Planetary defense preparedness: Identifying the potential for post-asteroid impact time delayed and geographically displaced hazards

A considerable amount of effort has been done to quantify impact effects from the impact of an asteroid. The effects usually considered are: blast, overpressure shock, thermal radiation, cratering, seismic shaking, ejecta, and tsunami (e.g. Hills & Goda, 1993; Collins et al., 2005, Rumpf et al., 2017). These first-order effects typically are localized in time and diminish with increased distance from the impact (or air burst) location.
However, there are delayed effects that will propagate through time and occur in areas not immediately affected by the initial impact. These delayed effects include, but are not limited to, down-stream and down-wind effects. Down-stream effects could occur months after the impact as sediment and debris are washed into reservoirs, potentially impacting water quality for populations not originally affected by the impact event. Down-wind effects could deposit dust and debris hundreds or thousands of kilometers down-wind, reducing insolation and ultimately settling out over large areas which could include cropland. Depending on when this occurs, significant damage could occur to croplands, thus reducing or eliminating whole sections of the global food chain. In addition, depending on the amount of ashfall, the deposition of dust and debris could cross watershed boundaries and thus affect water quality for a larger population than just those who live in the initial impacted watershed.
For most smaller asteroid impacts, these delayed effects can be neglected. However, there are likely a class of impacts (e.g. impactor size and composition, impact location and time of year) where failure to consider these effects could complicate post-impact relief and recovery efforts. For example, evacuation of the population within the initial damage zone from an impact to a city down-stream could exacerbate water quality issues and water usage months later. An impact in western Nebraska might have minimal civil defense requirements for evacuation (due to the low population density) but the down-wind effects could disrupt both the economic health of the American Midwest while threatening global food security. Understanding when these time-delayed and geographically displaced effects become relevant is key to successful civil defense and recovery planning.