Ronnie Wiens, age 11, of Beamsville, Ont., Canada, for his question:

How can they tell the surface gravity of the planets?

Gravity is a natural force built into every body of matter. It depends upon the mass and the size of the object, and it decreases with distance from the center of a heavenly body. The mass of an object is the amount of matter it contains, and larger bodies may be 1ess or more massive than smaller bodies.

When we speak of gravity, we mean the force at the surface of a planet, such as the earth, the pulling power that gives us weight and holds us tightly to the face of the globe. This depends upon the mass of the globe, the amount of matter packed into its solid form and its size, which gives us the distance from the center to the surface. The surface gravity of each of the planets also is calculated from its mass and its size.