Asteroids that could collide with the Earth are listed on the publicly available Near Earth Object (NEO) hazard web sites maintained by the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). The risk of 69 potentially threatening NEOs that produce 261 dynamically distinct impact instances, or Virtual Impactors (VIs), has been calculated using the Asteroid Risk Mitigation and Optimization Research (ARMOR) tool. ARMOR calculates the impact risk in terms of expected casualties based on three factors: impact probability, exposure and vulnerability. First, the impact probability of each VI is projected onto the surface of the Earth as a spatial probability distribution. The projection considers orbit solution accuracy and the global impact probability. Second, the global population distribution is introduced and represents the exposure to the hazard. Finally, the vulnerability of the population to the physical impact effects produced by a colliding asteroid is calculated. Impact effects are calculated based on asteroid size, impact speed and impact angle and the effects are: crater formation, thermal radiation, seismic shaking, overpressure shock wave, strong winds and the deposition of an ejecta blanket. Population vulnerability is determined based on the severity of the impact effects at a given distance from the impact site. Factoring together impact probability, exposure and vulnerability allows calculation of the risk for each VI as well as the combined risk of the 69 asteroids. To account for the uncertainty in the impact effect models, ARMOR produces three scenarios that represent the least harmful, the expected and the worst case outcomes. Because the risk calculation is dependent on the current impact probability, the risk calculation is subject to significant variability based on the availability of new asteroid observations. The calculated risk expresses the current best estimate of expected casualties that are associate- with each asteroid. The method has the potential to form the basis of a new impact hazard threat scale similar to the Torino or Palermo scale. The results are presented in the form of global spatial risk distributions and as quantitative analysis.
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