Interestingly, the Egyptians are thought to have worshipped meteorite fragments, imbuing them with godly significance because they had been sent down from the heavens and were apparently made of strange metals.
Whatever their social significance, the implications of a large meteorite hitting earth are certainly worth considering. There are a number of questions we have to address to fully understand the threat to life on Earth, as explained below.
How Likely is Earth to be Hit?
The heavens are not short of lumps of rock in the shape of comets and asteroids, and they do occasionally turn up within Earth's orbit. Considering that the Shoemaker-Levy comet hit Jupiter, we know that we are almost as vulnerable, and the Tunguska Incident occurred just over 100 years ago.
Comets are often wandering around the Asteroid Belt, and it's not too unlikely that one might knock an asteroid out of its orbit and send it our way. Even though these distances are vast, and there is a huge amount of empty space around the Earth, we do still have quite a strong gravitational field that can pull asteroids in...
Having said that, you can get a pretty good indication of the current 'state of play' with nearby asteroid showers, by looking at the Moon's surface. It's very badly cratered, but it has been relatively impact-free for the past few million years, as far as we can see.
What Damage Will They Do?
There are many theories (and quite a few movies!) outlining what will happen if a large asteroid hits Earth. There are several factors to consider: the size of the asteroid, the speed of the asteroid, the angle at which it hits the surface, whether it impacts on the land or the sea, and the composition of the asteroid. Many combinations of these factors can result in the extinction of life on this planet, but many are not quite so bad - maybe just catastrophic failure of crops and food for a few years while the dust clears to let the Sun through again.
Is There Nothing We Can Do?
Faced with a piece of rock weighing several hundred thousand tonnes, there's not much action that we could take. Many people have suggested launching nuclear missiles at it to break it up, and this might conceivably work. The main problem is that we would have to have quite a lot of notice that it was on its way here, and we just can't be sure that we would get it. The first thing we might know about the 'extinction level' asteroid is seeing it enter our atmosphere, then we'd have about 20 seconds.
To aid in the early detection of 'Earth-crossers', as they are known, there is an organisation called Spaceguard which is busy cataloguing as many asteroids as it can, largely with the help of thousands of professional and amateur astronomers who each search small sectors of the sky.
The largest asteroid currently known about, Ceres, is the size of a small country - 800km across. If something that big turned up unexpectedly, there would be nothing we could do, even given enough time. Large ones are easier to spot though, so it's less likely for something that destructive to hit us. There are still estimated to be quite a few thousand that we don't know about, though, and any of them can be a few kilometres across.
It's enough to really take the edge off your day.