(X-posted to the DN blog). For me there are several main take-away points: 1) It continues to amaze me that Patrick can photograph a grey colored the size of a school bus at 122,000 miles. 2) More recent modeling of the 1908 Tungska airblast reduces estimates of the blast size from 20 Megatons to between 3 to 5 Megatons and can occur on the scale of several hundred years. Even these relatively smaller blasts can have ground effects. 3) Mortality models for a repeat of a Tungska like event take into consideration recent disasters, e.g. the Indonesian tsunami. Past discussions have centered around a scenario of Tungska-like airburst over land. More recent thinking, reflected in the NAS report, center on a 3 to 5 megaton airburst over water - the blast wave of which reaches the surface of the ocean and generates a tsunami. Because of higher population levels near coastlines (compared to 1908), such a tsunami would, although relatively small in size, would wash ashore and drown about 9,100 people once every 100 years. 4) The comparision of the estimates for risk of death by shark (3 to 5 people every 1) or death by asteriod (91 per year) to death by automobile (+1 million per year) is not properly equivocable. Two the estimates are infrequent by daily ordinary incidents. The third is a castrophic event. The above info may be useful for responding to "death from above" questions at this year's star parties. Bad Astronomy's Phil Plait's 2008 book - Death from the Skies!: These Are the Ways the World Will End - looks like a good entertaining read on this topic, but I haven't bought a copy.. Clear Skies - Kurt National Research Council, National Academy of Sciences. Jan. 23, 2010. Press Release on Issuing of Final Report titled "Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies" at pages 19-20. ----------------- Our understanding of the immediate damage produced by land impacts capable of producing craters is reasonably mature because their effects are constrained by nuclear weapon tests as well as craters on planetary surfaces. For airbursts, however, a lot of work is needed to improve our understanding of their consequences. For example, many groups have studied the 1908 Tunguska blast. Using insights from nuclear blast data as well as seismograms and barograph records of the Tunguska event, scientists estimated that the height of the explosion was about 10 km and that the energy yield was 10 to 20 MT (Chyba et al., 1993). According to the new estimate of size distribution made by Harris (2009), the average interval between such events on Earth would be on the order of one every 2,000 years. Work by Boslough and Crawford (1997; 2008), however, indicates that a much lower yield could produce the same effects. They found that asteroid airbursts do not act like point explosions in the sky (e.g., like a nuclear bomb explosion) but instead are more analogous to explosions along the line of descent. In an airburst, kinetic energy (see Appendix E) is deposited along the entry path, with significant downward momentum transferred to the ground. Accordingly, they suggest that smaller explosions with net yields of 3 to 5 MT may be sufficient to produce Tunguska-like impact events. If true, the average interval between Tunguska-like events using the Harris (2009) size distribution (see Figure 2.4) would be on the order of a few hundred years. These results would increase the calculated hazard from smaller objects, perhaps as small as 30 meters or so. Further research is needed to better characterize this threat. -----------------