OK, I am scared.
As of writing this, asteroid 2004 MN4 is going to hit earth with probability 2.2%. Impact would be equivalent to an explosion of 1300 megatons of TNT; by contrast the bomb dropped on Hiroshima had explosive strength of 0.013 megatons.
On the upside, if it does hit, it will happen in 2029 so there will be plenty of time left to eat, drink, and be merry.
Update: New estimate as of Dec. 27 - 2.7% chance of impact. The media seems to be ignoring the story (maybe its liberal bias - the media hates America so they are not reporting the substantial chance that the human race, including America, will die out in 25 years!). Seriously, this is huge. This wikipedia article on asteroid deflection may be of some interest; but given that no one has ever tried to deflect the course of an asteroid in real life, there is little ground for overconfidence.
Update: Further observations have ruled out the possibility that the aforemention asteroid, 2004 MN4, will hit the earth. Humanity survives for another day.