Frequency of Impacts

Because there are far fewer large NEOs and long-period comets in space than smaller ones, the chances of a collision decrease rapidly with increasing size. The impact-hazard community—primarily scientists with an interest in the issue— has defined a global catastrophe to be an impact that leads to the death of one-fourth

Meteor Crater (or Barringer Crater), Arizona, U.S., a pit 1.2 km (0.75 mile) in diameter excavated about 50,000 years ago by the explosive impact of an object with the composition of a nickel-iron asteroid and a diameter of perhaps 50 metres (160 feet), at the low end of the size range for destructive impacts. Estimates of the energy released by the impact range between 15 and 40 megatons. D.J. Roddy/U.S. Geological Survey Manicouagan or more of the world's population. An impact by a 1-km- (0.6-mile-) diameter NEO, the smallest believed capable of causing such a catastrophe, is estimated to occur about once per 100,000 years on average, based on the assumed population in space of such objects. On the other hand, an impact by a 100-metre (300-foot) NEO, the smallest believed capable of causing regional devastation, is estimated to occur about once every 1,000 years on average. The hazard posed by long-period comets is less certain because fairly few such objects are known, but it is thought to be perhaps as high as 25 percent of that for NEOs.

The major difference between the threat posed by the impact of an asteroid or comet and that posed by other natural disasters is the extent of the damage that could be done. In some parts of the world at high risk for floods or earthquakes, the chances of dying in such an event are 100 to 200 times greater than the risk of dying from a

Average interval between impacts of NEOs of different diameters kinetic energy released (megatons of TNT)

100 100,000 100,000,000



10m global catastrophe ' threshold


1 km diameter

Estimated average times between impacts on Earth for near-Earth objects (NEOs) over a range of sizes and equivalent amounts of released kinetic energy. Because there are far fewer large NEOs than smaller ones, the chance of an impact drops off rapidly with increasing size. Objects of the size thought to have resulted in the explosion over the Tunguska region of Siberia in 1908 and in the Chicxulub crater off Mexico's Yucatán Peninsula 65 million years ago are located on the curve for reference. Encyclopedia Britannica, Inc.

cosmic impact. What distinguishes the impact hazard, however, is that it is the only known natural disaster, with the possible exception of an exceedingly large volcanic eruption, that could result in the death of a significant fraction of Earth's population and, in the most extreme case, the extinction of the human species.

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