areas (notably the oceans) rather than in populated regions. Unlike the other hazards listed in Table 2.2, the hazard statistics for NEOs are dominated by single events with potentially high fatalities separated by long time intervals. Should scientists identify a large life-threatening object on a collision course with Earth, tremendous public resources to mitigate the risk would almost certainly be brought to bear. However, options for effective mitigation become much more limited when threatening objects are identified with only months to years, rather than decades or centuries, before impact. Thus, one of the greatest elements of risk associated with NEOs is the publicís expectation that governments will provide protection against any threat from NEOs, even as governments and agencies have been unwilling so far to expend public funds in a concerted effort to identify, catalog, and characterize as many potentially dangerous NEOs as possible, as far in advance of a damaging impact event as feasible.

Given these issues, there are a number of concerns that can be addressed by an NEO detection, characterization, and mitigation program:

  1. The statistical risk to human life and property associated with impacts of NEOs is real, but it falls outside the everyday experience of most of humanity. This risk must therefore be communicated effectively to the community at large in the context of other natural disasters, particularly those that the local community is likely to encounter. Scientists must carefully assess and explain the hazard so that appropriate public policy measures, commensurate with the level of risk, can be put into action.

  2. There must be an assessment of the statistical risk from NEOs that is reasonable and acceptable to the general public. The mandate of discovery of 90 percent of objects 140 meters in diameter or greater in the George E. Brown, Jr. Near-Earth Object Survey Act of 2005 was based on many assumptions about impact hazards. However, periodic reassessment of the impact threat needs to be performed as the knowledge base on NEO populations, their physical characteristics, and impact-associated processes increases.

  3. It is important to assess the length of time that the public is prepared to wait for scientific surveys to reach target goals of detection and characterization and for mitigation technologies to reach the desired maturity. Whereas surveys will never be 100 percent complete given the diversity of the objects, their origins, and their orbits, surveys should be as close as feasible to 100 percent complete in order to assure the public that all reasonable precautions are being taken.

  4. An assessment is needed of the levels of expenditure that the public is prepared to accept in order to reach such goals for detection, and similarly for characterization, and mitigation. Although the costs (other than for advanced mitigation strategies) are almost vanishingly small relative to other elements of the federal budget, public support for such activities may be absent lacking demonstration of a clear and present threat.

Undoubtedly issues 2, 3, and 4 above are strongly interrelated, as higher mandated percentage detections of increasingly smaller objects over shorter time periods would drastically increase cost. Equally, a comprehensive near-term mitigation strategy to address the full spectrum of possible NEO threats would be more expensive than a phased program of technology development. In the following chapters, various scales of NEO detection, characterization, and mitigation programs are presented that seek to identify a greater percentage of potentially threatening objects and to expeditiously develop the knowledge and capability to mitigate the risk associated with those objects. In addition, a program of research activities is presented to provide better constraints on the threat by various classes of NEOs impacting in diverse environments.

REFERENCES

Asphaug, E., and W. Benz. 1994. Density of Comet Shoemaker-Levy 9 deduced by modelling breakup of the parent “rubble pile.” Nature 370:120–124.

Baldwin, R.B. 1985. Relative and absolute ages of individual craters and the rate of infalls on the Moon in the post-Imbrium period. Icarus 61:63–91.

Boslough, M., and D. Crawford. 2008. Low-altitude airbursts and the impact threat. International Journal of Impact Engineering 35:1441–1448.

Boslough, M.B.E., and D.A. Crawford. 1997. Shoemaker-Levy 9 and plume-forming collisions on Earth. Near-Earth Objects, the United Nations International Conference: Proceedings of the International Conference held April 24-26, 1995, in New York, N.Y. (J.L. Remo, ed.). Annals of the New York Academy of Sciences 822:236–282.



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