should enable the use of a variety of methods for NEOs up to two times larger than is possible with current launch vehicles.

DISRUPTION

Both the kinetic impact and the nuclear detonation mitigation methods are capable of including larger changes in the velocity of the NEO than those discussed above, particularly for smaller objects; in those cases, however, these methods deliver so much energy that there is a likelihood of totally disrupting the NEO (i.e., fragmenting it). Disruption has been widely proposed as a mitigation option, but disruption could make the situation worse. Specifically, if the hazardous object breaks into a small number of large fragments with only a very small spread in velocity, the multiple impacts on Earth might cause far more damage than a single, larger impact. Thus, disruption or fragmentation is a sensible strategy only if it can be shown that the hazard is truly diminished. In the case of a very large impactor (e.g., a 10-kilometer-diameter, civilization-destroying NEO) discovered without many years of warning, adequate orbital change may not be possible, leaving disruption as the only option for mitigation. This option would likely require a system on standby at all times and a decision to disrupt made long before the probability of impact was high. Even in this situation one would want assurance, from previous studies, that disruption would both succeed and reduce the hazard.

Numerous studies of the catastrophic disruption of asteroids, undertaken in order to increase the understanding of the evolution of the asteroid belt, have shown that the energy required for catastrophic disruption per unit of mass of an asteroid has a minimum for bodies with diameters of a few hundred meters (e.g., Holsapple, 2002). These calculations, of course, assume physical properties for the asteroids, and those properties are not well known in any particular case. Early laboratory experiments and subsequent basic physical and numerical simulations (Housen and Holsapple, 1990; Michel et al., 2004) show that when an asteroid is catastrophically disrupted, only one large fragment remains, and the size of that fragment shrinks with increasing energy of the impact. Furthermore, energy arguments imply that most of the other fragments disperse with velocities comparable to or greater than the escape velocity from the original body, that is, >1 meter per second for a kilometer-sized NEO. To the extent that these calculations and laboratory experiments are relevant, they suggest that disruption might leave one much smaller object on an impact trajectory, with most of the other pieces spreading out over a cross section much larger than Earth within less than a year.

Thus disruption might be a useful mitigation technique. However, the uncertainties in the structure of NEOs are sufficiently large that this committee does not now have high enough confidence in the disruption approach to recommend it as a valid technique for mitigation at this time. Additional research, including a suite of independent calculations and laboratory experiments, but particularly including experiments on real comets and asteroids, might show that disruption is well enough understood to use as a mitigation technique.

To avoid disruption, both kinetic impact and nuclear detonation approaches to orbit change benefit dramatically from using multiple events. (They also allow the effective orbit change of larger NEOs, but disruption is rarely an issue in that case.) This strategy also allows for the adjustment of the total effect when the hazardous object’s response to an event is not accurately predictable in advance.

SUMMARY

Figure 5.5 summarizes the range of parameter space in which each of the four types of mitigation could be considered primary, emphasizing the still-significant uncertainty in the boundaries between the various regimes. Other parameters (density of the NEO, details of the NEO’s orbit, probability of impact at a given warning time, etc.) all play a role in the uncertainty. Furthermore, civil defense should play a role in all of the regimes, and one might choose to apply multiple methods in a given case, thus further blurring the distinctions. Toward the left edge of the figure, representing short warning times, one would likely be able to carry out nothing but civil defense, unless disruption was shown to be reliable; toward the right edge of the figure, representing long warning times, the uncertainty in the prediction might discourage action. Toward the right half of the figure, there would often be time to design, build, and launch a mitigation mission. Toward the left half, one might need a mission ready to



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