Just by coincidence, the name of this blog site "Notes Experiences and Opinions" produces an acronym which also matches "Near Earth Object", which I find an interesting subject, in particular after the cancelled doomsday, allegedly predicted by the Mayan calendar.
A NEO (Near Earth Object) is a celestial body which orbits closer enough to Earth and which is big enough to threaten Earth in a regional or global scale. An example is an event which happened in Tunguska in 1908, which caused a big devastation due an explosion of something between 5 and 30 megatons.
If you are disappointed with the Doomsday which was a complete fiasco, you might be pleased to know that there's an asteroid which will approach us very very closely next year, more exactly on 16/Feb/2013. It will flyby 45,000km from Earth, which is approximately 10 times closer to us than the Moon is (see orbit). Despite very close, it will not collide to us.
This sort of near miss is relatively common but collisions are very, very rare. For the future, calculations show that we are not at risk for centuries to come.
If you like Astronomy you may also consider supernovas or other mighty celestial bodies emitting lethal gamma rays and so on. Don't worry: a supernova would be a threat to us only if closer than some 25ly (light-years) from us. As far as we know, there are no candidates for such cataclysm sufficiently near to us. Our Sun will become a nova some day and will certainly destroy this planet... but don't worry: it still will take 5 billion years to happen.
Looks like there's no astronomical cataclysms which can offer any danger to this planet anytime soon. In this case, we are forced to face a very simple and hard reality: we are still the biggest threat to this planet.