But then there's the impact threat.
Sure, the chances of civilization (or even a single human) being sent off to the Great Asteroid Belt in the Sky are very low, but they're not zero, and they're high enough that we should be looking into this threat.
And astronomers do. There are observatories (though not enough IMO) dedicated to scanning the skies for incoming rocks, and NASA's Jet Propulsion Laboratory in Pasadena, California is one place where they take this seriously. In fact, they've created The Asteroid Watch webpage loaded with info on such dangerous objects. They have news, multimedia, quizzes, and more all about the danger from space.
They even have a way cool widget which will keep you up-to-date on which rocks are passing when, with a handy drawing of a building or a bridge to give you an idea of the size of the interplanetary interloper.
Not only that, they started a couple of Twitter feeds: AsteroidWatch with general info, and LowFlyingRocks which is a wonderful way of feeding into your paranoid fantasies of civilization-ending astronomical threats: it tells you what rocks just passed by the Earth.
All this is fun to me, because you should know the odds of getting killed in an impact event are roughly equal to getting killed on an amusement park ride: that is, pretty low. But remember, we have laws on amusement ride safety to protect us, so we should also have plans in place for cosmic cannonballs, too. JPL's efforts here are a great and fun step in raising public awareness about impacts, and that's the first step toward making politicians take it seriously, too.