Uncertainty and Risk at the Catastrophe Threshold

Abstract

The planetary defense enterprise is justified by a cost/benefit analysis comparing long-term average losses from unmitigated NEO impacts to the expense of preventing them. Probabilistic risk assessment is used because the risk is dominated by NEOs that have not yet been discovered and are on unknown trajectories. The number of lives saved has historically quantified the benefit side, but there is no unambiguous means to compare this quantity to the monetary cost of planetary defense. Nevertheless, it is a useful metric for intercomparison of various risk-reduction strategies. There is a power-law distribution of NEO sizes, and the consequences are a strong function of size with a presumed threshold for global catastrophe. Therefore, the most cost-effective means for reducing risk has been to focus first on surveys designed to discover asteroids large enough to exceed the catastrophe threshold. The possibility of a NEO-caused global catastrophe has nearly been eliminated, reducing the overall assessed risk by an order of magnitude so far. Eliminating the remaining risk will be more difficult and the returned benefit will be smaller. Barring the discovery of an asteroid on a collision course, at some point the planetary defense cost will no longer be justified by the marginal benefit.