Scientific America's Ask the Experts

Some scientists were seriously concerned about the possible high density of objects in the asteroid belt, which lies between the orbits of Mars and Jupiter, when the first robotic spacecraft were scheduled to be sent through it. The first crossing of the asteroid belt took place in the early 1970s, when the Pioneer 10 and Pioneer 11 spacecraft journeyed to Jupiter and beyond. The danger does not lie in the risk of hitting a large object. In fact, such a risk is minuscule because there is a tremendous amount of space between Mars and Jupiter and because the objects there are very small in relation. Even though there are perhaps a million asteroids larger than one kilometer in diameter, the chance of a spacecraft not getting through the asteroid belt is negligible.

Even if there were 100,000 sizable asteroids (more than a few kilometers in size) in the asteroid belt -- and the real number is quite likely about 10 times less -- the average separation between them would be about five million kilometers. That is more than 10 times the distance between the earth and the moon. If you were standing on one of those asteroids and looked up, you would not see a sky full of asteroids; your neighbors would appear so small and dim that you would be quite lucky to even see one, let alone hundreds.

In some ways, the asteroid belt is actually emptier than we might like. In the early 1990s, the National Aeronautics and Space Administration wanted the Galileo space-craft to encounter an asteroid while it was passing through the asteroid belt on its way to Jupiter. But it took some effort to find an object that was located even roughly along Galileo's path. Special targeting was required to reach this object, but the result was the first close-up view of an asteroid, the one called Gaspra.

The number of objects in the asteroid belt increases steeply with decreasing size, but even at micrometer sizes the Pioneer spacecraft were hit only a few times during their passage. That is not to say that asteroids cannot pose any danger, however. It is worth noting that for a large planet like Earth, over a long period of time, there is an appreciable chance of being hit. This hazard comes from the fragments of mutual collisions in the asteroid belt; after their break-up, some of these fragments move toward the earth under the gravitational action of Jupiter.

An asteroid about 12 kilometers in diameter crashed into the earth 65 million years ago, killing nearly 90 percent of the animals, including the dinosaurs. Such major impacts are very rare events, but for smaller objects the likelihood of impact increases; the chance of the earth being struck by an object approximately one kilometer in size is about one in 5,000 in a human lifetime. An object one kilometer across would still be large enough to cause a global disaster because of the enormous energy it would release upon impact:at east a million times the energy of the bomb dropped on Hiroshima in 1945.