Unless you are living in a cave, you will have heard by now of the huge meteor exploding over Russia. Except that it wasn't particularly huge, and it didn't explode. The Chelyabinsk meteor weighed about ten tons, [UPDATE: Nature says it was 7000 tons. That's a big discrepancy, but maybe they meant 7000 kilograms. I'm trying to find out which source was wrong. and the "explosion" was just the shock wave that it produced as it plowed through the atmosphere at supersonic speeds. If the meteor was made of iron (which it almost certainly was) it would have been about a meter or two in diameter. [UPDATE2: Nature was right, it was 7-10 thousand tons. That would make it about 10 meters in diameter.]
Those are not the scary rocks.
The scary rocks are the ones that are 100 meters or so in diameter, about the size of a football field. Those are scary because they are too small to be seen with ground-based telescopes, but when -- not if -- one of those hits it will without a doubt ruin your day no matter where on earth you happen to be. Watch the videos of the Chelyabinsk event and then imagine a rock that weighs a million times more. Such a rock hits the earth on average once every 10,000 years. A Chelyabinsk-sized rock hits every 100 years or so. The last one was in 1908, so this one was right about on schedule. (Tunguska was almost certainly a comet, not an asteroid, but the energy released was comparable -- Tunguska was probably a little higher. [UPDATE] I got this wrong. Tunguska was a LOT bigger, but because it disintegrated in the atmosphere over an unpopulated area it didn't cause much damage other than a lot of toppled trees.).
The asteroid that wiped out the dinosaurs was about ten kilometers in diameter. Those hit every hundred million years or so. They're pretty rare, and they're big enough to see, so we know that we aren't in any imminent danger. (Still, it's a little sobering to think that about forty of those have hit the earth since it was formed!)
The little rocks, like the one that hit Chelyabinsk, mostly burn up in the atmosphere or cause minor damage.
But, like I said, the 100-meter-sized rocks are the scary ones. There are a lot of them, and they're hard to see so we don't know where the vast majority of them are. An impact by a 100-meter asteroid would not be an extinction-level event, but it could be a civilization-ender. It would without a doubt be the worst natural disaster in recorded history by a huge margin. If it hit in water (the most likely scenario) it would cause 1000-meter-high tsunamis. For comparison, the tsunami that destroyed the Fukushima nuclear plant was 10 meters high.
There is about a one percent chance that this will happen in your grandchildren's lifetime.
Take a moment to think about that.
Happily, there is actually something we can do about it. We have the technology to find and even divert 100-meter-sized asteroids. And it isn't even very expensive in the grand and glorious scheme of things, in the neighborhood of $500 million or so. That's less than half the cost of a single B-2 bomber.
I don't proselytize much here on the Ramblings, but in this case I'll make an exception: if you care about the future of civilization, I urge you to go to the web site of the B612 Foundation (the name comes from the home of the Little Prince) and, at a minimum, sign up for their mailing list. B612 was founded by a pair of NASA astronauts, Ed Lu and Rusty Schweickart. They know what they are doing, and they have a plan that will work -- if it can get funded. The more people they have on their mailing list, the more likely they are to be able to attract the attention of a person or institution capable of providing that funding.
So do your bit to save the planet: sign up for B612's mailing list. Do it now. And if you're really feeling motivated, make a donation. Every little bit helps.