Evaluating Radiation Risk on Planetary Surfaces

1. Radiation on Planetary Surfaces M. S. Clowdsley1, G. DeAngelis2, J. W. Wilson1, F. F. Badavi3, and R. C. Singleterry1 1 NASA Langley Research Center, Hampton, VA 2Old Dominion University, Norfolk, VA 3Christopher Newport University, Newport News, VA Solar and Space Physics and the Vision for Space Exploration Meeting Wintergreen, Virginia October 16-20, 2005

2. Outline • Requirements for Evaluating Risk Due to Radiation on Planetary Surfaces • Description of the free space radiation environment near the planet (types of particles and their energy spectra) • Model of planetary magnetic field (if one exists) • Models of planetary surface material and atmosphere (if planet has an atmosphere) • Radiation transport code or codes • Guideline defining how much of each type of radiation is too much • Examples Calculations • The Moon • Mars • Callisto • Conclusions

3. Free Space Radiation Environment • Galactic Cosmic Rays (GCR) • Made up of heavy ions as well as alpha particles and protons • Modeled using the Badhwar-O’Neill formulation • Modulated by the solar wind • Vary with the solar cycle • Dependant on distance from the sun • Solar Particle Events (SPE) • Made up of a large number of particles, mostly protons • Correspond to large coronal mass ejections • Large SPE rare • Last only a few hours • Could result in fatality

4. Free Space GCR Environments at 1 AU 1977 Solar Minimum (solid) 1990 Solar Maximum (dashed)

5. Solar Sunspot Numbers and Deep River Neutron Monitor Count Rates (Measured and Predicted) Deep River Neutron Monitor Solar Sunspot Number

6. Free Space Solar Particle Event Proton Spectra at 1 AU

7. Mars Induced Fields Planetary Surface Material and Atmosphere GCR ion High energy particles Diffuse neutrons (Simonsen et al.)

8. Radiation Transport Codes • Monte Carlo Codes: MCNPX, HETC, FLUKA, TIGRE • Accurately model the transport of neutrons, protons, and other light ions (and electrons in the case of TIGRE) • GCR ions being added • Require large amounts of computer time • Deterministic Codes: HZETRN, GRNTRN, Electron Transport Code (Nealy et. al.) • Accurately model the transport of neutrons, protons, light ions, and GCR (and electrons in the case of the electron transport code) • Provide rapid transport calculations

9. Lunar Surface GCR Environments 1977 Solar Minimum (solid) 1990 Solar Maximum (dashed)

10. Lunar Surface “Worst Case SPE” Environment

11. Dose Equivalent on Lunar Surface Due to GCR

12. Mars Surface GCR Environments 1977 Solar Minimum (solid) 1990 Solar Maximum (dashed)

13. Mars Surface Neutrons

14. Mars Surface “Worst Case SPE” Environment

15. Dose Equivalent on Mars Surface Due to GCR

16. Mars Surface Mapping Charged Ions – 1977 Solar Minimum from Space Ionizing Radiation Environment and Shielding Tools (SIREST) web site http://sirest.larc.nasa.gov

17. Mars Surface Mapping Neutrons – 1977 Solar Minimum from Space Ionizing Radiation Environment and Shielding Tools (SIREST) web site http://sirest.larc.nasa.gov

18. Mars Surface Mapping Low Energy Neutrons – 1977 Solar Minimum from Space Ionizing Radiation Environment and Shielding Tools (SIREST) web site http://sirest.larc.nasa.gov

19. Mars Surface Environment

20. Model for Mars Atmosphere • Atmospheric chemical and isotopic composition modeled using results from in-situ Viking 1 & 2 Landers measurements for both major and minor components: CO2 % 95.32 N2 % 02.70 Ar % 01.60 O2 % 00.13 CO % 00.08

21. Model for Mars Surface • The surface altitude, or better the atmospheric depth for incoming particles, determined using a model for the Martian topography based on the data provided by the Mars Orbiter Laser Altimeter (MOLA) instrument on board the Mars Global Surveyor (MGS) spacecraft. • The Mars surface chemical composition model based on an averaging process over the measurements obtained from orbiting spacecraft, namely the Mars 5 with gamma-ray spectroscopy, and from landers at the various landing sites, namely Viking Lander 1, Viling Lander 2, Phobos 2 and Mars Pathfinder missions.

22. Model for Mars Surface SiO2 % 44.2 Fe2O3 % 16.8 Al2O3 % 08.8 CaO % 06.6 MgO % 06.2 SO3 % 05.5 Na2O % 02.5 TiO2 % 01.0 • The adopted Mars surface chemical composition:

23. Model for Mars Surface • The composition, different with respect to the regolith (e.g. CO2 ice, H2O ice), of seasonal and perennial polar caps has been taken into account by modeling the deposition of the possible volatile inventory over the residual caps, along with its geographical variations all throughout the Martian year, for both the Mars North Pole and South Pole, from results from imaging data of orbiter spacecraft and from groundbased observations • No 3D time dependent models for the Martians polar caps was previously available for radiation studies

24. Callisto Surface GCR Environments 1977 Solar Minimum (solid) 1990 Solar Maximum (dashed)

25. Dose Equivalent Rate on Callisto Due to GCR for Jan. 1, 2047

26. Sample ISS Calculations Directional Dose distributions Dose Maps Ray-trace Mesh Directional Dose

27. Conclusions • Surface radiation calculations have been performed for the Earth’s moon, Mars, and Callisto • These calculations show that radiation shielding will be an important consideration in planning of long term missions to these surfaces • These calculations also demonstrate the large variation in exposure rates due to solar cycle • The advantages of using shielding materials containing hydrogen were demonstrated • The ability of the HZETRN code to calculate the radiation environment on the surface of any planet or moon has been demonstrated

28. Table 1 – Dose Equivalent Limits (Sv) Table 2 – Career Dose Equivalent to BFO Limits (Sv) Age at Exposure BFO 25 Eye 35 Skin 45 55 Career Table 2 4.0 6.0 Male 1.5 2.5 3.2 4.0 1 Year 0.50 2.0 3.0 Female 1.0 1.75 2.5 3.0 30 Day 0.25 1.0 1.5 Exposure Limits for LEO Operations (NCRP 98) Limits defined in terms of dose equivalent (H) H =  Q(L) DL dL where DL is the dose (energy absorbed per unit mass) from particles with linear energy transfer between L and L+dL and Q(L) is a quality factor. Based on 3% excess career fatal cancer risk ALARA – In addition to the above limits, radiation exposure must be kept “as low as reasonably achievable.” Note: limits not yet defined for missions beyond LEO

29. Table 2 – Career Effective Dose Limits (Sv) Table 1 – Gray Equivalent Limits (Gy-Eq) Proposed Exposure Limits for LEO Operations (NCRP 132) Age at Exposure BFO 25 Eye 35 Skin 45 55 Career Table 2 4.0 6.0 Male 0.7 1.0 1.5 3.0 1 Year 0.50 2.0 3.0 Female 0.4 0.6 0.9 1.7 30 Day 0.25 1.0 1.5 • New radiation protection quantities • Gray equivalent to BFO, eyes, and skin used to evaluate risk due to deterministic effects • Gy-Eq = i RBEi Di • Whole body effective dose used to evaluate health risk due to stochastic effects • E= wTHT Based on 3% excess career fatal cancer risk ALARA – In addition to the above limits, radiation exposure must be kept “as low as reasonably achievable.” Note: limits not yet defined for missions beyond LEO

30. Table 1 – Gray Equivalent Limits (Gy-Eq) BFO Eye Skin Career REID 4.0 6.0 1 Year 0.50 2.0 3.0 30 Day 0.25 1.0 1.5 Possible Exposure Limits for Lunar Missions(NASA-STD-3000 Vol. VIII - Feb. 1, 2005 Draft) REID: “Occupational radiation exposure is limited to not exceed 3% probability of radiation exposure induced death (REID). NASA will assure that this risk limit is not exceeded at a 95% confidence level using a statistical assessment of the uncertainties in the risk projection calculations…” ALARA – In addition to the above limits, radiation exposure must be kept “as low as reasonably achievable.”