Nicolas Palestini, age 11, of Portland, Maine, for his question:
Why don't the planets bump into each other?
People began to accept the the true picture of the Solar System about 400 years ago. Some must have wondered and perhaps worried about the possibility of planetary collisions. For almost two centuries, no one was sure. Then the problem was solved with a brilliant explanation of why our planets cannot possibly bump into each other.
It took hundreds of years and the brainwork of several geniuses to figure out the laws and structure of our Solar System. Early in the 1600s, the great Galileo figured out the positions of the sun and six of the orbiting planets. Later in the same century, Newton figured out the laws of gravitation that govern the motions and traffic lanes of the heavenly bodies. The knowledge of these orderly cosmic forces was very comforting. It suggested that planets and other heavenly bodies must obey orderly rules and regulations. Thoughtful people hoped that this made planetary collisions unlikely. Then early in the 1800s, Pierre Simon Laplace proved that such dreadful disasters are downright impossible.
Laplace was an astronomer and a mathematical genius. His interest was celestial mechanics, those cosmic forces that govern the motions of planets and all other heavenly bodies. He used Newton's laws of gravitation, calculus and other complex math to explain how the planets are kept in their separate orbits by a sort of tug of war between opposing cosmic forces. The tugs in opposing directions are evenly balanced. Neither can win. This is what keeps the planets in their places. It makes it impossible for them to leave their separate traffic lanes and risk bumping into each other.
In any case, most people stopped worrying after we began to realize the size of the planets compared with the vast reaches of space between them. The Solar System is more than 7,000 million miles wide and this vast saucer of space is occupied by only nine orbiting planets. The nearest never comes closer to us than 26 million miles. The farthest is more than 3,000 million miles away. If we scale .down the system to a smallish model, it is easier to grasp the size of the planets compared with the immense spaces between them.
If the sun is reduced to the size of a beach ball two feet wide, then the farthest planet would be a pea sized ball at a distance of more than three miles. Earth and Venus would be pea sized planets with about 73 feet between their traffic lanes. Even if a planet did stray from its orbit, it would be unlikely to cross the vast reaches of space and just happen to bump into a neighbor. But Laplace figured out that a planet cannot stray from its orbit. On the one hand, it is pulled toward the sun by the mighty force of solar gravitation. It is pulled away from the sun by the force of its own whirling orbital speed. These cosmic forces are perfectly balanced to keep each planet in its separate traffic lane, whirling at the correct speed at the correct distance from the sun. The strict old Solar System is very set in its ways.
The governing factor that keeps the Solar System stable is balance. The force of gravity depends upon the mass of a heavenly body and its gravity decreases with distance. Newton showed that these ratios are strictly precise. Actually, the gra¬vitational attraction between the sun and a planet pull the two together. But this pull is offset by the force of the planet's orbital velocity. In each case, the opposing forces are evenly balanced and the planet is pulled into a curved path around the sun. And cosmic traffic violations are impossible.