Top 3 apocalypses - part I

I was talking yesterday about the possible End of Days in 2012, but let's have a look at the most realistic threats to human civilization. In my opinion there are 3 major catastrophes that may occur and modify forever this world we're living in.
My first catastrophic event is an asteroid hitting the Earth, event that nowdays is so vehiculated regarding the extinction of the dinosaurs. Of all the 3 events I've chosen for a probable Apocalypse this is the one with the smallest chances of hapenning. After all, each human has more chances of winning the big pot at the lottery 3 times in a life than a big asteroid destroying our planet.

We are hit every day by hundreds of small cosmic rocks, but 99,99% burn entirely in the atmosphere and only a few reach the Earth surface where can be collected and studied. We can even see "rains of stars" when the Earth is bombarded by thousands of asteroids showing a true magical show.
However this is the bright side, as the Earth was and surely will be struck by bigger asteroids that not only do they pass our safety measures identified by the atmosphere, but also they manage to hit the surface with devastating force that can have fierce effects. Last century, in 1908, a huge explosion devasted a huge surface of Siberian woods, with the equivalent of 1 000 Hiroshima atomic bomb. This event shows how real and close to us is this danger and only the luck made that asteroid hit a unhabitated area.

Probably that's why the big economical powers of the world have decided to invest some serious money in developing a global defence programme that uses many satelittes and observatories all around the world. Thier main goal is to identify any possible threat to the Earth and to do in time to let the humanity prepar for the worst.