"Without the messiness of a nuclear bomb?" Yes, a nice clean comet impact is far preferable. If someone wanted to depopulate the planet - and I can't think of any particularly good reasons for doing so - certainly they could come up with a better way to do it than smashing it with a comet. And even if that was the plan, do these conspiracy theorists really think that figuring out how to impact a comet to precisely change its path so that it will hit the earth is an exact science? Even by CT standards, this one is pretty far out there.

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"Scientific progress goes 'boink'?"