Duck and cover: how expensive does impact safety have to be?

This week is Tunguska week: on June 30 1908 a large meteoroid or comet exploded with the force of 5-30 megatons above the Tunguska River in Russia. The journal Nature celebrates it with several articles about impacts, ranging from a discussion of a controversial meteorite artwork to the confirmation that most of the northern hemisphere of Mars is a gargantuan crater.

From an ethics perspective the most interesting issue is how we should protect ourselves from rare but very destructive events. Had the Tunguska impactor hit an inhabited area it could easily have killed
millions, and larger impacts could imperil our species. But big impacts are rare: how much should we pay to detect and avoid them?

The good news is that the danger has been reduced by about 90% over the last years for a low budget and with little official support. NASA and other agencies have since the late 1990’s undertaken the Spaceguard Survey, attempting to find the NEOs (Near Earth Objects, asteroids that cross the Earth’s orbit). According to Alan Harris it has identified about 79% of the NEOs that could pose a danger. Since the largest are also the easiest to detect the survey appears to have reduced the total risk (measured in potential human deaths) by 90%. As Harris notes:

"Before the Spaceguard Survey, it was thought to be comparable to the risk of dying in a commercial aeroplane accident. Currently, however, the residual risk from the remaining undiscovered NEOs is more comparable to the risk of death from a fireworks accident"

Further surveys can be done, but would cost more since there are many more small NEOs. This might still be worthwhile from a scientific perspective. Yet recent detonations in the atmosphere suggest that a badly placed impact could be locally disastrous or trigger nuclear conflicts due to misunderstandings.

As an editorial in Nature points out, the unlikely matters. In many situations it is not the average that causes losses but extreme cases. Unlikely events also have unexpected and unprecendented effects that are hard to plan for since there is so little previous experience. A "black swan" can ruin your whole day, as the dinosaurs discovered.

This makes it more rational for politicians to fund further surveys. The cost is modest, while the risk of being remembered as the one who dismissed a disaster that later happened is real. However, cognitive biases such as our tendency to subjectively overestimate low objective probabilities, risk aversion and availability bias may actually make the decisionmakers overly willing to fund further searches. Conversely, of course, other cognitive biases may counteract these. All in all, we should not expect the decisions in this area to be unbiased or perfectly rational.

Protecting the Earth from future impacts will require impact readiness plans and scanning for new threats.  Projects to divert threatening NEOs would be costly but could be implemented with enough lead time: even a small push will have turned into a large shove years later. But the cheapest and most realistic way of dealing with not too large impacts is to let them happen. If the location is known sufficiently well in advance it could be evacuated if in an inhabited area, and tsunami preparations done if it will be in the sea. Even a few hours warning could be enough to have people take shelter in cellars or bomb shelters, likely reducing fatalities significantly. Impacts with larger climate effects can partially be countered by storing up grain stockpiles and seed banks if the lead time is a few years.

Scanning the heavens will still be necessary. Even if all major NEOs are accounted for long-periodic comets pose a problematic threat: they can occur at any time, they have somewhat unpredictable orbits due to outgassing, and would not give much lead time.

Much of this could be done for a very small budget ahead of time. The irony is that most public attention is focused on heroic diversion projects rather than boring civil defence and meticulous orbit tracking. Just like impacts, most risk reduction work is not about dramatic giant projects but a persistent rain of planning and observation projects, like falling stars. But it still requires us to look upwards and recognize what we need.

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