-1 Estimated Crewmember Radiation Dose Equivalent During the Lunar Reference Mission with the Estimated Shield Masses
3.0-2 Estimated Crewmember Radiation Dose Equivalent During the Mars 600 Day Surface Stay Reference Mission with Estimated Shield Masses
3.0-3 Estimated Crewmember Radiation Dose Equivalent During the Mars 150 Day Surface Stay Reference Mission with Estimated Shield Masses

Tables

Number and Title
2.1-1 Radiation Exposure Limits for Astronauts in Low Earth Orbit
2.1-2 Mars Atmospheric Protection in the Vertical Direction
2.3-1 Annual Dose Equivalents for GCR at Solar Minimum in Free Space at 1 Astronomical Unit from the Sun
2.3-2 Annual Dose Equivalents for GCR at Solar Maximum in Free Space at 1 Astronomical Unit from the Sun
2.3-3 Dose Equivalents for the Sum of 1989 Large SPEs in Free Space at 1 Astronomical Unit from the Sun
2.3-4 Modeling Approximations
3.0-1 Reference 45 Day Surface Stay Lunar Mission Analysis
3.0-2 Reference 600 Day Surface Stay Mars Mission Analysis
3.0-3 Reference 150 Day Surface Stay Mars Mission Analysis

Abstract

Human space exploration missions require shielding of crewmembers against ionizing space radiation emanating from the sun and from galactic sources. Complex calculations of radiation attenuation by various materials have been performed in other studies, and this study attempts to integrate those results into a simple model which may be used to quickly estimate the radiation shielding mass necessary for various space exploration habitable elements. This model may be used as a mission planning tool in that it automates part of the mission design process in which the designer estimates crew vehicle mass based on mission parameters.

1.0 Introduction

The Crew Habitable Element Estimation of Radiation Shielding (CHEERS) Model has been developed for preliminary quantification of the radiation shielding needed to protect humans on Lunar and Mars missions. The following subsections describe the background and purpose for this tool as well as a summary of its development.

1.1 Background

The Space Exploration Initiative (SEI) has been defined as a long-term United States effort to explore the Earth's moon and the planet Mars. Human missions to the moon will take place prior to sending crews to Mars. All SEI missions will encounter ionizing space radiation from which crewmembers must be protected.

1.2 Purpose

The study documented herein was conducted as part of an effort to define SEI crew habitation systems requirements for both Lunar and Mars missions. A major habitation requirement for these missions is that of radiation protection. It carries with it a potentially large mission mass impact and therefore merits early study to maximize benefits in later program phases.

1.3 Scope

This study was defined to estimate the mass of space radiation shielding for SEI habitable elements. It attempts to consider all potential human Lunar and Mars missions, including possible mission durations, types of space radiation encountered, radiation dose limits for humans, and the protective qualities of potential shield materials.

1.4 Approach

The study approach included research of previous studies in this area, extraction of relevant information, development of a model which integrates results from previous studies, sensitivity analysis on the various model parameters, and analysis of representative Lunar and Mars mission scenarios.

2.0 Model Development

The following subsections describe information used to generate the shielding model as well as model inputs, algorithms, and outputs.

2.1 Reference Information

Figure 2.1-1 illustrates a conceptual process for integrated mission analysis for space crew radiation protection. This process was used as a reference for the development of the radiation shield estimation model described in the following subsections. As shown in the figure, radiation shielding definition is dependent on the prior definition of the mission, radiation environments, biological effects of environmental radiation, natural shielding effects of planetary bodies, size of habitable elements, radiation from nuclear propulsion or power systems, shielding material properties, and human exposure limits.

Current radiation exposure limits for astronauts in low Earth orbit are shown in Table 2.1-1 (reference 1).

Reference 2 was used as the source of dose equivalents versus shielding thickness for various materials. Reference 3 was used as the source of this information for aluminum shielding of a solar particle event. Reference 4 was used as the source of the Mars atmospheric shielding as shown in Table 2.1-2. Reference 5 was used as the source of dose equivalent versus shielding thickness for liquid hydrogen and galactic cosmic ray (GCR) radiation at solar minimum. Reference 6 was used as the source of dose equivalent versus shielding thickness for liquid hydrogen and solar particle event (SPE) radiation.

Figure 2.1-1 Crew Radiation Management Analysis Process

Table 2.1-1 Radiation Exposure Limits for Astronauts in Low Earth Orbit

* Dependent on crewmember sex and age.

Table 2.1-2. Mars Atmospheric Protection in the Vertical Direction

Altitude (km)	Low-Density Model (g CO2/cm2)	High-Density Model (g CO2/cm2)
0	16	22
4	11	16
8	7	11
12	5	8

2.2 Model Inputs

The following inputs are made by the user of the CHEERS model:

Crew size:

the number of crewmembers in the habitable element which is being analyzed.

Stay time:

the number of days the crewmembers remain in the element which is being analyzed.

Environment:

the location at which to analyze required radiation shielding. (Lunar surface, Mars surface, Earth-moon space, near-Mars space, or near-Venus space)

Radiation type:

the particular class of radiation against which the crew is to be shielded. (Solar particle event (SPE), galactic cosmic radiation (GCR) at solar cycle minimum, or GCR at solar maximum)

Allowable radiation dose equivalent:

the allowable number of centi-Sieverts (cSv) to the blood forming organs (5 cm depth) from the given radiation type over the given stay time.

Shield material:

the specific chemical composition of the radiation shield to be analyzed. (aluminum, water, polyethylene, Lunar regolith, or liquid hydrogen)

Shielded area:

the volume which is radiation-protected. (entire habitable element, solar storm shelter)

Storm shelter free volume per person:

the number of cubic meters of volume available for the crew in the solar storm shelter divided by the number of crewmembers.

Portion of habitable element mass effective as shielding:

the fraction of the mass of the habitable element which affords radiation protection to its crew. On a planetary surface, any habitable element mass below floor level does not provide any shielding benefit to the crew because of the 2-pi shielding provided by the planet. (0 to 1)

Habitable element pressurized volume:

the number of cubic meters of volume inside the pressure shell of the habitable element to be shielded.

Habitable element mass:

the number of metric tons in the mass of the basic habitable element to be shielded, excluding the dedicated shield mass being calculated by the model and excluding any external vehicle systems which do not provide a shielding effect.

2.3 Model Data and Algorithms

Tables 2.3-1 through 2.3-3 illustrate the data base of dose equivalent versus shielding which was integrated based on the results of previous studies (references 2-6).

The algorithm used to estimate shield mass is shown in the appendix.

Approximations included in the model are a mix of conservatism and non-conservatism. Table 2.3-4 lists some of these approximations and their expected effects. The overall effect is expected to be conservatism in the estimation of habitable element shield mass required to achieve a given degree of crew radiation protection.

Table 2.3-1 Annual Dose Equivalents for GCR at Solar Minimum in Free Space at 1 Astronomical Unit from the Sun

	Body Dose Equivalent (cSv/yr)		Shielding
MATERIAL:	0 cm depth	5 cm depth	g/cm2
Aluminum:	35.51	31.4	50
	43.51	36.33	25
	61.42	46.87	10
	74.78	54.17	5
	86.59	60.37	2
	91.11	62.85	1
	111.45	65.6	0
H2O:	22.56	21.86	50
	30.29	27.62	25
	49.6	40.16	10
	65.6	49.6	5
	81.11	58.11	2
	87.67	61.63	1
	111.45	65.6	0
Lunar Regolith:	27.81	25.68	75
	30.32	27.64	50
	38.4	32.93	25
	57.25	44.43	10
	71.73	52.57	5
	84.83	59.57	2
	90.04	62.43	1
	111.45	65.6	0
Polyethylene:	15.99	16.18	50
	21.92	20.99	25
	40.81	33.9	10
	58.74	45.06	5
	77.22	55.74	2
	85.45	60.34	1
	111.45	65.6	0
Liquid Hydrogen		8	50
		12.5	20
		17	10
		25	5
		37	2.5
		65.6	0
		65.6	0

Table 2.3-2 Annual Dose Equivalents for GCR at Solar Maximum in Free Space at 1 Astronomical Unit from the Sun

	Body Dose Equivalent (cSv/yr)		Shielding
MATERIAL:	0 cm depth	5 cm depth	g/cm2
Aluminum:	17.75	16.17	50
	20.96	17.78	25
	26.07	21.11	10
	29.09	23.12	5
	31.28	24.69	2
	32	25.27	1
	40.02	25.92	0
H2O:	10.97	10.69	50
	14.28	13.16	25
	21.31	18.11	10
	25.92	21.3	5
	29.59	23.86	2
	30.93	24.86	1
	40.02	25.92	0
Lunar Regolith:	14.51	13.4	75
	15.33	13.93	50
	18.32	15.92	25
	24.27	19.94	10
	27.96	22.43	5
	30.68	24.36	2
	31.63	25.17	1
	40.02	25.92	0
Polyethylene:	7.39	7.6	50
	9.94	9.65	25
	17.36	15.1	10
	23.15	19.26	5
	28.17	22.87	2
	30.16	24.05	1
	40.02	25.92	0
Liquid Hydrogen

Table 2.3-3 Dose Equivalents for the Sum of 1989 Large SPEs in Free Space at 1 Astronomical Unit from the Sun

	Body Dose Equivalent (cSv)		Shielding
MATERIAL:	0 cm depth	5 cm depth	g/cm2
Aluminum:		3.2	50
		13	25
		50	10
		118	5
		215	2
		280	1
		391.5	0
H2O:	0.3199	0.3803	50
	12	7.29	25
	105.6	42.88	10
	411.4	105.6	5
	1922	218.6	2
	5099	291.8	1
	19800	391.5	0
Lunar Regolith:	0.89	0.7	75
	3.13	2.24	50
	20.71		25
	164	57.03	10
	615.9	127.2	5
	2792	239.5	2
	7435	306.2	1
	19800	391.5	0
Polyethylene:	0.254	0.3012	50
	10.02	6.3	25
	90.89	39.05	10
	360.3	99.23	5
	1706	212.1	2
	4553	287.5	1
	19800	391.5	0
Liquid Hydrogen	0.0708	0.0589	50
	1.26	0.963	25
	18.7	11.4	10
	91.9	41.4	5
	509	130	2
	1490	215	1
	19800	391.5	0

Table 2.3-4 Modeling Approximations

Conservative:

Low density Mars atmosphere at 5 km altitude: 10 g/cm2 CO2 overhead (hemispherically integrated effect of Mars atmosphere is greater)

Representation of Mars CO2 atmosphere as aluminum shield (equal number of g/cm2 CO2 has more stopping power than Al)

Representation of habitat mass as aluminum (typical habitat will contain water and hydrocarbons which have higher stopping power than aluminum)

Mass external to the habitable element, such as propellant tanks and power systems, is not accounted as shielding

Representation of EMU as having zero shielding effect

Representation of BFO dose equivalent as 5 cm depth water slab (human body actually provides more shielding to BFO)

Representation of skin dose equivalent as 0 cm depth (human body provides shielding to skin from underneath)

For Mars mission, assume one large SPE just after trans-Mars injection (TMI), one during Mars surface stay, and one just prior to Earth entry or at Venus swingby, whichever is worse

GCR at solar cycle minimum (worst case GCR)

SPE represented as sum of large flares during August - October 1989

Non-conservative:

Neglected Van Allen Belt radiation doses (2 cSv?)

SPE intensity is inversely proportional to the square of the distance from the sun (not precisely known)

ICRP26 quality factors for GCR dose equivalent through liquid hydrogen (versus ICRP60)

Representation of habitable element mass as a constant-thickness shell completely surrounding the crew

Representation of the solar storm shelter shield as a spherical shell (non-spherical shapes would require more mass)

Straight line log-linear extrapolation of dose equivalent versus shielding for polyethylene and water above 25 g/cm2 for SPE

2.4 Model Outputs

The user interface to the CHEERS model is illustrated in Figure 2.4-1. User inputs may be modified in any order, resulting in immediate recalculation of the model outputs.

Model output includes a list of contributions to habitable element radiation shielding and an estimate of the mass of dedicated shielding material necessary to provide adequate protection to the crew.

Shield mass estimate: the number of metric tons of dedicated shield material to be added to the habitable element analyzed to protect the crew to the dose equivalent level input by the user.

Inherent shielding of Mars atmosphere: True when a Mars surface element is being analyzed, otherwise False.

Inherent shielding provided by planet: True when a Lunar or Mars surface element is being analyzed, otherwise False.

Total habitable element shielding required: the number of g/cm2 of the user-selected material required to protect the crew to the dose equivalent input by the user.

Inherent shielding provided by the habitable element mass and the Mars atmosphere: the equivalent number of g/cm2 of the user-selected material represented by the effective shielding of the element itself and the Mars atmosphere.

Additional shielding required: the number of g/cm2 of the user-selected material which must be added to the habitable element to protect the crew to the dose equivalent input by the user.

Figure 2.4-2 illustrates the variation of estimated habitable element shield mass with dose equivalent from GCR at solar maximum. It is seen that a larger habitable element requires proportionately more radiation shielding to achieve equivalent crew exposure. As crew exposure is reduced, shield mass estimates increase non-linearly to the point that the shield mass can become larger than the mass of the original element itself.

Figure 2.4-3 illustrates the variation of estimated habitable element shield mass with dose equivalent from GCR at solar minimum. It is seen that shielding requirements increase significantly due to the more intense GCR environment at solar minimum relative to solar maximum. This does not reflect the influence of SPE shielding requirements.

Figure 2.4-4 illustrates the variation of estimated habitable element shield mass with dose equivalent from a large SPE at 1 Astronomical Unit (AU) from the sun, or Earth's orbit distance. Again, shield mass rises non-linearly as crew exposure is reduced.

Figure 2.4-5 illustrates the variation of storm shelter estimated shield mass with dose equivalent from a large SPE at 1 AU. It is seen that shielding only a 12 m3 storm shelter reduces shield mass estimates significantly relative to shielding the entire habitable element from SPE. It is also seen that a more massive habitable element may provide a degree of reduction in the storm shelter dedicated shield mass requirement.

Figure 2.4-1 CHEERS Model User Interface

Figure 2.4-2 Variation of Estimated Habitable Element Shield Mass

with Dose Equivalent from GCR at Solar Maximum

Figure 2.4-3 Variation of Estimated Habitable Element Shield Mass

with Dose Equivalent from GCR at Solar Minimum

Figure 2.4-4 Variation of Estimated Habitable Element Shield Mass

with Dose Equivalent from a Large SPE

Figure 2.4-5 Variation of Estimated SPE Storm Shelter Shield Mass

with Dose Equivalent from a large SPE

3.0 Lunar and Mars Mission Analyses

The following subsections describe CHEERS analyses of several Lunar and Mars mission scenarios.

3.1 Lunar Mission Analysis

A reference mission to the moon was analyzed. This mission was assumed to involve four crewmembers and a 45 day Lunar surface stay. The crew module element was assumed to have an internal volume of 8.5 m3 and mass of 7.5 mt. The surface habitat element was assumed to have an internal volume of 100 m3 and a mass of 25 mt. Twenty Extravehicular Activity (EVA) periods of eight hours each per crewmember were assumed. A large SPE was assumed to occur during the Lunar surface phase. The mission radiation analysis is summarized in Table 3.0-1. Estimated shield mass for the crew module was 330 kg polyethylene and for the surface habitat was 630 kg water. Figure 3.0-1 shows the profile of the estimated crewmember radiation dose equivalent during the 45 day surface stay Lunar reference mission with the estimated shield masses.

Table 3.0-1. Reference 45 Day Surface Stay Lunar Mission Analysis

Figure 3.0-1 Estimated Crewmember Radiation Dose Equivalent During the Lunar Reference Mission with the Estimated Shield Masses

3.2 Mars 600 Day Surface Stay Mission Analysis

Two reference Mars missions were analyzed using the shielding model. A transit habitat with internal volume of 200 m3 and mass of 50 mt, a surface habitat of the same size, and a crew module of 8.5 m3 internal volume and 7.5 mt mass were assumed. Fifty Mars surface EVA periods per crewmember were assumed. Table 3.0-2 illustrates the results of the analysis of a 600 day surface stay Mars mission. The model results were iterated to produce a total mission radiation budget which meets monthly, yearly, and career dose equivalent limits for the crew. Estimated shield masses for the crew module were:

Transit Habitat: 1280 kg polyethylene to shield entire element

Surface Habitat: 1280 kg polyethylene to shield entire element

Crew Module:0

EMU:0.

Figure 3.0-2 shows the profile of the estimated crewmember radiation dose equivalent during the 600 day surface stay Mars reference mission with the estimated shield masses.

Table 3.0-2. Reference 600 Day Surface Stay Mars Mission Analysis

Figure 3.0-2 Estimated Crewmember Radiation Dose Equivalent Duringthe Mars 600 Day Surface Stay Reference Mission

with Estimated Shield Masses

3.3 Mars 150 Day Surface Stay Mission Analysis

Table 3.0-3 shows the results of a 150 day surface stay Mars mission with a Venus swingby return trajectory. Estimated shield masses for the elements were:

Transit Habitat: 9600 kg polyethylene

(6800 kg to shield entire element + 2800 kg for SPE shelter)

Surface Habitat:1280 kg polyethylene to shield entire element

Crew Module:0

EMU:0.

The mission total dose equivalent is significantly less than the smallest career dose limit of 100 cSv, but is constrained by the annual limit of 50 cSv during the final year of the mission.

Figure 3.0-3 shows the profile of the estimated crewmember radiation dose equivalent during the 150 day surface stay Mars reference mission with the estimated shield masses.

Table 3.0-3. Reference 150 Day Surface Stay Mars Mission Analysis

Figure 3.0-3 Estimated Crewmember Radiation Dose Equivalent During the Mars 150 Day Surface Stay Reference Mission with Estimated Shield Masses

4.0 Conclusions

The following subsections describe the significant findings from the development and initial use of the CHEERS model.

4.1 Programmatic

Radiation management for Lunar and Mars mission crewmembers can be initiated early in the planning process as an integral part of the concurrent engineering of missions and systems.

Parametric analysis of radiation shielding and its programmatic impacts can be accomplished to allow a broad optimization of multiple missions and systems.

4.2 Mission

For short duration Lunar missions, such as the 45 day surface stay reference mission analyzed here, radiation shielding and dose management are primarily driven by the possibility of a large solar particle event.

For Mars missions, both large SPE and GCR drive some aspects of the radiation shielding and dose management. A mission with long Mars surface stay time, such as the 600 day surface stay reference mission analyzed here, may require less total radiation shielding mass than a short surface stay mission due to the shorter periods of free space exposure to GCR and due to the differences in trajectories. A large SPE during Venus swingby on a short surface stay Mars mission can be the driving factor in sizing the transit habitat radiation shield mass.

4.3 Future Work

Specific issues related to habitable element sizing and crew radiation protection can be analyzed using the CHEERS model. Both parametric analyses and mission analyses will be performed as new mission concepts and habitable element concepts are developed.

5.0 References

1. National Council on Radiation Protection and Measurement, Guidance on Radiation Received in Space Activities, NCRP Report No. 98, July 31, 1989.

2. Simonsen, L. C., Nealy, J. E., and Townsend, L. W., "Concepts and Strategies for Lunar Base Radiation Protection: Pre-Fabricated Versus In-Situ Materials", SAE 921370, 22nd International Conference on Environmental Systems, July, 1992.

3. Nealy, J. E., Simonsen, L. C., and Townsend, L. W., et al, "Radiation Exposure and Dose Estimates for a Nuclear-Powered Manned Mars Sprint Mission", manuscript prepared for Eighth Symposium on Space Nuclear Power Systems, January 1991.

4. Simonsen, L. C., Nealy, J. E., Townsend, L. W., and Wilson, J. W., "Space Radiation Dose Estimates on the Surface of Mars", Journal of Spacecraft and Rockets, Volume 27, Number 4, July-August 1990.

5. Simonsen, L. C., and Nealy, J. E., "Radiation Protection for Human Missions to the Moon and Mars", NASA Technical Paper 3079, January 1991.

6. Nealy, J. E., Unpublished data, NASA Langley Research Center, September 1992.

6.0 Bibliography

NASA Lunar and Mars Exploration Programs Office, "Proceedings of the Tutorial on Space Radiation and the Space Exploration Initiative", December 14, 1990.

Nealy, J. E., Striepe, S. A., and Simonsen, L. C., "MIRACAL: A Mission Radiation Calculation Program for Analysis of Lunar and Interplanetary Missions", NASA Technical Paper 3211, May 1992.

Santoro, R. T., and Ingersoll, D. T., "Radiation Shielding Requirements for Manned Deep Space Missions", Oak Ridge National Laboratory, ORNL/TM-11808, April 1991.

Stanford, M., "Space Radiation Hazards", McDonnell Douglas.

Striepe, S. A., Nealy, J. E., and Simonsen, L. C., "Radiation Exposure Predictions for Short-Duration-Stay Mars Missions", AAS Paper 92-107, February 1992.

Striepe, S. A., Nealy, J. E., and Simonsen, L. C., "Radiation Exposure Predictions for Long-Duration-Stay Mars Missions", AIAA/AAS Astrodynamics Conference, August 1992.