John Gierl, age 9, of Milwaukee, Wisconsin, for his question:

Can we predict the future of the moon?

Our astronauts took rock samples from the moon and made all sorts of tests on the spot. But so far, nobody can prove how the moon was born or what happened in its past. However, guessing games are fun    even to scientists. They have guessed several things that might have happened to the moon long ago. And some experts have made a few guesses about what might happen to the moon in the far future.

We won't be around to see whether these predictions come true. But let's have fun pretending. Let's suppose we are on earth for another five billion years. If this prediction comes true, the moon would look paler and one third smaller than it looks now. This is because it would be about 100,000 miles farther away. If we lingered here for still another five billion years, we might see it come closer again. It might come so close that the earth's gravity would shatter it to fragments. Then our planet would have shiny rings, like those around Saturn.

Of course this prediction into the far future may never come true. But experts have reasons for thinking that it might. Some of these reasons are based on things that are happening right now. We know that the moon causes our mighty tides. Careful measurements prove that these tugging tides make the earth spin slower and the days longer. We don't notice these slight changes. But today is 25 billionths of a second longer than yesterday. If this goes on at the same rate, in five billion years the earth days would be 36 hours long.

As the earth slows down, it lets the moon move around faster. The moon moves farther away to a larger orbit. In the last 30 years, it has moved away one foot. If this keeps on for the next five billion years, the earth and moon will be about 100,000 miles farther apart. This is roughly half again as far as our present distance. At this point, you might expect the moon to keep on going and finally fade out of sight.

However, the experts predict that something, else will happen to bring our wandering moon back home. In five billion years, the sun is expected to swell up and get hotter. On earth, the long hot days and long cool nights are likely to stir up a lot of wild, stormy weather. This is expected to speed up the spinning days and make them shorter again. The faster spinning and the hotter sun would pull at the moon.

Then it is expected to slow down and move into a smaller orbit, closer to home.  If there are people here in this far future, they will not pay much attention to the antics of the moon. For then the old sun will be dying. After its last flare up, scientists expect that its worn out furnace will grow dim and finally go out altogether. Long before this tragedy, people will move to a new planet near a younger sun. And no doubt their space ships also will take along the earth's precious plants and animals, to enjoy a new life on a new world.