Imagine the asteroid belt as a disk-shape region with its inner edge at twice Earth’s distance from the Sun (186 million miles). The outer edge reaches to 3 times Earth’s solar distance (279 million miles), and the disk is about 9 million miles thick. The patch of space containing the asteroid belt has a huge volume — 1 trillion trillion cubic miles. That’s a lot of space to fill.
There are hundreds of thousands of main-belt asteroids known, and many more await discovery. If, as some estimates suggest, a million objects lie in the asteroid belt, then the average distance between asteroids is 730,000 miles (1.1 million km).
So, the asteroid belt is hardly a dense debris field. It’s easy for spacecraft to miss asteroids, given such large gaps between them. —Bill Cooke, NASA/Marshall Space Flight Center, Huntsville, Alabama