Managing Space Radiation Risk in the New Era of Space Exploration (2008)

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Committee on the Evaluation of Radiation Shielding for Space Exploration Aeronautics and Space Engineering Board Division on Engineering and Physical Sciences 

THE NATIONAL ACADEMIES PRESS   500 Fifth Street, N.W.   Washington, DC 20001 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance. This study was supported by Contract No. NNH05CC16C between the National ­Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, find- ings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the organizations or agencies that provided support for the project. International Standard Book Number-13:  978-0-309-11383-0 International Standard Book Number-10:  0-309-11383-0 This report is available in limited supply from the Aeronautics and Space Engineering Board, 500 Fifth Street, N.W., Washington, DC 20001; (202) 334-2858. Additional copies are available from the National Academies Press, 500 Fifth Street, N.W., Lockbox 285, Washington, DC 20055; (800) 624-6242 or (202) 334-3313 (in the Washington metropolitan area); Internet, http://www.nap.edu. Copyright 2008 by the National Academy of Sciences. All rights reserved. Printed in the United States of America 

The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a man- date that requires it to advise the federal government on scientific and technical matters. Dr. Ralph J. Cicerone is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Charles M. Vest is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examina- tion of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Harvey V. Fineberg is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy’s purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the Na- tional Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Ralph J. Cicerone and Dr. Charles M. Vest are chair and vice chair, respectively, of the National Research Council. www.national-academies.org 



COMMITTEE ON THE EVALUATION OF RADIATION SHIELDING FOR SPACE EXPLORATION JAMES D.A. VAN HOFTEN, Chair, Bechtel Corporation (retired), San Francisco, California SALLY A. AMUNDSON, Columbia University Center for Radiological Research, New York, New York SAMIM ANGHAIE, University of Florida, Gainesville WILLIAM ATWELL, The Boeing Company, Houston, Texas BENTON C. CLARK, Lockheed Martin Astronautics, Denver, Colorado MARC M. COHEN, Northrop Grumman Integrated Systems, El Segundo, California PATRICK J. GRIFFIN, Sandia National Laboratories, Albuquerque, New Mexico DAVID G. HOEL, Medical University of South Carolina, Charleston TATJANA JEVREMOVIC, Purdue University, West Lafayette, Indiana WALTER SCHIMMERLING, NASA (retired), Arlington, Virginia LAWRENCE W. TOWNSEND, University of Tennessee, Knoxville RONALD E. TURNER, ANSER Corporation, Arlington, Virginia ALLAN J. TYLKA, Naval Research Laboratory, Washington, D.C. GAYLE E. WOLOSCHAK, Northwestern University, Chicago, Illinois Staff KERRIE SMITH, Study Director (through December 2007) BRIAN DEWHURST, Study Director (from January 2008) SARAH CAPOTE, Program Associate (from April 2007) FRANKLIN SLATON, Senior Program Assistant (through March 2007) HEATHER LOZOWSKI, Financial Associate  

AERONAUTICS AND SPACE ENGINEERING BOARD RAYMOND S. COLLADAY, Chair, Lockheed Martin Astronautics (retired), Golden, Colorado CHARLES F. BOLDEN, JR., Jack and Panther, LLC, Houston, Texas ANTHONY J. BRODERICK, Aviation Safety Consultant, Catlett, Virginia AMY BUHRIG, Boeing Commercial Airplanes Group, Seattle, Washington PIERRE CHAO, Center for Strategic and International Studies, Washington, D.C. INDERJIT CHOPRA, University of Maryland, College Park ROBERT L. CRIPPEN, Thiokol Propulsion (retired), Palm Beach Gardens, Florida DAVID GOLDSTON, Princeton University, Princeton, New Jersey R. JOHN HANSMAN, Massachusetts Institute of Technology, Cambridge PRESTON HENNE, Gulfstream Aerospace Corporation, Savannah, Georgia JOHN M. KLINEBERG, Space Systems/Loral (retired), Redwood City, California RICHARD KOHRS, Independent Consultant, Dickinson, Texas ILAN KROO, Stanford University, Stanford, California IVETT LEYVA, Air Force Research Laboratory, Edwards Air Force Base, California EDMOND SOLIDAY, United Airlines (retired), Valparaiso, Indiana Staff MARCIA S. SMITH, Director (from January 2007) GEORGE LEVIN, Director (through January 2007) vi 

Preface The risks faced by human explorers will increase significantly as humans expand the exploration realm b ­ eyond low Earth orbit to revisit the Moon and venture on to Mars. In addition to the normal risks of mechanical, e ­ lectrical, and human failure, exposure to the space radiation environment outside the protective magnetic field of Earth presents a risk with which humans have limited practical experience. My introduction to space radiation came firsthand as a crew member aboard the space shuttle Challenger in April 1984. “What the heck was that?” I blurted out after seeing what looked like a white laser passing quickly through my eyes. “Oh, that’s just cosmic rays,” said Pinky Nelson, my spacewalking partner and a space physicist. The thought of extremely high energy particles originating from a distant cosmic event passing easily through the space shuttle and subsequently through my head made me think that this could not be all that healthful. The truth of the matter is that it is not. Other than the short lunar missions of the Apollo program, astronauts and cosmonauts have flown in the relative safety of low Earth orbits, protected by Earth’s local presence. NASA is now, however, planning to return to the Moon, establish permanent bases, and gain the experience needed for longer missions to Mars. At the request of NASA’s Exploration Systems Mission Directorate, the National Research Council (NRC) assembled the Committee on the Evaluation of Radiation Shielding for Space Exploration to further understanding of the risks of radiation to crews of lunar and martian missions and of how best to protect those crews, and to recommend areas for future technology investments. The statement of task for the committee is given in Appendix A. Based on NASA’s current progress in mission planning, this report focuses on exploration of the Moon and touches only on general aspects of Mars exploration. The report covers current knowledge of the radiation environment; the effects of radiation on biological systems, electronic systems, and missions; current plans for radiation protection; and a strategy for mitigating the risks to astronauts. The committee members ranged from space scientists to biologists to architects. Brief biographical sketches appear in Appendix B. The committee met four times from late 2006 through mid-2007 (see Appendix C for committee meeting agendas and a full list of speakers). During the first three meetings, NASA and the contractor community briefed the committee on the most recent plans for the Constellation program, including the design basis in relation to the galactic cosmic ray and solar particle event environment, current shielding plans, and the latest thinking on the biological impacts of such an environment. This report summarizes the committee’s findings on the state of vii 

viii PREFACE the program and presents recommendations on a strategy for mitigating the radiation risks and where to invest to improve technological understanding of the radiation issue. Since the designs for the actual vehicles are in a state of flux, the committee was forced to base its assessments on early designs and implore the community to continue to fund research to verify follow-on designs. James van Hoften, Chair Committee on the Evaluation of Radiation Shielding for Space Exploration 

Acknowledgment of Reviewers This report has been reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the Report Review Committee of the National Research Council (NRC). The purpose of this independent review is to provide candid and critical comments that will ­assist the institution in making its published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We wish to thank the following individuals for their review of this report: Thomas Borak, Colorado State University, Raymond Colladay, Lockheed Martin (retired), Louis Lanzerotti, New Jersey Institute of Technology, Noelle Metting, Department of Energy, George Paulikas, Aerospace Corporation, Jeffrey Schwartz, University of Washington, Margaret Shea, Air Force Research Laboratory (retired), and Mary Helen Sparks, White Sands Missile Range. Although the reviewers listed above have provided many constructive comments and suggestions, they were not asked to endorse the conclusions or recommendations, nor did they see the final draft of the report before its release. The review of this report was overseen by William M. Beckner, National Council on Radiation Protection and Measurements (retired). Appointed by the NRC, he was responsible for making certain that an independent examination of this report was carried out in accordance with institutional procedures and that all review com- ments were carefully considered. Responsibility for the final content of this report rests entirely with the authoring committee and the institution. ix 



Contents SUMMARY 1 1 INTRODUCTION 7 Hazards of Radiation, 7 Study Process, 8 Organization of This Report, 8 Overview of Mission Architecture, 9 NASA’s Space Radiation Program, 9 Overview of Radiation Protection, 10 Overview of Guidance on Radiation Limits Provided to NASA, 15 Historical Guidance, 15 Space Radiation Research Recommendations of 2006, 16 Current Radiation Limits and Guidance, 16 References, 18 2 CURRENT KNOWLEDGE OF THE RADIATION ENVIRONMENT 19 State of Radiation Environment Knowledge, 19 Galactic Cosmic Radiation, 21 Solar Particle Events, 25 Energy Spectra, 27 Composition, 28 Frequency of Events, 30 Near-Term Forecasts and Nowcasting, 30 Statistical Analyses of SPEs, 32 Space Radiation Climatology, 32 Ice-Core Results on Galactic Cosmic Radiation, 32 Ice-Core Results on Solar Particle Events, 32 Knowledge Gaps, 35 Trapped Radiation, 39 xi 

xii CONTENTS Secondary Radiation, 40 Knowledge Gaps, 43 Radiation from Nuclear Ground Power, 43 Development and Use of Nuclear Ground Power, 43 Radiation from Terrestrial Nuclear Ground Power Plants, 44 Radiation Shielding and Control, 44 Regulations and Policy, 44 References, 45 3 RADIATION EFFECTS 49 Effects on Humans, 49 Some Elementary Concepts in Radiation Biology, 49 Knowledge Gaps, 55 Effects on Materials and Devices, 58 Effects on Mission, 60 References, 60 4 SHIELDING APPROACHES AND CAPABILITIES 62 Basic Shielding Concepts, 62 Shielding for Project Constellation’s Orion, 62 ���������������� Lunar Lander, 64 Surface Infrastructure, 64 ������������ Habitats, 65 Robots, 65 Unpressurized Rover, 65 Pressurized Rover, 66 In Situ Shielding, 66 Extravehicular Activity, 68 Nuclear Power, 70 Nuclear Propulsion, 74 Alternative Methods, 75 Active Shielding, 75 Radioprotectants, 77 Summary, 78 References, 78 5 STRATEGY FOR RADIATION RISK MITIGATION 80 Technology Investments to Enable Lunar Missions, 80 1.  Radiation Biology Research, 80 2.  Radiation Protection in Orion, 82 3.  Validation and Verification of Transport Calculations, 82 4.  Research on Solar Particle Events, 83 5.  Empirical Data for Shielding Design, 83 6.  In Situ Monitoring and Warning, 84 7.  Multifunctional Materials, 86 8.  In Situ Shielding Tradeoffs, 87 9.  Review of Existing Neutron Albedo Datasets, 87 10.  Surface Fission Power Demonstration—Nuclear Power for Mars, 88 Strategies for Keeping Radiation Risk Within NASA Guidelines, 89 Transition from Research to Operations, 89 

CONTENTS xiii Human Capital Infrastructure, 90 Integration of Radiation Protection into Design of Vehicles and Missions, 91 References, 93 6 FINDINGS AND RECOMMENDATIONS 95 APPENDIXES A Statement of Task 103 B Biographies of Committee Members 104 C Committee Meeting Agendas and Speakers 108 D Glossary and Acronyms 111