Live from the shooting gallery: what price impact safety?

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?

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