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SPACE SCIENCE IN THE TWENTY-FIRST CENTURY IMPERATIVES FOR THE DECADES 1995 TO 2015 PLANETARY AND LUNAR EXPLORATION Task Group on Planetary and Lunar Exploration Space Science Board Commission on Physical Sciences, Mathematics, and Resources National Research Council NATIONAL ACADEMY PRESS Washington, D.C. 1988

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National Academy Press 2101 Constitution Avenue, N.W. . Washington, D. C. 20418 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 report has been reviewed by a group other titan the authors according to procedures approved by a Report R.,riew Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The National Academy of Sciences is a priorate, 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 mandate that require it to advise the federal government on scientific and technical matters. Dr. Franlc Pme is president of the National Academy of Sciences. The National Academy of Engineering was established in 19434, 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 spo~ore engineering programs aimed at meeting national needs, encourag" education and rcacarch, and recognizes the superior achievement of enginecre. Dr. Robert M. White is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Scienc" to "cure the avarices of eminent members of appropriate profemione in the Lamination of DolicY matters Dcrtainin. to the health of the Dublic. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal go~rcrnment and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Samuel O. Thier 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 ad~rising 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 National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academi" and the Institute of Medicine. Dr. Franlc Pre" and Dr. Robert M. White are chairman and Rice chairman, respectively, of the National R - Arch Council. Support for this project was provided by Contract NASW 3482 between the National Academy of Sciences and the National Aeronautics and Space Administration. Library of Congress Catalog Card Number 87-43335 ISBN 0-309-03885-5 Printed in the United States of America First Printing, June 1988 Second Printing, November 1988

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1 TAS1[ GROUP ON P LANETA1LY AND [UN AR EXP [O1lATIO N Donald M. Hunten, University of Arizona, Chairman Arden L. Albee, California Institute of Technology David C. Black, NASA Headquarters Jacques Blamont, C.N.E.S. William V. Boynton, University of Arizona Robert A. Brown, Space Telescope Science Institute A.G.W. Cameron, Center for Astrophysics Thomas M. Donahue, University of Michigan Larry W. Esposito, University of Colorado Ronald Greeley, Arizona State University Eugene H. Levy, University of Arizona Harold Masursky, U.S. Geological Survey David D. Morrison, University of Hawaii at Manoa George W. Wetherill, Carnegie Institution of Washington Paul F. Uhlir, Staff Officer Anne L. Pond, Secretary ...

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STEERING GROUP Thomas M. Donahue, University of Michigan, Chairman Don I`. Anderson, California Institute of Technology D. James Baker, Joint Oceanographic Institutions, Inc. Robert W. Berliner, Pew Scholars Program, Yale University Bernard F. Burke, Massachusetts Institute of Technology A. G. W. Cameron, Harvard College Observatory George B. Field, Center for Astrophysics, Harvard University Herbert Friedman, Naval Research Laboratory Donald M. Hunten, University of Arizona Francis S. Johnson, University of Texas at Dallas Robert Kretsinger, University of Virginia Stamatios M. Krimigis, Applied Physics I,aboratory Eugene H. Levy, University of Arizona Frank B. McDonald, NASA Headquarters John E. Naugle, Chevy Chase, Maryland Joseph M. Reynolds, The Louisiana State University Frederick L. Scarf, TRW Systems Park Scott N. Swisher, Michigan State University David A. Usher, Cornell University James A. Van Allen, University of Iowa Rainer Weiss, Massachusetts Institute of Technology Dean P. Kastel, Study Director Ceres M. Rangos, Secretary 1V

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SPACE SCIENCE BOARD Thomas M. Donahue, University of Michigan, Chairman Philip H. Abelson, American Association for the Advancement of Science Roger D. Blandford, California Institute of Technology Larry W. Esposito, University of Colorado Jonathan E. Grindisy, Center for Astrophysics DonaId N. B. Hall, University of Hawaii Andrew P. Ingersoll, California Institute of Technology William M. Kaula, NOAA Harold P. Klein, The University of Santa Clara John W. Leibacher, National Solar Observatory Michael Mendillo, Boston University Robert O. Pepin, University of Minnesota Roger J. Phillips, Southern Methodist University David M. Raup, University of Chicago Christopher T. Rudely, University of California, Los Angeles Blair D. Savage, University of Wisconsin John A. Simpson, Enrico Fermi Institute, University of Chicago George L. Siscoe, University of California, Los Angeles L. Dennis Smith, Purdue University Darrell F. Strobel, Johns Hopkins University Byron D. Tapley, University of Texas at Austin Dean P. K"tel, Staff Director Ceres M. Rangos, Secretary v

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COMMISSION ON PHYSICAL SCIENCES, MATHEMATICS, AND RESOURCES Norman Hackerman, Robert A. Welch Foundation, Chairman George F. Carrier, Harvard University Dean E. Eastman, IBM Corporation Marye Anne Fox, University of Texas Gerhart Friediander, Brookhaven National Laboratory Lawrence W. E`unkhouser, Chevron Corporation (retired) Phillip A. Griffiths, Duke University J. Ross Macdonald, University of North Carolina, Chapel Hill Charles J. Mankin, Oklahoma Geological Survey Perry L. McCarty, Stanford University Jack E. Oliver, Cornell University Jeremiah P. Ostriker, Princeton University Observatory William D. Phillips, Mallinckro~t, Inc. Denis J. Prager, MacArthur Foundation David M. Raup, University of Chicago Richard J. Reed, University of Washington Robert E. Sievers, University of Colorado Larry L. Smarr, National Center for Supercomputing Applications Edward C. Stone, Jr., California Institute of Technology Karl K. Turekian, Yale University George W. Wetherill, Carnegie Institution of Washington Irving Wladawsky-Berger, IBM Corporation Raphael G. Kasper, Executive Director Lawrence E. McCray, Associate Executive Director V1

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Foreword Early in 1984, NASA asked the Space Science Board to un- dertake a study to determine the principal scientific issues that the disciplines of space science would face during the period from about 1995 to 2015. This request was made partly because NASA expected the Space Station to become available at the beginning of this period, ant} partly because the missions needed to implement research strategies previously developed by the various commit- tees of the board should have been launched or their development under way by that tune. A two-year study was called for. To carry out the study the board put together task groups in earth sciences, planetary and lunar exploration, solar and space physics, astronomy and astrophysics, fundamental physics and chemistry (relativistic gravitation and microgravity sciences), and life sci- ences. Responsibility for the study was vested in a steering group whose members consisted of the task group chairmen plus other se- nior representatives of the space science disciplines. To the board's good fortune, distinguished scientists from many countries other than the United States participated in this study. The findings of the study are published in seven volumes: six task group reports, of which this volume is one, and an overview report of the steering group. ~ commend this and all the other task group reports to the reader for an understanding of the challenges . V11

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that confront the space sciences ma the insights they promise far the next century. Tbe omcis1 reco~endatlons of the study are those to be Fund in the steerlog graphs overflew. Oboe M. Don~e, Cb~rm~ Space Sconce Board . .. mu

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Contents 1. INTRODUCTION 2. SCIENTIFIC GOALS AND RECOMMENDATIONS Goals of Planetary Exploration, 4 A Balanced Planetary Program, 7 A Mars Focus, ~ Summary of Recommendations, 9 Scientific Investigations, 10 Technical Developments, 17 International Collaboration, 18 3. STATUS OF PLANETARY SCIENCE IN 1995 Overview, 19 State of Planetary Exploration as of 1995, 19 Scientific Questions ~ of 1995, 23 Planetary Geosciences, 25 Scientific Objectives for Planetary Geosciences, 26 The Inner Solar System, 28 Rocky Satellites, 37 Atmospheres, 41 Earth, Mars, and Venus, 41 Titan, 44 lo and the Plasma Torus, 46 tic 1 19

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Jovian Planets, 47 Rings, 49 Interiors of the Giant Planets, 53 Planetary Magnetism, 57 Generation of Planetary Magnetic Fields, 58 Earth's Magnetic Field, 61 Studying Planetary Magnetic Fields, 62 Primitive Bodies and the Origin of the Solar System, 64 The Origin of the Sun and Planets, 64 Search for and Study of Other Planetary Systems Asteroids, Small Satellites, and Meteorites, 73 Comets, 80 General Characteristics, 80 State of Knowledge in 1995, 82 , 70 4. FUTURE PROGRAMS Proposed Missions, 88 Programs for Planetary Geosciences, 88 Programs for the Outer Solar System, 94 Future Missions and Programs for Primitive Bodies and the Origin of the Solar System, 96 A Program for Intensive Exploration of Mars, 99 Why Mars?, 99 Scientific Objectives for a Mars Focus, 102 Role of Humans in Intensive Mars Exploration, 104 A Phased Approach, 105 Recommendations, 106 Exploration of the Solar System, 106 Exploration of Mars, 111 x 88