Space: A blizzard of rocks?

When people think of the asteroids, they either think of this scene from Star Wars, with its cloud of close packed boulders...

Above: A Star Wars asteroid belt. Couyrtesy of Lucasarts. Or Disney. Or someone....

...or, if they've read a little bit more, they think of a doughnut shaped belt of a few thousand rocks going around the Sun.

OK, not that much like a doughnut shape. Mmmm, doughnut.

It's not like that. 

Take a hailstorm, and separate each of the hailstones by kilometres -  but keep the number of hailstones the same, so the storm is now huger than worlds. Scale it all up again, so the 'hailstones' can be up to hundreds of kilometres wide, and travelling at tens of thousands of kilometres an hour.

Now keep the storm running for billions of years and fly in circles through it. That's what you (and me, and all of us) are doing right now - planet Earth is zooming in circles through space, through a storm of flying rocks.
While the odds of getting hit by ones of the asteroid 'hailstones' at any given moment is small, over time it becomes pretty much certain we'll catch a few in the planetary windshield. When we do the results have ranged from spectacular fireworks...

Video above: The Leonid meteor shower - the Leonids are a stream of dust and debris left behind by comet Tempel-Tuttle, so what you're seeing here are pieces of comet raining out of the sky. That's fine, and beautiful. Whole comets raining out of the sky would be more of a problem... Video courtesy of Space.com.

... to an explosion that damages a city and injures thousands of people...

Above: A van-sized asteroid explodes over the city of Chelyabinsk, Russia.
 

....to the once in a hundred thousand years mega impact, or even bigger, that can kill off whole species. But asteroids aren't simple bad news: They also delivered the water and organic chemistry that enabled life to get a toehold on ancient Earth. They may even have warmed Mars enough to make it habitable, once.

Today they bring us a free way of exploring worlds billions of miles away, and even worlds that were destroyed long ago. And some people are hoping the could become a source of material we can use to expand our space economy.

Image above: A cut and polished nickel-iron meteorite, showing the strange Thomson structure that occurs when the metal has cooled very, very, slowly, under microgravity - which means in the dying core of a protoplanet. Bits of the iron cores of protoplanets just fall out of the sky! But it's too rare to justify always wearing a hard hat. And a hard hat wouldn't be much good against most of these things anyway. 
But, if you just like hard hats, feel free... Image courtesy of The National Museum of Wales
  

What does this hailstorm of asteroids look like? I ran across this video a long while ago, but it still shows it well: New discoveries in white, main belt asteroids in green, non-threatening Earth approachers in orange, potentially hazardous Earth crossers in red. Enjoy! Or possibly be afraid...

Video above: A map of the inner solar system, showing the discovery of asteroids from 1980 to the present day. That's a lot of rocks. What really strikes me, though, is that by the end of the animation all the terrestrial planets are embedded in a swirling disk of flying rocks! It's worth remembering:  These blips are the remnants of the Suns protoplanetary disk  that gave birth to all the planets... Video courtesy of Scott Manley