Robert Stojevich, age 10, of Duluth, Minnesota, for his question:

How do they figure weights on other planets?

Physicists know the factors that determine the weights of objects on different planets. But figuring out the fine details is quite a complex task. The main factor is the force of gravity. This force depends on the mass of a planet. And mass is the amount of material packed into the volume or size of the planet. The more massive planets exert more gravity.

Actually, weight is the force of gravity that a planet exerts on objects on its surface. If the earth's volume were to increase without change in its mass, we on the surface would weigh less than we do now. With these factors in mind, experts start computing the surface gravity on other planets. Giant Jupiter, for example is 318 times as big as the earth. You might expect its surface gravity to be 318 times stronger so that objects weigh 318 times more than they do on earth. This is not so. The big planet's materials are less dense and a 100 pound earthling would weigh only 264 pounds on Jupiter.