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--> Exploring the Trans-Neptunian Solar System Committee on Planetary and Lunar Exploration Space Studies Board Commission on Physical Sciences, Mathematics, and Applications National Research Council NATIONAL ACADEMY PRESS Washington, D.C.1998
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--> 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. 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 mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce Alberts 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. William A. Wulf 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 examination 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. Kenneth I. Shine 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 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 Academies and the Institute of Medicine. Dr. Bruce Alberts and Dr. William A. Wulf are chairman and vice chairman, respectively, of the National Research Council. Support for this project was provided by Contract NASW 96013 between the National Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the sponsor. International Standard Book Number 0-309-06041-9 Cover: Multiple views of the current configuration of NASA’s proposed Pluto-Kuiper Express spacecraft. Courtesy of Robert Staehle and the Jet Propulsion Laboratory. Copies of this report are available free of charge from Space Studies Board National Research Council 2101 Constitution Avenue, NW Washington, DC 20418 Copyright 1998 by the National Academy of Sciences. All rights reserved. Printed in the United States of America
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--> COMMITTEE ON PLANETARY AND LUNAR EXPLORATION RONALD GREELEY, Arizona State University, Chair JAMES ARNOLD,* University of California, San Diego FRANCES BAGENAL, University of Colorado JEFFREY R. BARNES, Oregon State University RICHARD BINZEL, Massachusetts Institute of Technology WENDY CALVIN, U.S. Geological Survey PHILIP R. CHRISTENSEN,* Arizona State University RUSSELL DOOLITTLE, University of California, San Diego HEIDI B. HAMMEL, Massachusetts Institute of Technology LARRY HASKIN, Washington University BRUCE JAKOSKY, University of Colorado GEORGE McGILL, University of Massachusetts HARRY McSWEEN, JR. University of Tennessee TED ROUSH, San Francisco State University JOHN RUMMEL, Marine Biological Laboratory GERALD SCHUBERT, University of California, Los Angeles EVERETT SHOCK, Washington University EUGENE SHOEMAKER, Lowell Observatory DARRELL F. STROBEL,* Johns Hopkins University ALAN T. TOKUNAGA,* University of Hawaii ROGER YELLE,* Boston University MARIA T. ZUBER,* Massachusetts Institute of Technology Staff DAVID H. SMITH, Study Director ALTORIA B. ROSS, Senior Program Assistant ELAINE E. HARRIS, Interim Program Assistant STEPHANIE ROY, Research Assistant JACQUELINE D. ALLEN, Senior Program Assistant * Former member.
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--> SPACE STUDIES BOARD CLAUDE R. CANIZARES, Massachusetts Institute of Technology, Chair MARK R. ABBOTT, Oregon State University JOHN A. ARMSTRONG,* IBM Corporation JAMES P. BAGIAN,* Environmental Protection Agency DANIEL N. BAKER, University of Colorado LAWRENCE BOGORAD, Harvard University DONALD E. BROWNLEE, University of Washington JOHN J. DONEGAN,* John Donegan Associates, Inc. GERARD W. ELVERUM, JR., TRW Space and Technology Group ANTHONY W. ENGLAND, University of Michigan DANIEL J. FINK,* D.J. Fink Associates, Inc., Potomac, Md. MARILYN L. FOGEL, Carnegie Institution of Washington MARTIN E. GLICKSMAN,* Rensselaer Polytechnic Institute RONALD GREELEY, Arizona State University WILLIAM GREEN, former member, U.S. House of Representatives NOEL W. HINNERS,* Lockheed Martin Astronautics ANDREW H. KNOLL, Harvard University JANET G. LUHMANN,* University of California, Berkeley JOHN H. McELROY,* University of Texas, Arlington ROBERTA BALSTAD MILLER, CIESIN BERRIEN MOORE III, University of New Hampshire KENNETH H. NEALSON, University of Wisconsin MARY JANE OSBORN, University of Connecticut Health Center SIMON OSTRACH, Case Western Reserve University MORTON B. PANISH, AT&T Bell Laboratories (retired) CARLÉ M. PIETERS, Brown University THOMAS A. PRINCE, California Institute of Technology MARCIA J. RIEKE,* University of Arizona PEDRO L. RUSTAN, JR., U.S. Air Force (retired) ROLAND W. SCHMITT,* Rensselaer Polytechnic Institute (retired) JOHN A. SIMPSON, Enrico Fermi Institute GEORGE L. SISCOE, Boston University EDWARD M. STOLPER, California Institute of Technology RAYMOND VISKANTA, Purdue University ROBERT E. WILLIAMS, Space Telescope Science Institute MARC S. ALLEN, Director (through December 12, 1997) JOSEPH K. ALEXANDER, Director (as of February 17, 1998) * Former member.
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--> COMMISSION ON PHYSICAL SCIENCES, MATHEMATICS, AND APPLICATIONS ROBERT J. HERMANN, United Technologies Corporation, Co-chair W. CARL LINEBERGER, University of Colorado, Co-chair PETER M. BANKS, Environmental Research Institute of Michigan WILLIAM BROWDER, Princeton University LAWRENCE D. BROWN, University of Pennsylvania RONALD G. DOUGLAS, Texas A&M University JOHN E. ESTES, University of California at Santa Barbara MARTHA P. HAYNES, Cornell University L. LOUIS HEGEDUS, Elf Atochem North America, Inc. JOHN E. HOPCROFT, Cornell University CAROL M. JANTZEN, Westinghouse Savannah River Company PAUL G. KAMINSKI, Technovation, Inc. KENNETH H. KELLER, University of Minnesota KENNETH I. KELLERMANN, National Radio Astronomy Observatory MARGARET G. KIVELSON, University of California at Los Angeles DANIEL KLEPPNER, Massachusetts Institute of Technology JOHN KREICK, Sanders, a Lockheed Martin Company MARSHA I. LESTER, University of Pennsylvania NICHOLAS P. SAMIOS, Brookhaven National Laboratory CHANG-LIN TIEN, University of California at Berkeley NORMAN METZGER, Executive Director
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--> Foreword At the distance of Neptune, the Sun is 900 times fainter than at Earth and only 400 times brighter than our full Moon. Beyond Neptune lies Pluto with its moon, Charon, and a vast frozen region that recent observations show is teeming with icy remnants of the nebula that formed the solar system, the Kuiper Belt. Although the intrepid Voyager 2 spacecraft zoomed past Neptune in 1989 and continues to send signals from several times that distance, its orbital dynamics did not allow a detailed inspection of Pluto or the Kuiper Belt. This report considers the scientific imperatives and priorities for further study of the trans-neptunian system, including Neptune's own moon Triton. It considers both ground-based observations and space missions. The report recognizes that technology is the key to cost-effective, in situ exploration of Pluto, Charon, and the Kuiper Belt. Studying these remote objects requires small spacecraft, lightweight enough to be boosted to the outer solar system with modest-sized rockets yet suitably instrumented to perform meaningful science when they arrive. The trans-neptunian system contains the most primitive and undisturbed remnant of the material from which our planet formed. A major reward for studying and exploring these distant regions is the understanding it can give about the origin and evolution of our home in the solar system. CLAUDE R. CANIZARES, CHAIR SPACE STUDIES BOARD
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--> Preface In the last decade, our knowledge of the outer solar system has been transformed as a result of the Voyager 2 encounter with Neptune and its satellite Triton and from Earth-based observations of the Pluto-Charon system. However, the planetary system does not simply end at the distance of Pluto and Neptune. In the past few years, dozens of bodies have been discovered in near-circular, low inclination orbits near or beyond the orbit of Neptune. These bodies are now believed to be directly related to each other and to Pluto, Charon, and Triton. As a class they define and occupy the inner boundary of a hitherto unexplored component of the solar system, the trans-neptunian region. We have just begun to characterize the nearest and larger members of the population of bodies beyond Neptune to at least 100 AU, known as the Kuiper Belt. These bodies have low albedos, are about a hundred to a few hundred kilometers in diameter, and number in the tens of thousands. Smaller Kuiper Belt objects are presumably vastly more numerous. Because the inner part of the Kuiper Belt is unstable with respect to gravitational perturbations by Neptune, the smaller objects are suspected to be the major source for short-period comets that enter regions closer to the Sun and that can collide with the planets. The largest known comet, Chiron, orbits between Saturn and Uranus. It is about 170 km across, has low albedo, and exhibits a complex coma and jet structure. Its orbit strongly suggests that it is a former Kuiper Belt object, and its size indicates that it is similar to those objects recently discovered beyond Neptune. The sizes of the largest Kuiper Belt objects are unknown, but it is likely that the Pluto-Charon system, stabilized in a gravitational orbital resonance with Neptune at the inner edge of the Kuiper Belt, is a large surviving member of the primordial Kuiper Belt population. Exploration of the Pluto-Charon system by spacecraft is a prime objective of NASA's planetary and lunar exploration program in the 21st century. NASA's present plan is to study and design an integrated instrument suite and spacecraft to explore Pluto-Charon under extreme cost constraints. Mission development is closely connected to the New Millennium advanced spacecraft technology development program and will aim for a new start in FY 2000. Neptune's Triton is very similar to Pluto in size, density, and surface and atmospheric composition. Although Triton is a planetary satellite, it was probably captured and thus may be related in origin to Pluto and Kuiper Belt objects. The Voyager flyby of Triton revealed it to be a geologically and meteorologically remarkable body. It exhibits a wide array of geological terrains, present-day geological activity, and a variety of atmospheric and seasonal processes that imply dramatic climatic variations on longer time scales. Although NASA has no planned missions to return to the Neptune system, Pluto-Charon and Triton could be studied synergistically to great effect. As a result of these developments, the Space Studies Board charged COMPLEX to review the state of scientific knowledge of the trans-neptunian region of the solar system and address the following questions:
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--> • What is the present understanding of the origin, composition, and physical characteristics of these bodies and the interrelationships among Kuiper Belt objects, Chiron, the Pluto-Charon system, and Triton? • What ground-based and Earth-orbiting telescopic observations are needed to characterize the Kuiper Belt and the complex worlds, Pluto-Charon and Triton? • What observations would clarify the highest-priority scientific questions concerning Pluto-Charon and Triton, and would identify new targets of high scientific interest? • What are the likely opportunities for relatively inexpensive flyby or rendezvous missions to Pluto-Charon, Triton, Chiron, or Kuiper Belt bodies? • What are the priority scientific questions for such missions, and what instruments are necessary to answer them? • What enabling technologies are needed to make these missions affordable? This project was formally initiated in October 1995, and the bulk of the material was written in the latter part of 1996. This material was extensively revised, updated, and reviewed in the summer of 1997. Although many COMPLEX members past and present worked on this report, the bulk of the task of assembling and editing their many individual contributions was performed by Fran Bagenal with the assistance of Heidi Hammel, Ted Roush, Gerald Schubert, Darrell Strobel, and Roger Yelle. The work of this writing team was made easier thanks to the invaluable assistance rendered by Dale Cruikshank (NASA Ames Research Center), Harold Levison (Southwest Research Institute), William McKinnon (Washington University), Robert Staehle (NASA Jet Propulsion Laboratory), and Paul Weissman (NASA Jet Propulsion Laboratory). COMPLEX also wishes to acknowledge additional assistance given by Donald Brownlee (University of Washington), Karen Meech (University of Hawaii), Robert Pepin (University of Minnesota), Marcia Rieke (University of Arizona), and Peter Stockman (Space Telescope Science Institute). This report has been reviewed by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council's (NRC's) Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the authors and the NRC in making the 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 contents of the review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. COMPLEX thanks reviewers Reta Beebe (New Mexico State University), Robert H. Brown (University of Arizona), Marc Buie (Lowell Observatory), A.G.W. Cameron (Harvard-Smithsonian Center for Astrophysics), William Kaula (University of California, Los Angeles), Margaret Kivelson (University of California, Los Angeles), and Jane Luu (Harvard-Smithsonian Center for Astrophysics) for many constructive comments and suggestions. Responsibility for the final content of this report rests solely with the authoring committee and the NRC.
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--> Contents Executive Summary 1 1 The New Worlds Beyond 30 AU 7 Trans-Neptunian Objects, 9 Exploration of New Territory, 10 Reservoirs of Primitive Materials, 11 Processes That Reveal the Solar System's Origin and Evolution, 13 Links to Extrasolar Planets, 15 Prebiotic Chemistry, 15 Effects of the Trans-Neptunian Region on the Inner Solar System, 16 References, 16 2 Current Knowledge and Outstanding Issues 18 Triton, 18 Pluto and Charon, 21 Kuiper Belt Objects, 23 Centaurs, 26 References, 28 3 Key Measurement Objectives 30 Orbits, 30 Bulk Properties, 31 Surfaces and Chemical Compositions, 32 Atmospheres, 33 Plasma Interactions, 33 Laboratory Studies, 34 Theoretical Studies, 35 References, 35
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--> 4 Technology Issues 36 Telescopic Observations, 36 Spacecraft Missions, 38 References, 40 5 Conclusions and Recommendations 42 Spacecraft Missions, 42 Telescopic Observations, 44 Research and Analysis, 45 Reference, 45 Glossary 46