There are many ways the world could end. Nuclear winter, alien invasion, super-volcanoes, the sun going supernova, everyone plugging in hair dryers at the same time. And it will happen some day - maybe not for billions of years, long after the human race has died out, or maybe it will happen tomorrow out of the blue.
None of the outcomes scares me as much as a rock the size of Texas slamming into the planet at hundreds of thousands of miles per hour.
Sure, alien invasions and zombie apocalypses would suck. But at least we could try to fight back. And most likely, those things would be our fault, that could have been avoided, so at least we have ourselves to blame. But if the world ends because of a freak cosmic collision with no warning that... just seems anticlimactic.
It's happened before. It is widely accepted that a massive asteroid impact caused the extinction of the dinosaurs some 65 million years ago. Scientists just recently discovered evidence that a much larger rock (estimated to be 1000km across) smashed into the Earth about 3 billion years ago that probably shaped the geological face of the planet. It's fair to assume another impact could reshape it again, and we as humans are pretty happy breathing oxygen, thank you.
There are untold billions (trillions?) of rocks floating out in space, travelling at massive speeds. NASA is hard at work mapping what they can, but they can't spot everything. Large asteroids pass close to the Earth all the time with little-to-no warning. It's inevitable that one of them will eventually pass close enough, and be large enough, to cause significant damage on impact. And it may happen so suddenly that we have just a few weeks or even days to say goodbye before life as we know it is obliterated in the blink of an eye.
There's no reason to panic. The "inevitable" part is a huge stretch of time in cosmic terms. Yes, that could be tomorrow, but it could just as easily be a billion years from now. And since such an event would be so sudden and cataclysmic, it's hardly worth worrying about. We can't really stop it or plan for it. It's just something that might... big might... happen someday.
Still, makes you wonder, doesn't it?