Asteroid Fields

I have been asked by friends and family how we get probes and satellites through the asteroid belt that lies between Mars and Jupiter. The simple answer is that, contrary to Star Wars and other science fiction portrayals of them, asteroid fields are rather... empty.

Now, we don't know exactly how many objects are in the asteroid belt. It isn't particularly important for this demonstration, so lets err on the side of caution and say there are 10 million asteroids more than half a mile across in the asteroid belt. How likely are you to run into one passing through the asteroid belt?

The middle of the asteroid belt is around 2.8 astronomical units from the sun. One astronomical unit is 92,955,887.6 miles, the distance between the Earth and the sun. Some quick math (circumference tells us that the circumference of a circle with a diameter of 2.8 astronomical units is 1.64 * 10^9 miles. Written out, that is a circle 1,640,000,000 miles around. If there were 10 million objects larger than half a mile across, there would be 164 miles between each one. And this is if every object was arranged on a ring. The real asteroid belt is a 'donut', a full astronomical unit across, and it isn't completely flat. There is just so much space and so little of everything else that actually running into -anything- is very unlikely.

The odds of succesfully navigating the asteroid belt are significantly better than 3720 to 1, in any case.