The Asteroids Are Coming!
And there is a chance one might kill you!
An extremely unlikely chance, says Mike Brown. About 1 in 74 million, meaning that there are 73,999,999 other ways that are equally likely to knock you off.
Of course the occasional biggie can flatten a city or even a continent, but Brown says we know there aren't any of the continent smashers headed our way for at least several hundred years. Consequently, there is no urgency whatsoever to rush to build a zillion dollar asteroid defense system. We should continue to study asteroids, mainly for other reasons, but just monitor the situation. If one is headed here in 500 years, we can hope that our descendants will be a lot better prepared to deal with it then.
Comets are another problem. Partly because they are moving much faster, so they have a lot more kinetic energy. Mostly, though, the problem is that we won't have, can't have, much warning. A year or two at most. Consequently, our prospects for doing anything about a comet with our name on it are approximately zero.
Or, more to the point, don't panic*. And don't spend tens of billions panicking about them.
*But you might want to carry a towel.