You may have to risk a catastrophe to prevent one

In Richard Posner's 2004 book, Catastrophe: Risk and Response (which must be 6 or 7 books ago for him by now), he discusses a number of far-fetched if not completely implausible catastrophic scenarios. Among them is the spread of "Grey Goo", i.e., the uncontrollable spread of a culture of self-replicating matter-consuming nanomachines.

Posner also discusses the prospect of catastrophic meteor impacts. In the long run, there is a high probability of a potential impact that would kill many millions, were it to happen today. I say "potential" because we might develop the means to avert such a disaster.

We've seen nuclear bombs used in Armageddon and Deep Impact (comet, same difference). We might instead attach rockets to a meteor in order to alter its velocity, as Posner mentions. What Posner fails to consider in this context is a potential application of grey goo. A capsule full of mineral-consuming nanomachines could be launched to intercept the meteor. By the time it reached Earth, the approaching body could be consumed, its elements now comprising a larger clump of grey goo. Being a more diffusive substance, it would, perhaps, completely incinerate in our atmosphere before crashing into the planet and eating it.