to their impact with Earth. The discovery of such objects shortly before impact provides an opportunity to save lives by evacuation or by suitable sheltering rather than by human changing of their orbits.
Based on these results, one could argue that a change is needed in the minimum diameter of the object to be included in the search, say, from 140 meters down to 50 meters. Nevertheless, the committee concluded that work on detecting these smaller objects should not be at the expense of detecting objects 140 meters and greater in diameter (see the recommendation at the end of Chapter 3). Additional information could change the relative statistical hazard associated with the various size ranges of NEOs as the following data are obtained:
Orbital distributions and collision probabilities for subkilometer-sized impactors;
More reliable estimates of the effects of Tunguska-like and larger impacts, including tsunami damage; and
Maps that more realistically account for human population distribution and growth.
As was clearly stated in the Stokes et al. (2003) and NASA PA&E (2006) studies, the completion of the survey as currently conceived will result in a significant amount of the residual statistical risk residing with the long-period comet population.
A key issue associated with the hazard from NEOs is that the length of time needed to execute a mitigation strategy involving orbit change is likely to require acting before the knowledge of the trajectory is sufficiently accurate to know with high confidence that an impact would occur without mitigation. It is possible, therefore, that action to mitigate could be deferred until it is too late if plans are not already in place to act when the probability of impact reaches some level that is well below unity. As addressed in Chapter 5, the time required to mitigate optimally (other than only by means of civil defense) is in the range of years to decades, but this long period may require acting before it is known with certainty that an NEO will impact Earth.
Chodas and Chesley (2009) have simulated the discovery of objects that would impact within the 50 years starting at the beginning of the next generation of surveys (see Chapter 3), using estimates of the (decreasing) orbital uncertainty as observations are accumulated. Although there are many assumptions in this approach, the most important is whether or not the surveys and the follow-up programs to determine the orbits will be funded and will operate as assumed. Chodas and Chesley (2009) assume that an NEO is declared “truly hazardous” and worthy of mitigation preparations when the probability of hitting Earth reaches 0.5 (any other assumption regarding the decision point is also easily simulated). In this simulation, about 90 percent of impacting NEOs larger than about 140 meters in diameter are discovered in a 10-year survey. The temporal distribution of discoveries in this simulation showed that several percent of the 140-meter-sized objects that impact do so before discovery, but the total number of impactors per century is not large, so that a few percent represents an exceptionally unlikely event. Most of the impactors in this size range are discovered to be truly hazardous within several years of discovery, typically at the next time that the object is in a location in which it is viewable, thus providing warning times of a decade to several decades. By contrast, more than 10 percent of the objects larger than 50 meters in diameter that would impact within 50 years do impact before discovery, and there are many more of these than there are of the larger objects. Such smaller objects would generally be found to be truly hazardous within weeks to months before impact. Objects in the size range of 10 to 50 meters in diameter make up the majority of all potentially hazardous NEOs larger than 10 meters. The damage that could be caused by one of these smaller objects is less than for a larger object, but those smaller ones that are detected are likely to be found, at most, hours to months prior to their final plunge, with civil defense then being the only plausible mitigation strategy.
Currently, by far the most probable scenario is that of a small impactor, likely to cause at most only local destruction. However, the assessed probability of any particular scenario is changing with time as the next-generation surveys discover most of the larger objects and the understanding of impact processes, such as airbursts and tsunami generation, improves. Thus, planning for mitigation must continue to evolve over time. Furthermore, when working with the statistics of small samples, and particularly when less likely scenarios have outcomes that