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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies 6 Research Dealing with the hazards of near-Earth object (NEO) impact is complicated because it involves balancing the imprecisely known risks of this hazard against the costs, risks, and benefits of proposed responses. Since the NEO impact risk is partly probabilistic in nature, it is difficult to grasp and difficult to communicate unless and until an object is discovered that will hit Earth at some definite date not too far in the future. However, the probabilistic risk is similar to that for other types of natural disasters like earthquakes. Scientists have an idea of the likelihood that an earthquake of a given magnitude will strike a given region within a given time. The fundamental reasons why earthquakes occur are known (they are associated with plate tectonics), and it is known that the risks from earthquakes are particularly high in certain specific regions (e.g., near plate boundaries, in certain types of soil). However, no one can predict with confidence the date of the next great earthquake of magnitude 7 or larger that will strike San Francisco or Tokyo. Nevertheless, it is known from experience that such disasters will occur, and moreover experts can assess the likely damage. The United States and other countries around the world have responded to the risk of earthquakes by committing to various civil-defense and mitigation programs, including research programs. The U.S. federal and state governments dedicate resources to earthquake research in order to improve the understanding of the causes of the hazard, to better quantify risks and to improve the capabilities for prediction, and to increase the effectiveness of mitigation measures. Likewise, an appropriate and necessary aspect of mitigation of the NEO impact hazard is a research program. The scope of this research program on NEO impact hazards would ideally be targeted to address all of the areas in which uncertainties stemming from a lack of knowledge and/or understanding hamper scientists’ ability to quantify and mitigate the NEO impact risk. For instance, there is uncertainty as to the magnitude of the impact risk for several reasons. One reason is that the populations of small potential Earth impactors are poorly understood, so there is uncertainty even about the average impact rates by objects greater than 140 meters in diameter or greater than 50 meters in diameter. Another reason for the uncertainty with respect to the magnitude of the risk is that the fundamental natures of these bodies are not known: what they are made of, or to what extent they may be intact objects as opposed to heavily fractured, or even completely separate, components traveling together as loose, gravitationally bound aggregates. Some 15 percent of known NEOs have one or more satellites. Furthermore, even given knowledge of the size, impact energy, and fundamental nature of an impacting object, the effects of the impact on Earth are uncertain. They depend on whether and how high in the atmosphere the impactor may break up before hitting the surface, and on whether an impact occurs on shallow water, on deep water, or on land, or on any of the rock types found there. In addition, the impact effects would not necessarily be limited to local
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies or regional effects near the time and place of the impact, but could include, for large impacts, global climate change or tsunamis. But how large an impact and what kind of impact could cause these effects is still uncertain. A research program is needed to address all of these issues in order to assess and quantify the risks associated with the NEO impact hazard. The ability to mitigate the impact hazard, or even to define appropriate strategies for mitigating the hazard, likewise depends on the acquisition of the new knowledge and understanding that could be gained through a research program. Even if the only viable mitigation approach to an impending impact is to warn the population and to evacuate, better information is needed for making sound decisions. Under what conditions should warning be provided and when, and who should evacuate? If, however, there are available active mitigation options, like changing the orbit of an impactor, again better information is needed: One must be able to predict with confidence the response of an impactor to specific forms of applied forces, impacts of various types and speeds, or various types of radiant energy, such as x rays. The required information goes beyond the basic physical characterization that determines the size and mass of the impactor and includes surface and subsurface compositions, internal structures, and the nature of their reactions to various inputs. Just as the scope of earthquake research is not limited only to searching for and monitoring earthquakes, the scope of NEO hazard mitigation research should not be limited to searching for and detecting NEOs. A research program is a necessary part of an NEO hazard mitigation program. This research should be carried out in parallel with the searches for NEOs, and it should be broadly inclusive of research aimed at filling the gaps in present knowledge and understanding so as to improve scientists’ ability to assess and quantify impact risks as well as to support the development of mitigation strategies. This research needs to cover several areas discussed in the previous chapters of this report: risk analysis (Chapter 2), surveys and detection of NEOs (Chapter 3), characterization (Chapter 4), and mitigation (Chapter 5). The committee stresses that this research must be broad in order to encompass all of these relevant and interrelated subjects. Recommendation: The United States should initiate a peer-reviewed, targeted research program in the area of impact hazard and mitigation of NEOs. Because this is a policy-driven, applied program, it should not be in competition with basic scientific research programs or funded from them. This research program should encompass three principal task areas: surveys, characterization, and mitigation. The scope should include analysis, simulation, and laboratory experiments. This research program does not include mitigation space experiments or tests that are treated elsewhere in this report. Some specific topics of interest for this research program are listed below. This list is not intended to be exhaustive: Analyses and simulations of ways to optimize search and detection strategies using ground-based or space-based approaches or combinations thereof (see Chapter 3); Studies of distributions of warning times versus sizes of impactors for different survey and detection approaches (see Chapter 2); Studies of the remote-sensing data on NEOs that are needed to develop useful probabilistic bases for choosing active-defense strategies when warning times of impacts are insufficient to allow a characterization mission (see Chapter 4); Concept studies of space missions designed to meet characterization objectives, including a rendezvous and/or landed mission and/or impactors; Concept studies of active-defense missions designed to meet mitigation objectives, including a test of mitigation by impact with the measurement of momentum transfer efficiency to the target (see Chapter 5); Research to demonstrate the viability, or not, of using the disruption of an NEO to mitigate against an impact; The technological development of components and systems necessary for mitigation; Analyses of data from airbursts and their ground effects as obtained by dedicated networks, including military systems and fireball (brighter than average meteor) observations; also analyses and simulations to assess
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies the following: where, why, and how objects break up in the atmosphere; what the effects of airbursts are, including pulses of electromagnetic energy and consequences for communications and other infrastructure; and what the effects of target material properties for land or water impacts are; Detailed, realistic analytical analyses and simulations to determine the risks of tsunami generation from water impact or airbursts of various types and sizes of impactors; Joint analyses, when possible, of available data on airbursts and data on the corresponding surviving meteorites to establish ground truth; Laboratory study of impact phenomena for a wide variety of impacting and impacted material (i.e., of various physical structures and properties) at speeds of collision up to the highest attainable so as to study, for example, the transfer of momentum to the target due to ejecta of material from it; Leadership and organizational planning, both national and international; The economic and political implications of an NEO impact; and Behavioral research (including national and international workshops) for studying people’s perception of impact risks, including their mental models, and for increasing the understanding of their possible misconceptions and/or lack of knowledge, needed to develop appropriate plans and simulation exercises in preparation for a possible impact event.