Of the two generic approaches to the mitigation of an impact hazard—(1) active orbital change or the destruction of the incoming body, and (2) passive, traditional natural-disaster mitigation based on “all-hazards” protocols for evacuation, sheltering, response and recovery, and so on—people in contemporary society would very likely be faced with evacuation and sheltering rather than orbital change or destruction during their lifetimes. The most probable event will be a very late warning of a small NEO (tens of meters in diameter or less). At the opposite end of the size spectrum for impacts approaching or exceeding the level of “civilization-threatening impacts” (100 to many hundreds of meters in diameter), there are inadequate precedents. For comparable events, one might think of the Black Death, world wars, or the fictional end-of-the-world stories in On the Beach (Shute, 1957) or, more relevantly, Lucifer’s Hammer (Niven and Pournelle, 1977). Whether human civilization would be fragile or robust in the face of such an event is unclear to us.
Although civil defense is the most likely response to any impact hazard, the committee did not possess the expertise needed to address fully the political and economic aspects of even a small-asteroid impact. This issue requires additional study.
There is a spectrum of potential events that might invoke one or more of the social, scientific, and emergency-management approaches to disaster mitigation. Some typical examples of such potential events, in rough order of increasing severity, include the following:
News media reports of a low-probability near- or long-term impact, warranting appropriate response informed by lessons in risk communication. Such occasions have happened frequently in the past decade and require no further societal action.
The prediction of an unusually high likelihood of a major impact (such as the Apophis case in 2004-2005 [Giorgini et al., 2008]) at some point in future decades. As planning was developed for a rational approach to orbit change, the “risk corridor” for locations where the impact might occur would be known. There could be some immediate economic and political implications (e.g., concerns about property values in potentially threatened locations), although further astronomical observations may change the probability of impact to zero.
The prediction of an imminent impact (in hours to days) impact by a very small object (1 to 10 meters in size) on an impact trajectory. This type of warning might begin to occur once every few years if telescopic searches are optimized for discovering such imminent impactors. Although it is very likely that such an impact would be harmless for people on the ground, prudent people near ground zero should stay indoors, away from windows, and perhaps not gaze at the atmospheric explosion. Such events might rain down meteorites or cause an on-ground explosive cratering event as in the case of the Carancas impact event in Peru in 2007, and could possibly break windows. Practicing risk communication would be important and would need to be planned in advance (see, too, Chapter 7).
A prediction with a short-term warning (days to weeks) of an impact by a small NEO (10 to 25 meters in diameter). Such an event is likely to occur during this century. Such impacts are near the threshold of causing significant and potentially lethal damage in a zone a modest few tens of kilometers wide near ground zero, warranting prudent evacuation if the impact occurred on or near land. Here, the approaches would be similar to established procedures for other predictable, localized natural disasters such as a flooding river or a volcano ready to explode. Of course, first responders would lack knowledge of the characteristics of such devastating events in locations where floods, volcanoes, and so on are not relevant. Thus plans should be made to ensure adequate knowledge transfer from experienced first-responders in the event that such a circumstance might materialize.
An unpredicted destructive impact by a modest-sized (10 to 100 meters) NEO. This case is about as likely as the previous one. Such an event could have modest to severe local consequences, but the customary response-and-recovery methodologies employed after natural and human-made disasters would generally be as applicable in this case as in any generic disaster. The kinds of damage from a modest-sized NEO in the atmosphere or impact into the ground would be similar to the kinds of damage from other natural disasters, including building collapse, fires, social confusion, injuries, and death. Of course, the cause of this particular type of disaster is unusual and would possibly evoke uninformed, exaggerated responses, such as fears that the impact was a harbinger of more