Eric Lenz, age 9, of Berkeley, California, for his question:

How high does the atmosphere go?

It is at least 3,000 miles and most likely 4,000 from sea level to the top of the atmosphere. It is hard to mark the exact boundary because the air in the upper level of the atmosphere grows thinner and thinner and finally is lost in space. But perhaps you would like to know what the air is like up there    plus all of the details. So let's plan an imaginary trip    up and away to the very top of the earth's atmosphere!

Let's imagine a suitable craft and plan to launch it from a beach at sea level. It must be sealed to keep out both heat and cold and to keep in the oxygen we shall need as the air will grow too thin for breathing. The atmosphere changes as we rise,. so we will need some instruments to check the details. Let's take an altimeter to measure our rising altitude, a thermometer to check temperatures and a barometer to measure the air pressure    this is the weight of air pressing down from above. At sea level the pressure is 15 pounds per square inch. Air samples can tell us a lot, so let's take some instruments to study them as we go. Also, we shall need notebooks to record our findings of the five different layers of the atmosphere. And since the trip up and down is about 8,000 miles, let's take enough food and water.

Now we are ready for our imaginary journey. The different layers of the atmosphere are ball shaped shells called spheres. The lowest one is the troposphere, which reaches 10 miles above the equator and dips to 5 miles over the poles. As we rise, our barometer shows that pressure is falling as the air grows thinner. The temperature also is falling    about 3 degrees Fahrenheit with every 1,000 feet. Our air samples are crowded with gas molecules, dust and moisture. Outside there are clouds and strong winds.

At the top of the troposphere, we enter the beginning of the stratosphere. The pressure keeps falling, but the temperature stays fairly constant    somewhere between 80 degrees and 100 degrees below zero. For the next 10 to 15 miles, the thin, cold air is calm. Then we enter the mesosphere and the temperature gradually rises to 50 degrees Fahrenheit. Outside there are moderate winds, less oxygen and more ozone. When we reach 50 miles above the earth, we leave the thin mesosphere for the thinner, but very turbulent ionosphere.

Up here the sun's radiation tears electrons from their atoms and the air is filled with electrically charged particles called ions. Although this realm is thinner than the air in a light bulb, its energetic particles bend and bounce our radio waves back to the earth. The ionosphere acts like a sounding board and helps broadcast our radio signals around the world. This layer of thin air ends somewhere between 350 and 600 miles above the earth. Then we enter the even thinner exosphere, where gas particles are very rare. And as we rise to 3,000 miles, the temperature rises to 2,000 degrees Fahrenheit. Finally, on we go, approaching 4,000 miles, where the pressure is about zero and the last gaseous molecule of the atmosphere is diffused in space.

The air, of course, is a mixture of gaseous molecules. It is held by gravity and gravity gets weaker as we rise above the earth. About 3/4 of the total molecular atmosphere is crowded into the slim troposphere    the first layer of the atmosphere. The ionosphere, teeming with energy, is 300 to more than 600 miles high. But it contains only a fraction of the total molecular atmosphere. And at the top of the thin exosphere, a few air molecules escape the pull of gravity and breeze off into outer space.