... there is a one in 909,000 chance of asteroid 2003 QQ47 impacting our planet...The rock is said to measure approximately 1.2 kilometres (less than a mile) across - only one tenth of the size of the impactor thought to have wiped out the dinosaurs 65 million years ago... "In theory such an asteroid could cause devastation across an entire continent," Christine McGourty says.
This is not necessarily a sterilizing event, but wiping out a continent would pretty much end our civilization.
So there's a 1 in one million probability of losing our civilization from this one rock. There are, however, many more of them. Based on what we've seen in the past few years, and applying some very rough and ready stats, the odds of a civilization ending impact in the next 100 years is probably on the order of 1/100,000.
The Economist had a good discussion on this statistic several years ago. Based on what we spend on prevention of typical risks (pesticide management, food poisoning, head injury, airplane crashes) we should be spending several billion dollars a year to develop a strategy to monitor asteroids and develop an avoidance strategy.
Human brains don't work well with managing this kind of risk though ...