Welcome to You Ask Andy

Norman D. Jentner, age 10, of Fairview Park, Ohio, for his question:

What keeps the planets in their orbits?

Some people suspect that planets can come off their courses and crash into each other. After all, orbiting artificial satellites often crash down to the ground. But experts assure us that planets cannot leave their orbits, and WE never need to worry about planetary collisions.

The planets are kept in their places by strict traffic regulations. These cosmic laws were figured out by Kep1er, Newton and other great men, and we know that they cannot be broken. One law is that of universal gravitation, the pulling power in every particle of matter. Another law controls all moving bodies suns and satellites, bullets and bouncing balls.

These two forces cause an endless tug of war between the sun and each of its planets. The speed of the moving planet takes it in a direction away from the sun. Gravitation pulls it toward the sun. The two opposite forces are evenly balanced, and the moving planet is compelled to take a curved path. Neither side can win the tug of war, and the curved path takes the planet in orbit around and around the sun.

The exact orbit depends upon a number of interlocking factors. Most of them have to do with the nature of gravitation. This built in pulling power varies with mass, and mass is the amount of matter packed into the size or volume of an object. Its force weakens with the distance from the center of an object's mass. and out in space between the: massive heavenly bodies, the universal law of gravitation works on a very grand scale.

he mighty gravitation of the massive sun reaches out to Pluto and beyond, getting weaker as it goes. The weaker gravitation of the less massive planets reaches the sun. The sun pulls at Each planet and each planet pulls at the sun. But the sun is affected only slightly by all the tugging of its children.

The basic pulling power depends upon the mass of the sun and the mass of a planet. This force is changed by distance and by the planet's speed. So to figure out an orbit, we must know the masses of the sun and planet, the speed of the moving planet and its distance from the sun.

The sun is about 333,000 times more massive than the Earth and about 93 million miles away. The Earth's orbital speed is roughly 66,600 miles an hour. These factors interlock to keep the world in orbit. But if it speeded up, it would move father from the sun in a larger orbit. If it slowed to a stop, it would spiral down into the sun. But the laws of moving bodies say that these things can never happen.


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