David Hallam, age 9, of Vancouver, B.C, for his question:

 What is escape velocity?

Escape velocity means the speed necessary to escape the gravity of the earth. When you throw a ball, you give it speed energy to go sailing through the air. For a short while it defies the law of gravity. But it soon spends this energy in flight. The pull of gravity begins to drag it down to the ground.

This powerful force of gravity hugs us to the face of the earth. A plane copes with it by making use of air cushions. But if its engines and propellers stop, it plummets to the ground. When we swim we make use of the water in much the same way. Swimming and floating are tricks to keep us from sinking down through the water.

The pull of gravity pulls everything down. Even the atmosphere is hugged to the earth by the pull of gravity, There are tricks to defy gravity, and speed is one of them. At a certain speed, an object is traveling fast enough to escape the pull of gravity. The object may be a tiny molecule of gas or a space ship. The escape velocity from the earth is always the same.

This speed depends upon the power of the gravity. And the strength of gravity depends on the mass, or weight of the earth. A small light body like the moon has less gravity. The speed needed to escape it is also less.

The escape velocity of the moon is 1.5 miles a second ‑ 5400 miles an hour. Air molecules speed up with heat. The day side of the moon is scorching hot ‑ hot enough to make gas molecules go faster than 5400 miles an hour. Because of low gravity and daytime heat the moon could never hold an atmosphere. The gas molecules would reach escape velocity and fly off into space.

Jupiter is a big and weighty planet. Its escape velocity is 38 miles a second. Mars is much smaller and less massive than the earth. Its escape velocity is 3.2 miles a second. The experts have even figured the escape velocity of the sun in order to escape the pull of cavity on the surface of the sun.