As I am writing this post, asteroid 2012 DA14 is sweeping past Earth, inside the synchronous orbit (in fact, I am watching it on live webcast). Earlier today, an unrelated impactor disintegrated above Chelyabinsk, producing some dramatic footage and some injuries from shattered glass due to the sonic boom. It might have been the largest impactor over the last century, clocking in at hundreds of kilotons. It is no wonder people are petitioning the White House to mount a vigorous planetary defense against asteroids and comets. But what is the rational and ethical level of defense we need against astronomical threats?

2012 DA14 is estimated to be around 50 meters in diameter and moves at about 7.8 km/s – a building size boulder moving 26 times the speed of sound. Yet plugging it into the Earth impact effects program shows that it is unlikely to reach the ground: if it is made of rock it will begin to break up at an altitude of a few tens of kilometers, and a few kilometers from the ground it explodes into an airburst of about one megaton. If it were solid iron it would hit, making a kilometer-sized crater. This assumes a 45 degree impact; a more vertical impact does more local damage. But even in the worst case scenario we are talking about damage being done mostly within a few kilometers of the epicenter: not unlike a nuclear explosion, but without the heat and radioactivity.

The Chelyabinsk impactor was likely far smaller (perhaps 10 tons) and faster. Still, it looks like it is the only meteor in historical times that managed to cause widespread personal injuries and material damage. That it occurred at the same day as 2012 DA14 is merely a weird coincidence (it came from a different direction) but that we are living in an era where meteors actually hurt people is not a coincidence: there are simply more humans around today and in more places than in the past, so we should expect impacts to have bigger effects (and if one likes anthropic reasoning, we should of course expect to live in an era with particularly large population).

Neither of these objects is a global threat. Even larger near Earth objects like 99942 Aphopis would merely be a local disaster. In order to start registering as a threat to humanity asteroids need to be about a kilometer in diameter, at which point the environmental impact starts mushrooming. Estimates suggest that such an impact could cause a billion fatalities; since such impacts occur roughly once every million years the average risk is a thousand dead per year. The total risk from impacts is dominated by this tail of large and rare events: most years (or even centuries) nobody is hurt, but occasionally a large number of people are threatened.

In fact, surveys have determined 90-98% of the largest objects that could pose a danger, and none of them are a threat over the next century. Emily Lakdawalla describes the current state:

Near-Earth object surveys have found (we think) 98% of the largest objects that present the most risk, reducing the actuarial risk due to asteroid impacts from 250 fatalities per year to 64 per year. Based on past discovery rates and projecting forward through proposed future projects, over the next 16 years, we should achieve 90% completion of discovery of asteroids larger than 140 meters in diameter. The effect of this 16 years of work — at a cost of roughly a billion dollars — will be to reduce the actuarial risk to 33 fatalities per year. If you see asteroid surveys as a form of insurance, then you’re spending about two million dollars per fatality avoided. From the point of view of insurance, this is a relatively expensive effort. Harris’ point: “The hazard stuff might sell the program,” but in fact, the benefit is questionable; the real value of survey programs is in the science they produce. “The scientific value of deep surveys is such a treasure trove that it’s worth it right there.”

In terms of reducing risk, going from about a thousand to merely 33 fatalities per year might seem small potatoes, especially in the light of 36,000 fatalities due earthquakes and 5 million due to tobacco. (See also the interesting controversy over whether climate change is responsible for 150,000 and should be on the list) It is also an epistemic risk reduction: we were safe before we looked too, it just that now we know we are safe. Yet that risk was largely of a different kind than the earthquake and tobacco risk: neither has any chance of ending humanity whatsoever, while asteroids actually does have the ability to cause existential risk. Extinction is permanent and might have a far heavier moral weight than merely 7 billion deaths: it encompasses loss of future generations, perhaps the loss of the achievements of past generations, or even the total loss of the only value-experiencing minds in the universe, depending on your axiological theory. So reducing the tail risk for a billion dollars might actually have been a bargain.

But once that is done, what remains (besides for looking for the very rare new tail risk objects)? In a sense we are a victim of our own success. We are now increasingly able to detect even small NEOs as they sweep by, and we will find a large number of them. In 2008 the 80 tonne 2008 TC3 was detected 20 hours before it hit northern Sudan, exploding in an explosions equivalent to a few kilotons. Impacts like this happens two to three times a year. Sooner or later they will be predicted to hit more densely inhabited areas, most likely merely hours ahead of time.

Knowing that something like the Chelyabinsk bolide will show up ahead of time might be useful. Even if there are only a few hours of warning, telling people to stay away from windows would likely save many from injury: “duck and cover” is not a bad strategy. There are also generic safety improvements that reduce both meteor risks and many other kinds of risks, such as keeping building codes strict (prevents collapse and fires) and having earthquake or other civil defense drills help people handle unpredictable events (and not to panic about them).

But how much should we spend beyond that? Spending two million dollars per fatality avoided is not too far away from normal statistical value of life estimates. There might be biases acting here making us more willing to spend on single disastrous events rather than ongoing processes. Few people see smoking as 140 times as bad as earthquakes and think we should spend 140 times as much on reducing it as we spend on earthquake protection. The naturalness and lack of agency on the other hand reduces our interest: terrorism on average cause a few hundred deaths per year, but due to sheer outrage we respond fiercely and very expensively. The cost of watching space is pretty minor compared to what we spend there, or even on earthquakes.

It seems to me that the rational response is to maintain a vigilant spaceguard – most of the time it will mainly gather interesting science, but it will save us from some injuries from time to time. It will also at regular intervals sound warnings reminding us that we are living in the middle of space. That might be even more important than the actual risk reduction.