Karen Roedel, age 13, of Omaha, Neb., for her question:

WHAT IS SOLAR ENERGY?

Plant and animal life on earth couldn't exist without solar energy. Green plants, for example, through a process called photosynthesis, store solar energy as food. During photosynthesis, solar energy is changed to chemical energy by plant cells that contain chlorophyll. Animals eat the green plants and absorb some of the energy stored by the plants.

Solar energy is energy given off by the sun. It is produced by atomic reactions inside the sun where hydrogen atoms are changed into helium atoms.

Every second the sun changes about 657 million tons of hydrogen into 653 million tons of helium. The other 4 million tons of matter are changed into energy. The sun started to give off energy about 5 billion years ago, and scientists believe that it will continue to give off energy for another 5 billion years at least.

To help you understand how much energy is given off by the sun, imagine that gasoline is flowing over Niagara Falls at the same rate that water flows over it. The volume of this flow is 5 billion gallons every hour. Now imagine that all of the gasoline that flowed over the falls for more than 200 million years could be collected. If all that collected gasoline could be burned, the energy from the fire could be duplicated by the sun in just one hour.

A unit called a kilowatt is used by scientists to measure power. The sun continuously produces 390 sextillion kilowatts of power. That number is written out by putting 21 zeros behind 390.

Since the sun's solar energy goes in every direction, only about two billionths of its energy reaches the upper  atmosphere of the earth. Even so, the sun sends the earth more than 1,500 quadrillion kilowatt hours of power each year. That number is written by putting 15 zeros behind 1,500.

Now imagine a train of coal cars that is long enough to stretch five times back and forth from the moon to the earth. If all that coal could be burned, it would produce the same amount of energy that reaches the upper atmosphere of the earth from the sun every hour.

More than 700 quadrillion kilowatt hours of power actually reach the earth's surface each year. Man now annually uses more than 50 trillion kilowatt hours for factories, vehicles and machines. This shows that in 40 minutes the sun delivers to the earth's surface as much energy as man uses in a year.

The idea of using solar energy here on earth goes back to ancient times. Solar energy research and development programs are now underway in many parts of the world. Some solar energy is being used nowp but not too efficiently. One of these days in the not too distant future scientists and engineers will have power plants available that can easily convert solar energy into electricity for home and industrial use.