Calibrating Asteroid Impact

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Science  29 Nov 2013:
Vol. 342, Issue 6162, pp. 1051-1052
DOI: 10.1126/science.1246250

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An asteroid impact on Earth about 65 million years ago caused a mass extinction, opening an opportunity for mammals and, eventually, human beings to evolve. We could suffer the dinosaurs' fate this century, but chances are extremely tiny. More realistic, though less catastrophic, threats come from much more numerous but much smaller near-Earth asteroids (NEAs). NEA impacts with the potential to kill millions of people, like the very largest floods, earthquakes, and hurricanes, occur far less than 1% as often as such natural terrestrial calamities. Indeed, truly dangerous NEA impacts, discounting mere meteorites that puncture roofs, occur so rarely that none have been reliably observed until this year. On page 1069, Popova et al. (1) describe the impact and atmospheric explosion of a 20-m-wide NEA over the Russian city of Chelyabinsk (population 1.2 million) on 15 February 2013, the first NEA impact disaster in modern history.