Jeffrey Johnston, age 10, of Kanata, Ont., Canada, for his question:
COULD TWO PLANETS EVER COLLIDE?
About 400 years ago, the great Galileo figured out that the planets orbit around the sun. Right away people began to worry. Suppose a planet left its orbit and bashed into a neighbor! At last, in the 1800s, another great astronomer proved that such a planetary collision is downright impossible. It just cannot happen.
In the 1600s, young Isaac Newton figured out the laws of gravitation. He explained in simple words just how this strange, built in force governs the traffic rules and regulations of the whole universe. It keeps the planets in their proper places and, what's more, it works all the time, weekdays and Sundays.
This is nice to know, for it means that the planets cannot escape from their traffic lanes and risk colliding with each other. But to feel really safe, people need to know more about how the heavenly traffic regulations work.
In the 1800s, a French genius named Pierre Simon Laplace went into the problem in great detail. He based his study on Newton's works, used calculus and other complex math to explain why the planets simply must stay in their proper places.
The remarkable system works like a stupendous tug of war between two cosmic forces. one is gravity. For example, the sun pulls all the planets and the planets pull back at the sun. If this were all, the planets would have fallen into the sun long ago. But something else works against the pull of gravity.
This is the orbiting speed of the planets. Just swinging around prevents them from falling into the sun. Gravity and orbital speed are so perfectly balanced that neither can win the tug of war. Each planet is forced to stay right there in its own traffic lane. Each planet orbits at the correct speed, which depends upon its mass and its distance from the sun. Nothing can change this perfect balance so no planet can come off course and bash into a neighbor.
The two outer planets are Neptune and then Pluto. Sometimes Pluto's oval orbit moves inside the more circular orbit of Neptune. But even those two planets are not likely to collide. If they ever came too close, Pluto would merely become one of Neptune's moons. Gravity and orbital speed would keep it at a safe new path.