OVERVIEW OF SOLAR ENERGETIC PARTICLE EVENT HAZARDS TO HUMAN CREWS

1. OVERVIEW OF SOLAR ENERGETIC PARTICLE EVENT HAZARDS TO HUMAN CREWS Lawrence W. Townsend University of Tennessee

2. OUTLINE • Environment • GCR Doses and Effects • Solar Particle Event Doses and Effects - Carrington Flare as a Worst Case Event • Mars and Lunar Surface Doses • Space Radiation Transport Code Development • Concluding Remarks

3. ENVIRONMENT • Space environment is complex • Van Allen belts important for LEO; also GCR important for high-latitudes (ISS) • Solar Energetic Particles (SEP) important for missions outside Earth’s magnetosphere • Acute and chronic exposures possible

4. ENVIRONMENT (cont.) • Galactic Cosmic Rays (GCR) important for missions outside Earth’s magnetosphere • Chronic exposures are at issue (unique effects?) • Acute effects not possible

5. CORONAL MASS EJECTION (SOHO Image)

6. Annual GCR Doses

7. DEEP SPACE GCR DOSES • Annual bone marrow GCR doses will range up to ~ 15 cGy at solar minimum (~ 40 cSv) behind ~ 2cm Al shielding • Effective dose at solar minimum is ~ 45-50 cSv per annum • At solar maximum these are ~ 15-18 cSv • Secondary neutrons and charged particles are the major sources of radiation exposure in an interplanetary spacecraft

8. GCR Risks • Clearly, annual doses < 20cGy present no acute health hazard to crews on deep space missions • Hence only stochastic effects such as cancer induction and mortality or late deterministic effects, such as cataracts or damage to the central nervous system are of concern. • Unfortunately, there are no data for human exposures from these radiations that can be used to estimate risks to crews • In fact, it is not clear that the usual methods of estimating risk by calculating dose equivalent are even appropriate for these particles

9. SOLAR PARTICLE EVENT DOSES • Doses can be large in deep space but shielding is possible • August 1972 was largest dose event of space era (occurred between two Apollo missions)

10. SKIN DOSE AUGUST 1972 SPE(1 g/cm2 Al shielding)

11. AUGUST 1972 SKIN DOSE RATE

12. EFFECTIVE DOSE AUGUST 1972 SPE

13. POSSIBLE ACUTE EFFECTSAugust 1972 SPE • Bone marrow doses ~ 1 Gy delivered in a day may produce hematological responses and vomiting (not good in a space suit) • Skin doses ~15-20 Gy could result in skin erythema and moist desquamation (in some cases) - doses inside nominal spacecraft might limit effects to mild erythema

14. ORGAN DOSE LIMITS (Gy-Eq)NCRP Report 132

15. October 1989 SEP

16. SOLAR PARTICLE EVENT DOSES (cont.) • Ice core data from the Antarctic indicate that the largest event in past ~ 500 years was probably the Carrington Flare of 1859 - fluence much larger than Aug 72 - actual spectrum energy dependence unavailable, assume both hard and soft spectra

17. Carrington Flare Dose Estimates

18. CARRINGTON FLARE DOSES(9/89 Spectrum) • Bone marrow doses ~ 1-3 Gy possible inside a spacecraft (life threatening) • “Storm” shelter of about 18 cm Al needed to shield to the applicable deterministic limits (30 d limits of 0.25 Gy-Eq) • Major problem for non radiation hardened electronics built with COTS components - up to 50 krads or more of total ionizing dose

19. CARRINGTON FLARE DOSES(8/72 Spectrum) • Bone marrow doses in spacesuit up to ~1.5 Gy; much lower inside a spacecraft ( not life threatening) • “Storm” shelter of about 10 g cm-2 Al needed to shield to the applicable deterministic limits (30 d limits of 0.25 Gy-Eq) • Major problem for non radiation hardened electronics built with COTS components unless they are shielded by at least 1 g cm-2 Al - up to 15 krads total ionizing dose for 15mils

20. Lunar Surface • Organ Doses and Dose Equivalents are ~ half those in deep space - 2 shadow shielding provided - Some neutron albedo from Lunar Surface • Inside a habitat the exposure is nearly all due to neutrons

21. Mars Surface(mainly protons and neutrons)

22. Hypothesis It has been proposed that • proton intensities on the stream-limited plateau present a minimal radiation hazard to astronauts • hazardous intensities occur upon CME-driven shock arrival at the spacecraft

23. Methodology - Data • Used the 5 largest events, in terms of accumulated dose, from years 1996-2001(July 14, 2000; November 8, 2000; September 24, 2001; November 4, 2001; November 22, 2001) • Differential and integral flux and fluence spectra measured on GOES-8 • Shock arrival times • ACE list of disturbances/transients (MAG and SWEPAM instruments) • SOHO/CELIAS solar wind data site • Discussions with NOAA SEC researchers • Stream Limited Intensities from Don Reames

24. Dose Calculations-July 14, 2000

25. November 4, 2001-Dose to Eye

26. September 24, 2001 – Dose to BFO

27. Implications for Event-Triggered Forecasting • Hazardous radiation levels do occur prior to shock arrival for large events for shielding thicknesses on the order of 3 g/cm2 of Al • This suggests that we should attempt to predict the temporal evolution of dose for the SEP event prior to shock arrival • The temporal evolution of the SEP event determines the available time for making decisions

28. HETC-HEDS Code Development • HETC has been extended to include the transport of high-energy heavy ions (HZE particles) in a new version now named HETC-HEDS • HZE particle event generator has been developed and incorporated into the code to provide nuclear interaction data • Minor revisions to the models and techniques used in the event generator are performed as needed based upon comparisons with laboratory beam data

29. HETC-HEDS Results Fragment Fluence for 2A GeV 56Fe on 10 g/cm2 of Polyethylene (HETC-HEDS vs. PHITS)

30. Status of FLUKA Development • Current version has the embedded event generators DPMJET • Four separate efforts on improvements to the event generators: - rQMD approach based on the constrained Hamiltonian formalism of Dirac (E. N. Zapp) - G. Xu is revisiting the original rQMD code - "after-burner" to the rQMD codes to reassemble the fragments (M. –V. Garzelli) - "Master Boltzmann Equation" approach (<100 MeV/A) (F. Cerutti)

31. HZETRN Code Development • Publicly-released version improved by incorporating better low energy treatment of interaction cross sections and better neutron transport • Meson and muon transport being incorporated • Green’s function techniques being developed for 3D transport

32. 1 A GeV iron ion beam validation NSRL Test Rig

33. CONCLUDING REMARKS • GCR exposures will be a problem for Mars missions due to large effective doses • Organ doses received from large SPEs can be hazardous to crews of vehicles in deep space - exposures that are survivable with proper medical treatment on Earth may not be survivable in space

34. CONCLUDING REMARKS (cont.) • Aside from acute effects, a single large SPE can expose a crewmember to an effective dose that exceeds their career limit • Due to their relatively soft energy spectra, most SPE doses can be substantially reduced with adequate shielding (several cmAl or equivalent) • A worst case event similar to the assumed Carrington Flare of 1859 could be catastrophic in deep space depending on spectral hardness and available shielding

35. CONCLUDING REMARKS (cont.) • Results presented only for aluminum • Other materials with low atomic mass numbers are better  LH2 reduces GCR dose equivalent by ~ one-half • In situ materials on lunar or Martian surface can be used to provide shielding (similar to Al in shielding characteristics) • Martian atmosphere is a relatively thick shield for operations on Mars surface ~ 16-20 g cm-2 CO2

36. QUESTIONS?

37. RBE VALUES FOR CONVERTING DOSE TO Gy-Eq (NCRP 132)

38. LEO DOSES • GCR and SAA protons dominate • About half and half at ~ 400 km altitude • Shuttle flights (28.5-62º; 220-615 km) - crew doses : 0.02 – 3.2 cGy • MIR (51.6º; ~ 400 km) - crew doses: 2.3 – 8.2 cGy • ISS (51.6º; ~ 400 km) - crew doses: ~ 5 cGy (solar max) • Rapid transits limit doses for deep space

39. SPE DOSE FORECASTING • At present it is not possible to forecast SPE fluences/doses before they occur • We are developing methods to forecast dose buildup over time based on the doses measured early in an SPE – “Nowcast” (supported by NASA LWS program) - Artificial Intelligence: Sliding Time Delay Neural Network - Locally-Weighted Learning - Bayesian Inference

40. NOVEMBER 2001 SPE Bayesian Methodology Dose Forecast at 2 hours into event

41. NOVEMBER 2001 SPE Bayesian Methodology Dose Forecast at 6 hours into event