Peter T. Bobrowsky Hans Rickman Comet/Asteroid Impacts and Human Society An Interdisciplinary Approach Peter T. Bobrowsky Hans Rickman (Editors) Comet/Asteroid Impacts and Human Society An Interdisciplinary Approach With 85 Figures, 46 in Color Editors Dr. Peter T. Bobrowsky Geological Survey of Canada Landslides and Geotechnics ESS/GSC-CNCB/GSC-NC/EDS Natural Resources Canada 601 Booth Street K1A 0E8 Ottawa, ON Canada E-mail: Preface The International Council for Science (ICSU) recently recognized that the societal implications (social, cultural, political and economic) of a comet/asteroid impact on Earth warrants an immediate consideration by all countries in the world. Given the paucity of information on this important issue, ICSU thus contacted the International Astronomical Union (IAU) and the International Union for Geological Sciences (IUGS) to address the topic on behalf of the global science community. This volume provides a summary of opinions regarding the controversy of fact vs. fiction in dealing with comet and asteroid impacts. Each contribution provides a timely state-of-the-art and state-of-the-science synthesis regarding the likelihood and impli- cations of past, present and future comet/asteroid impacts and their effect on human society. Individual chapters represent a wide range of disciplines, specialties and topics which are either directly or indirectly related to impact events. In this way, this book differs considerably from previous comet/asteroid impact books as well as most other natural hazard volumes that commonly focus on a single discipline of study. Our goal in compiling this volume was to ensure that representatives from ancillary disciplines (anthropology, archaeology, economics, geography, atmospheric sciences, political sci- ence, psychology and so on) had the opportunity to contribute to the discussion by astronomers and geologists and therefore broaden the restrictive vision normally ac- corded to topical discussions of natural hazards. Our aim is to widen the appeal of the subject of natural hazards to include specialists that deal with the subject but lack an appreciation of the related implications surfacing from other disciplines. Moreover, the papers were written with the non-scientist in mind, with the expectation to better inform and educate decision makers, politicians and the general public at large about the diverse nature of the physical and social consequences which have in the past, and will in the future, arise from an impact of a comet or asteroid with our planet Earth. This volume is clustered into three parts comprising 33 chapters. The focus of this book provides those individuals interested in multi-hazard interdisciplinary research a concise appraisal of what is currently known regarding the threat of comet/asteroid impacts, the likelihood and magnitude of such events in the future, an historic review of past impacts based on geological, archaeological and anthropological evidence, an elaboration on the likely physical effects of a significant impact, the ecological and atmospheric effects following an impact, the psycho-sociological implications associ- ated with risk, hazards and disasters as well as the financial, economic and insurance consequences of a catastrophic impact on our planet. VI Preface Part one covers the ancient (geology), prehistoric (archaeology) and historic (an- thropology) record of comet and asteroid events. This includes papers on popular culture and the use of tree ring studies in modern research as well as a review of the analogies of mega catastrophes resulting from volcanic eruptions. Part two contains contribu- tions focused on the status of near-earth object (NEO) surveys, current knowledge of NEO populations in space, physical properties of NEOs, the quantitative risk of im- pacts and risk reduction scenarios, the physical terrestrial effects of impacts, the atmo- spheric and oceanic (tsunami) effects of impacts, case studies including the Kaali meteorite and Tunguska events and cryometeors. Part three examines the social sci- ence of near-earth objects, perceptions of risk, dynamic risk assessment, social per- spectives on hazards, social vulnerability, the potential collapse of society, disaster planning, insurance coverage, economic consequences, communicating impact risk to the public, impact risk communication management, international policies on NEOs and the future of NEO research. In April 2004 Hans Rickman of the International Astronomical Union (IAU) and Peter Bobrowsky of the International Union for Geological Sciences (IUGS) met with a few key representatives of the comet/asteroid professional community in Paris under the auspices of the International Council for Science (ICSU). At that time, the group was encouraged by ICSU to consider collaboration in an interdisciplinary effort on the subject of comet/asteroid impacts and human society. ICSU was very interested in supporting a research proposal relevant to the topic that explicitly included individu- als in broadly allied fields of study that were not normally included in discussions on this subject. The intent of the proposal was to provide an open platform of discussion and interaction between astronomers, geologists, anthropologists, archaeologists, econo- mists, sociologists, geographers, psychologists, journalists and many others interested in natural hazards, disaster management, risk assessment and ancillary fields of study, but focussed specifically on the potential psycho-social and physical consequences of a catastrophic comet or asteroid impact on Earth. Following the initial meeting in April of 2004, IAU and IUGS coordinated a formal proposal submission to ICSU for a Class II grant. Representatives from allied unions including IUGG (International Union of Geodesy and Geophysics), IGU (International Geographic Union) and IUPsyS (Inter- national Union of Psychological Science) agreed to contribute to the working efforts of the project. Similarly, specialists in other disciplines including anthropology, archaeol- ogy, medicine, and so on, but not official representatives of their respective ICSU unions also agreed to contribute to such a project. Shortly thereafter, ICSU approved the grant proposal. An Advisory group consisting of the following individuals was struck: Harry Atkinson (UK NEO Task Force), Clark Chapman (Member at Large), Viacheslav Gusiakov (IUGG), Wing-Huen Ip (COSPAR), Michael MacCracken (SCOR) and Stefan Michalowski (OECD). Invitations were then sent to noted specialists in varied disci- plines to participate in a week long retreat which included technical presentations, break- out group discussions, interactive debates and a local field trip. The retreat was held in early December 2004 in La Laguna, Tenerife, Spain with the local support of Mark Kidger and the Instituto de Astrofisica de Canarias. The Editors are most grateful to Dr. Kidger and the staff and management of the institute for their kind support in facilitating this important meeting. Preface VII As an outcome of the workshop, a summation of the current state of the art and science on the subject and a discussion of related key political questions on the hazard lead to the development of a “white paper”. This compilation, aimed as a background document for politicians, is to appear as a separate published document. At the same time, all invited participants were asked to submit a technical manuscript summariz- ing their specialty, in a format that addressed the multi-disciplinary nature of the meeting. This volume represents the end product of this effort and thus addresses the outputs identified in the original proposal to ICSU. This volume represents the collective efforts of a great number of individuals. Most importantly, the Editors recognize the hard work of the contributing authors to clearly capture the key issues of their field of expertise and structure this information in a broadly informative nature readable by others outside their field of interest. The Edi- tors also appreciate the support and work of the editorial staff at Springer Verlag who helped them deal with the difficult process of managing modern techniques in copy- editing. Finally the Editors wish to thank all those individuals who kindly provided their time and effort as critical reviewers for the submitted papers; in some cases re- viewing several different papers. The critical reviews were important to us and the book, as they add a level of technical acceptability even when some of the opinions of some of the authors were contentious. Each manuscript was initially reviewed by Peter Bobrowsky and/or Hans Rickman and at least two other impartial persons. As a con- sequence of this referee process, several papers originally submitted to this volume were rejected and are not included in the published volume. The list of reviewers in alphabetical order were: Johannes Andersen, Joe Arvai, Mark Bailey, Elizabeth Barber, Tony Berger, John Birks, Bill Bottke, Edward Bryant, Andrea Carusi, David Carusi, Alberto Cellino, Clark Chapman, Rejean Couture, Curt Covey, John Davis, Robert Dimand, Eric Elst, David Etkin, Marten Geertsema, John Grattan, Richard Grieve, Peter Horn, David Huntley, Monica Jaramillo, Ruthann Knudson, David Kring, Howard Kunreuther, Jose Lozano, Brian Marsden, Bruce Masse, Jay Melosh, Patrick Michel, Millan Millan, Urve Miller, David Morrison, Jon Nott, Andrei Ol’khovatov, Effim Pelinovsky, Benny Peiser, Juri Plado, Alex Rabinovich, Barrie Raftery, Marko Robnik, Paul Slovic, Richard Spalding, Doug Stead, Duncan Steel, John Twigg, Juha Uitto, Giovanni Valsecchi, Don Yeomans, Fumi Yoshida, Ben Wisner, and Colin Wood. We acknowledge the support of our respective institutes (Geological Survey of Canada and Uppsala Astronomical Observatory), Unions (International Union of Geo- logical Sciences and International Astronomical Union) and families for providing us the valuable time needed to pursue this important activity. Peter Bobrowsky Hans Rickman November 2006 Contents Part I · Anthropology, Archaeology, Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 The Geologic Record of Destructive Impact Events on Earth . . . . . . . . . . . . . . . . . 3 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2 General Character of the Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2.1 Spatial Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2.2 Age Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2.3 Size Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2.4 Terrestrial Cratering Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2.5 Periodic Impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3 Recognition of Terrestrial Impact Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3.1 Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3.2 Geology of Impact Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.3.3 Geophysics of Impact Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1.4 Impacts in the Stratigraphic Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1.5 Impacts and the Biosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.5.1 Early Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.5.2 Coupling through the Atmosphere and Hydrosphere . . . . . . . . . . . . . . . . . . 13 1.5.3 Local and Mass Extinctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 1.5.4 Threat to Humanity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 1.6 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2 The Archaeology and Anthropology of Quaternary Period Cosmic Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.2 The Quaternary Period Cosmic Impact Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.2.1 Documented Impact Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.2.2 Validated Holocene Crater-Forming Impact Events . . . . . . . . . . . . . . . . . . . . 29 2.2.3 Airbursts, Tektites, and Impact Glass Melts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 2.2.4 A Sample of Current Studies of Potential Late Quaternary–Holocene Period Terrestrial Impact Sites . . . . . . . . . . . 34 2.2.5 Oceanic Impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 X Contents 2.3 Oral Tradition, Myth, and Cosmic Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 2.3.1 The Nature and Principles of Myth and Oral Tradition . . . . . . . . . . . . . . . . 40 2.3.2 Using Myth to Identify and Model South American Cosmic Impacts . . . 42 2.3.3 Modeling the Flood Comet Event – a Hypothesized Globally Catastrophic Mid-Holocene Abyssal Oceanic Comet Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 2.4 Epilog and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 2.4.1 Candidate Abyssal Impact Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 2.4.2 Post-Workshop Final Thoughts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 3 The Sky on the Ground: Celestial Objects and Events in Archaeology and Popular Culture . . . . . . . . . 71 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 3.2 The Archaeological Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 3.2.1 Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 3.2.2 Artifacts and Rock Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 3.2.3 Oral Tradition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 3.3 Celestial Objects in Popular Culture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 3.3.1 Astrology in Popular Culture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 3.3.2 Art and Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 3.3.3 Other Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 3.4 Garnering Public Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 3.4.1 Public Awareness and Support through Cinematic Film . . . . . . . . . . . . . . . 83 3.4.2 Public Education . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 3.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 4 Umm Al Binni Structure, Southern Iraq, As a Postulated Late Holocene Meteorite Impact Crater . . . . . . . . . . . . . . . . . . . . . 89 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 4.2 Geological Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 4.3 Origin of the Umm Al Binni Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 4.4 New Satellite Imagery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 5 Tree-Rings Indicate Global Environmental Downturns That Could Have Been Caused by Comet Debris . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 5.2 The Historical Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 5.3 Mythology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 5.4 What Actually Happened – the Global Consequences? . . . . . . . . . . . . . . . . . . . . . . . . 112 5.5 The Dust and Corrupted Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 Contents XI 5.6 The Scientific Prior Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 5.7 The AD540 Symptoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 5.8 Linkages to Other Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 5.9 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 6 The GGE Threat: Facing and Coping with Global Geophysical Events . . . 123 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 6.2 Volcanic Super-Eruptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 6.3 The Toba Super-Eruption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 6.4 Reassessment of the Super-Eruption Threat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 6.5 Collapsing Ocean-Island Volcanoes and Mega-Tsunami Formation . . . . . . . . 129 6.6 Volcano Instability and Structural Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 6.7 Environmental Triggers of Ocean-Island Volcano Collapse . . . . . . . . . . . . . . . . . . 130 6.8 Tsunami Generation from Ocean-Island Volcano Collapses . . . . . . . . . . . . . . . . . 131 6.9 Contemporary North Atlantic Mega-Tsunami Risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 6.10 High-Frequency GGEs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 6.11 Addressing the GGE Threat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 Part II · Astronomy and Physical Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 7 The Asteroid Impact Hazard and Interdisciplinary Issues . . . . . . . . . . . . . . . . . . 145 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 7.2 Near-Earth Asteroids (NEAs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 7.3 Consequences of NEA Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 7.4 Mitigation: Deflection and/or Disaster Management and Response . . . . . . . . 154 7.5 Perceptions of the Impact Hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 7.6 Societal Impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 7.6.1 The News Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 7.6.2 Religion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 7.6.3 The Military . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 7.6.4 Science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 7.7 Hazards Research/Disaster Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 8 The Impact Hazard: Advanced NEO Surveys and Societal Responses . . . 163 8.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 8.2 The Spaceguard Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 8.3 Sub-Kilometer Impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 8.4 Communication and Miscommunication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 8.5 Public Policy Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 XII Contents 9 Understanding the Near-Earth Object Population: the 2004 Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 9.2 Dynamical Origin of NEOs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 9.2.1 Near-Earth Asteroids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 9.2.2 Near-Earth Comets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 9.2.3 Evolution in NEO Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 9.3 Quantitative Modeling of the NEO Population . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 9.4 The Debiased NEO Population . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 9.5 Nearly Isotropic Comets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 9.6 NEA Size-Frequency Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 9.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 10 Physical Properties of NEOs and Risks of an Impact: Current Knowledge and Future Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 10.1.1 Key Questions before Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 10.1.2 The True Nature of NEOs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 10.2 Densities: from Feather to Lead? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 10.2.1 Determining Mass and Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 10.2.2 Typical Results on Densities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 10.2.3 Open Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 10.3 Structure: from Monoliths to Rubble Piles? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 10.3.1 Determining the Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 10.3.2 Outer Shape and Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 10.3.3 Porosity and Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 10.3.4 Comets Disruption and Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 10.3.5 Open Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 10.4 Surface Properties: from Sand Dunes to Concrete? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 10.4.1 Estimating the Surface Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 10.4.2 Typical Results on Surface Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 10.4.3 Open Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 10.5 Knowledge Expected from Future Science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 10.5.1 Remote Observations and Simulations under Development . . . . . . . . 198 10.5.2 Future Space Missions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 10.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 11 Evaluating the Risk of Impacts and the Efficiency of Risk Reduction . . . . 203 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 11.2 Near-Earth Objects Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 11.2.1 The Problem of Orbit Determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 11.3 Checking for Impact Possibilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 11.4 Eliminating Virtual Impactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 11.4.1 Decrease of the Risk Estimate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208