occurring on average every couple of centuries. They are also the events that are likely to have the least advance warning. For larger events, actively changing the orbit of the hazardous object is likely desirable. The choice among the three methods—the slow-push and -pull method, kinetic impact, and nuclear detonation—depends both on the mass of the NEO that has to be moved and on how early the NEO is determined to be hazardous, as well as on the details of the orbit. The mitigation options are laid out in Table 5.1, which lists the applicability of each option to a given threat. Table 5.2 shows the regimes in which each mitigation method is applicable. Note that Table 5.2 brings in an additional important aspect of the problem, international coordination, which is discussed in more detail in Chapter 7 of this report.

Although all of the primary mitigation strategy methods are conceptually valid, none is now ready to implement on short notice. Civil defense and kinetic impactors are probably the closest to deployable but even these require additional study before they can be relied on.

In all cases, the decision to initiate mitigation is a sociopolitical decision, not a technical decision. This decision is implicit in earlier sociopolitical decisions about which methods of mitigation to develop, and it also depends on the level of probability that is considered to require mitigation. The committee’s recommendations regarding the minimum approach to mitigation and more aggressive approaches are discussed later.

The subject of mitigation is rife with uncertainty. The effect on Earth of a given NEO depends critically on the velocity at which the NEO impacts Earth, a factor that is traditionally ignored in studies of the hazard. The decisions on mitigation must be based on the mass of the NEO rather than on its diameter, because mass is the quantity that most affects the effectiveness of any mitigation and the diameter for a given mass can vary by roughly a factor of two. The variation in diameter implies a factor-of-two variation, depending on the NEO’s density, of the size of an NEO that can be moved far enough to miss Earth. Clearly an earlier warning allows a smaller action to be sufficient, but quantifying this relation is very uncertain. The effectiveness of most but not all methods also depends critically on the physical properties of the NEO. Humanity’s ability to mitigate depends on the details of the intercepting trajectory. There are also significant differences depending on whether the discussion of mitigation is limited to current technology or includes likely future technology such as the next generation of heavy-lift launch vehicles. Thus the committee’s discussion of the range of applicability will show overlapping and uncertain ranges.

Realistic mitigation is likely to include more than one technique, if for no other reason than to provide confidence. In any case of mitigation, civil defense will undoubtedly be a component, whether as the primary response or as the ultimate backup.

Finding: No single approach to mitigation is appropriate and adequate for completely preventing the effects of the full range of potential impactors, although civil defense is an appropriate component of mitigation in all cases. With adequate warning, a suite of four types of mitigation is adequate to mitigate the threat from nearly all NEOs except the most energetic ones.

TABLE 5.1 Summary of Primary Strategies for Mitigating the Effects of Potential Impacting Near-Earth Objects


Range of Primary Applicability

Civil defense

(e.g., warning, shelter, and evacuation)

Smallest and largest threats.

Threat of any size with very short warning time.

Slow push

(e.g., “gravity tractor” with a rendezvous spacecraft)

A fraction (<10%) of medium-size threats.

Usually requires decades of warning time.

Kinetic impact

(e.g., interception by a massive spacecraft)

Most medium-size threats.

Requires years to decades of warning time.

Nuclear detonation

(e.g., close-proximity nuclear explosion)

Large threats and short-warning medium-size threats.

Requires years to decades of warning time.

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