The Cosmic Impact Hazard
Recognition of the hazard to life and property from impacts of comets and asteroids is currently leading to technical efforts to understand this hazard, estimate the resulting risk and plan for planetary defense. Public attention has been stimulated by the observed impact of Comet Shoemaker Levy 9 with Jupiter in 1994 and the destructive Earth impacts by small asteroids at Tunguska in 1908 and Chelyabinsk in 2013. The first task, to locate potential impactors, is well advanced, with telescopic discovery and orbit determinations for more than 15,000 near-Earth asteroids made in the past quarter century. No large asteroid has been found on an impact trajectory, but tens of thousands of small asteroids remain undiscovered. The current risk is dominated by a handful of undiscovered asteroids around 1-km diameter and by a much larger number with diameters 100–300 m. The larger task of developing technology to intercept, deflect or disburse any asteroid found to be on a collision course is much less mature. Planetary defense is an international problem, since all parts of the planet are at risk. Although at present, relatively few nations have the technology to contribute directly, broad international support for planetary defense is developing.
KeywordsPlanetary defense Impact hazard Asteroids Spaceguard Tunguska impact Chelyabinsk impact
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