Impact Risk Estimation and Assessment Scales

This chapter addresses the current state of the art in assessing the impact risk from any of the known near-Earth objects over the next century or so. The assessment is made by determining the orbits of potentially hazardous asteroids and comets using the latest sets of tracking measurements and then projecting into the future the possible positions for these objects during close approaches to the Earth. The chapter discusses various computerized risk assessment systems that perform these assessments, along with the scales that are used to assess these risks. The second part of the chapter addresses two important issues in accurately predicting the risk of impact on Earth that have been identified and addressed in recent years. The first of these is the “Yarkovsky effect,” which is the small recoil acceleration acting on an asteroid due to thermal emissions from its surface and which can now be detected, modeled, and accounted for. The second topic to be discussed is the phenomenon of “keyholes,” which are gravitational gateways that can take an asteroid from a close approach on one passage by the Earth to a later impact with our planet. Mapping the keyholes for a potential impactor is an important step in assessing the asteroid’s impact hazard and in planning a possible deflection mission.