Actual size: The asteroid

Corey Zehfus

All the geeks out there who have played “StarFox” or watched “Star Wars” know that asteroid belts are happening places, full of huge spiraling rocks which smash into each other and make it very difficult to fly deftly.
At least, that’s how they are in science fiction. In reality, we send unmanned probes sailing through the asteroid belt of our solar system with no problems. What exactly is the asteroid belt then, if it isn’t a hectic highway of rocky turbulence?
The asteroid belt takes up a very large area. Its inner edge is past Mars at about two astronomical units — one AU is the distance from the earth to the sun — and its outer edge is this side of Jupiter at about 3.25 AU. Suffice it to say, it would require a lot of rocks to fill this area, or even make it a frenzy of craggy collisions.
In reality, the asteroid belt has a mass of about 3-10^21 kg (1 kg is about 2.2 lbs. here on earth). This may seem to be a rather large number; however, it is only about 1/2000 of the mass of the earth.
In essence, if you were to smash up the moon into millions of little bits, throw away 95 percent of the pieces, and spread the rest out over the volume between 2 AU and 3.25 AU from the sun, you would have the asteroid belt.
There are probably a million or more asteroids with a diameter over one kilometer, but spread over an incredible vast area. So much for StarFox having all of that trouble in the asteroid belt — it turns out he didn’t have to do a barrel roll.
The epic crashing of huge chunks of rock does happen, but these cinematic occurrences are few and far between. The second largest asteroid in the belt, 4 Vesta, underwent a large collision about one billion years ago and lost approximately one percent of its mass.
Many of these pieces fell to Earth as what are call HED meteorites. Other than these rare occasions, the asteroid belt is mostly just empty space, populated by a few drifting frozen rocks.