Free Astronomy Magazine September-October 2019

31 SEPTEMBER-OCTOBER 2019 SMALL BODIES in their accuracy. The only thing known with certainty is the effects produced on the environment, of which there are many images and some videos. Being able to compare those effects with the aftermath of an even smaller event, but for which we have all the useful information to validate the models applicable to the Tunguska event, can get us a sufficiently precise idea of the mass, volume, density and composi- tion of the small asteroid that triggered the disaster. As we said at the beginning, the recent Chelyabinsk event offered this opportu- nity, becoming a sort of “Rosetta Stone” to better interpret the Tunguska event. Un- like previous studies that had taken into consideration a small subset of cases based on typical physical and dynamic properties, the new research has greatly expanded the series, including a new probabilistic aster- oid impact risk model, to better evaluate asteroid trajectory, explosion energy and environmental effects. The model pro- duced 50 million possible combinations, able to cover the entire range of proper- ties of the impactor. All possible scenarios are sampled from probabilistic distribu- tions representing our current knowledge of asteroid properties, entry trajectories, and size frequencies. The results show that Tunguska-type events can be produced by a wide range of impact scenarios and also suggest that objects with diameters of 70- 80 meters and initial energies of 20-30 T he illustration on the right shows the perime- ter of the Siberian area devastated by the Tunguska event with a red outline. The same perimeter was then superim- posed on the met- ropolitan areas of three major US cities, to give an idea of what could happen if an event like that of 1908 were re- peated today in a densely populated region. The ani- mation below also extends the com- parison to other major cities around the planet. [Asteroidday.org] megatons are more likely to cause Tun- guska-scale damage areas than objects at the lower end of the potential size range. Comparing these values to those previ- ously considered closer to reality, it can be Washington, D.C. San Francisco New York City