Mark Svacina, age 9, of San Diego, California, for his question:

What stops the planets from hitting each other?

Each planet swings around its own orbit, so far and no farther from the sun. The next planet has a traffic lane that is many millions of miles away. Even if one of them came off course a million miles or so, it could not come close enough to bash into a neighbor. Besides it cannot comes off course at all. The traffic laws of the Solar System are much too strict to let this happen. These laws are enforced by two mighty forces that pull against each other, somewhat like a tug of war.

On one side, the sun's mighty gravity pulls at the planets. On the opposite side, each planet orbits around to keep from falling into the sun. The strength of its speed exactly matches the sun's pulling power. The match is a draw and to keep it that way, every planet must stay right there in its traffic lane. This makes it absolutely impossible for two planets to hit each other.