In no case, however, is it wise to consider the application of techniques more than a few decades into the future. The technologies available at that time would likely be both more efficient and more effective, rendering present approaches obsolete. However, it is not wise to wait for those future technologies, leaving Earth unaware and threats to Earth unmitigated in the meantime.

The remainder of this report is devoted to a description of the various aspects of the hazard that the committee has considered, to its findings and recommendations in response to the charge, and to its prioritization of the recommendations in the context of the committee’s somewhat arbitrarily chosen alternative budget levels for funding an NEO program. In particular, Chapter 2 is directed toward clarifying, as well as is now feasible, the risks associated with asteroid and comet hazards and the uncertainties in current knowledge of those risks. These studies of risk include both small and large potential impactors, their various possible orbits, the effects of airburst and ocean impacts, and the key issue of warning time.

Chapter 3 contains the committee’s analysis of the survey and detection questions, including currently mandated goals, their possible modifications, and the possible means—ground- and/or space-based methods—of achieving them.

Chapter 4 addresses characterization, the gathering of information on the properties of asteroids and comets that form the pool of potential impactors. The emphasis is on asteroids and on properties that would importantly affect any attempts at an active defense of Earth against an impending impact. The various properties of relevance are listed and their importance explained. Methods are described for characterization, ranging from laboratory studies of meteorites, through detailed observations of airbursts, to ground- and space-based remote and in situ observations of samples from the pool. This chapter also devotes special attention to the role of radar observations, consistent with the study’s charge, and to the complementary nature of the various means for characterization. A vital issue is the wide variation in the key properties from one object to another.

Chapter 5 addresses mitigation, examining the available techniques and the situations for which each is applicable. The goal is to avoid a collision through changing the orbit of (or destroying) an impactor headed for Earth. The committee also examined the state of (un)readiness of each technique and discussed the developments and tests needed to establish confidence that the countermeasures would work when called on. As to the deployment of any countermeasure, a main guide is the ancient maxim “First, do no harm.” Obedience to this admonition is not so trivial as it might appear. With the years-long warning times likely needed to complete a mitigation mission successfully, the corresponding accuracy of prediction of the impact might well be poor. In particular, the error ellipse that describes the uncertainty in the prediction of impact might well not approach the near-certainty desired, indicating the need for caution.

The committee’s work uncovered many facets of the overall problem that need attention in order to enable the sensible planning and execution of the options that were considered. The committee therefore recommends a research program, discussed in Chapter 6, to address these issues. Included among these topics are airbursts from impactors in the decameter-size range, with various compositions and structures, as well as the current distribution in the sky of objects that could impact Earth over, say, the next century or so. This proposed research program should include peer evaluation of proposals.

The collision hazard posed by cosmic objects is, as noted, global. It therefore seems sensible to deal with this hazard in its international context. Also needed is national leadership and responsibility. Chapter 7 discusses such leadership, noting that the Office of Science and Technology Policy has been tasked with addressing this issue. In Chapter 7, the committee emphasizes international aspects—organization, coordinated activities and responsibilities, and means for settling disputes that might arise in the planning stages and especially from a failed mitigation effort.

The committee was asked to produce independent cost estimates of typical solutions that it considered for survey completion and mitigation. To this end, the NRC contracted with Science Applications International Corporation to use parametric models and other statistical techniques to produce estimates of these options. However, the committee notes that many of these options are technically immature and that cost estimates at this early stage of development are notoriously unreliable. At best, these cost estimates provide only crude approximations of final costs of pursuing any of these options, so the committee did not use these cost estimates in reaching its conclusions. The cost estimates are included in Appendix A.

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