tially hazardous NEO population of Stokes et al. (2003) and their assumed hazard due to airbursts and tsunamis, and (2) the recalculation based on the revised population curves shown in Figure 2.4 and the reassessed impact hazard for airbursts and tsunamis from Harris (2009), as illustrated in Figure 2.5, which, as noted earlier, may not be warranted.

Assuming that 85 percent of the NEOs with diameters larger than 1 kilometer have been discovered, which is close to the present state of affairs, Harris (2009) calculated the hazard statistics shown in Figure 2.7. Here the reassessed risk presented by the remaining 15 percent of the NEOs with diameters greater than 1 kilometer is comparable to that from all smaller objects. Figure 2.7 predicts that, in an actuarial sense, there is a long-term statistical average of about 91 fatalities worldwide per year due to impacts. Because the assessed statistical hazard from mid-range objects has dropped, the overall hazard has decreased as well. The drop from >1,000 to 91 expected fatalities per year clearly demonstrates the results of the Spaceguard Survey to date, which has “retired” the statistical risk from most objects above the assumed global catastrophe threshold. Using the Stokes et al. (2003) data for asteroids smaller than 1 kilometer in diameter yields a “humped” distribution with a peak near 300 to 400 meters. This hump may be significantly reduced when more realistic assessments of the effects of impact-driven tsunamis are available.

The residual hazard was used to establish the Stokes et al. (2003) goal that a future survey should try to identify 90 percent of the NEOs with diameters of 140 meters or greater. This limiting value, according to survey simulation of potentially hazardous NEOs, could remove a significant proportion of the remaining statistical hazard that still exists after the conclusion of the Spaceguard Survey. The completion of this survey does not change the probability of Earth impact for any undetected NEO. However, if none of the objects detected in the survey is on a collision course with Earth, the total statistical risk of impact is decreased as a result of the reduction in the total number of unknown potentially hazardous NEOs. Nonetheless, this survey may detect one or more NEOs on a collision course with Earth. (Carrying out a survey per se does not remove whatever risk there is; one just learns more about that risk.) In carrying out this survey, a substantial fraction of NEOs with diameters 50 meters and greater will also be discovered and catalogued. Although not specifically designed for the purpose, such surveys may also detect as many as half of the NEO “imminent impactors” larger than 10 meters in diameter in the hours to months prior

FIGURE 2.7 Estimated average fatalities per year for impacts by asteroids of various sizes calculated for the circumstances after 85 percent completion of the Spaceguard Survey. One histogram references the data used in the Stokes et al. (2003) study. The new data include changes resulting from newer estimates of the threat due to tsunamis and airbursts and from recent revisions to the size distribution of NEOs (Figure 2.4). SOURCE: Courtesy of Alan W. Harris, Space Science Institute.

FIGURE 2.7 Estimated average fatalities per year for impacts by asteroids of various sizes calculated for the circumstances after 85 percent completion of the Spaceguard Survey. One histogram references the data used in the Stokes et al. (2003) study. The new data include changes resulting from newer estimates of the threat due to tsunamis and airbursts and from recent revisions to the size distribution of NEOs (Figure 2.4). SOURCE: Courtesy of Alan W. Harris, Space Science Institute.



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