Dealing with the Impact Hazard

Abstract

The small fraction of asteroids with Earth-crossing or Earth-approaching orbits is of special interest to us because many will eventually impact our planet. The time-averaged impact flux as a function of projectile energy can be derived from lunar-cratering statistics, although we have little information on the possible variability of this flux over time. Alternatively, we can use current observations of near-Earth asteroids (NEAs) to derive the size distribution and flux of impactors. The effects of impacts of various energies can be modeled, using data from historic impacts (such as the K/T impactor 65 m.y. ago) and the observed 1994 bombardment of Jupiter by fragments of Comet Shoemaker-Levy 9. Such models confirm that the terrestrial biosphere is highly vulnerable to severe perturbation from impacts, so that even such a small event as the K/T impact (by a projectile 10–15 km in diameter) can lead to a mass extinction. Combining the impact flux with estimates of environmental and ecological effects reveals that the greatest contemporary hazard is associated with impactors near 1,000,000 MT energy. The current impact hazard is significant relative to other natural hazards, and arguments can be developed to illuminate a variety of public-policy issues. These include the relative risk of different impact scenarios and the associated costs and probabilities of success of countermeasures. It is generally agreed that the first step is to survey and catalog the larger NEAs. To that end, we review the status of the Spaceguard Survey, which has already discovered more than half of the NEAs larger than 1-km diameter, out of a total population estimated to be between 1000 and 1200. We compare the efficiency of survey approaches and consider the challenges of international coordination and the problems and opportunities associated with communicating the results with the press and the public. It is also important to reflect on how the impact hazard might be dealt with by both national governments and international decision-making bodies and to anticipate ways of mitigating the danger if a NEA were located on an apparent Earthimpact trajectory. As the most extreme known example of a natural risk with low probability but severe global consequences, the NEA impact hazard calls for the most careful consideration and planning.