Science fiction movies and TV series often depict encounters with asteroid belts or swarms in deep space. Terror ensues as the crew of the hapless ship pitches this way and that while attempting to dodge the rocky onslaught. Does that really happen? After watching these scenes play out for years, you can’t fault anyone for believing our own solar system’s asteroid belt — the familiar one between the planets Mars and Jupiter — must be just as hazardous.
It isn’t. While there are millions upon billions of space rocks out there that would destroy a galactic cruiser in an instant, they’re spread across an enormous volume of space. The main asteroid belt extends from about 185 million miles to 375 million miles from the sun and 50 million miles above and below Earth’s orbital plane. Imagine a very large life preserver ring and you’ve got the picture.
There are about 1.5 million big asteroids, ones a kilometer or larger (0.6 mile), with an average separation of 2 million miles or 8 times the distance of the moon. Nothing to worry about in a spaceship. At that distance, the asteroid would appear exactly like a star. What about the smaller ones? There are estimated 100,000,000,000,000,000 (quadrillion) asteroids larger than a yard (1 meter).
When we spread them across the “life preserver,” each asteroid would have about 1,735 square miles (4,500 sq. km) all to itself plus additional space above and below assuming each of their orbits is slightly tilted. That amounts to a very large box as wide as the distance between New York City and Denver for every asteroid one meter and up. At least in our solar system, you have to deliberately aim for an asteroid to hit one!
If you stood in the middle of the asteroid belt and looked out, it would look pretty much like empty space except for our shiny sun and a few tiny dots of planets. Asteroids occupy such a large volume of space that so far — knock on titanium — not one spacecraft passing through the belt has recorded an impact. Despite their great number, the asteroid belt contains just 4% of the mass of the moon, with about half of that contained in the four largest: 1 Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea.
One way to get a true swarm of asteroids would be via a catastrophic collision of a planet and an asteroid or two large asteroids. The resulting fragments might hold together through self-gravity to pose a hazard before ultimately dispersing. So we’ll be magnanimous and assume that some of those crazy swarms of rolling boulders in sci-fi movies that crank up our adrenaline were the result of recent break ups or collisions.