a verification mission that has a rendezvous with the NEO prior to the change in its orbit so as to measure this change; this approach should be implemented wherever possible.

The committee also notes that civil defense is likely needed in all mitigation scenarios, not just in those situations for which it is the most cost-effective approach. One aspect of civil defense is educating the public about the nature of the hazard and the manner in which individuals should respond. Public information about the hazard is crucial. For those impacts that cause very localized damage on the ground, there may nonetheless be peripheral effects on climate, probably small and of short duration but important enough that the public needs to understand them. There may also be effects on infrastructure, such as on communications, that extend well outside the area of direct damage. Dealing with these issues is all part of civil defense preparedness.

With the current uncertainty regarding both the properties of the NEOs themselves and the efficiency of an interaction with an NEO for kinetic and nuclear orbit change, and even from the general standpoint of confidence of success, functional redundancy is crucial. Instead of changing the orbit of an NEO with a single kinetic impactor, a series of impactors spread slightly in time provides much more reliability, and in some situations it might even allow assessment of the effect of the first impactor before the second arrived. Depending on the details of the specific orbit, it might be desirable and possible to divert later impactors, but the applicability of this concept needs further study. Alternatively, as long as there is a nuclear capability, one could consider readying a nuclear mission as a late-stage backup for a kinetic impactor that might, even with some very low probability, fail. Similarly, a kinetic impactor might be a backup for a gravity tractor on the chance that the gravity tractor might suddenly have a fuel leak or some other failure after a long but incomplete period of “pulling” the NEO.

A nuclear detonation approach, however implemented, is likely to raise significant public concern. If an NEO capable of massive death and destruction was discovered with certainty to be on a collision path with Earth and if there was no other way to stop it, presumably any concerns about the nuclear approach would be overridden. But in the early mitigation planning stages, public concern might inhibit development. This is primarily a public policy, rather than a technical question, and is therefore outside the scope of this committee’s task. Similarly, as noted above in the section on “Nuclear Methods,” the question of whether to maintain a nuclear stockpile for NEO mitigation purposes is not a technical question. In this report, the committee has assumed that a nuclear stockpile and nuclear development capability are on hand for other purposes.

Perhaps the most significant conclusion that can be drawn is the large uncertainty in the effectiveness of the mitigation techniques because of their dependence on the physical properties of NEOs that are not well known, and because of the difficulty of scaling any laboratory experiments to this regime. At this point it is not even possible to determine reliably the boundaries of applicability of the various approaches. In a later chapter the committee addresses organizational aspects of the decision-making process, but it still lacks information to guide that process. Any process must carry out a detailed study of where to draw the boundaries and what additional information would be needed. An applied research program, directed explicitly at the NEO hazard, could significantly reduce the uncertainties. At the lowest meaningful level of investigating the mitigation issues, this program would include both numerical simulations by multiple groups and laboratory experiments.

A much-larger-scale effort to address the mitigation of NEO hazards will likely include activities in space. The single most-significant step in this area appears to be a kinetic impact mission on a far larger scale than the Deep Impact mission, employing a much larger impactor on a much smaller target, with another spacecraft that has a rendezvous with the target well prior to impact to characterize the target and its orbit very precisely. This characterizing spacecraft would remain with the target until long after the impact in order to determine accurately the change in its orbit resulting from the impact. The Don Quijote mission that was studied by the ESA but is no longer under active consideration would have addressed most of these goals. Suggestions have been made to use the rendezvous spacecraft as a gravity tractor after the primary mission, but given the different design considerations it is not yet clear whether this is a good approach or not. A demonstration flight of a gravity tractor appears to be the second most significant step, since lesser knowledge of NEO behavior is needed for implementation. Both the kinetic impact and gravity-tractor approaches require significant engineering study, but more basic knowledge is needed for the kinetic impactor.

In cases of the late discovery of a hazardous NEO, the change in the NEO’s orbit that must be made for it to miss Earth can be so large that the required impact energy is comparable to or greater than the energy to disrupt



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