Voyage to the planets: what we're learning

Unmanned spacecraft have given man an incredible close-up look at Earth's planetary neighbors --to the giant gas-bag planets, Jupiter and Saturn, farther out. But when Voyager II takes a swing past Saturn next August, this era of planet-hopping will be over. The US exploration program will grind to a halt as its benefits are analyzed and new directions decided.

In a three-part series, Monitor natural science editor Robert C. Cowen tells how these interplanetary visits are providing a better understanding of our own Earth. Now that scientists have gained this "planetary perspective" -- thanks to wandering spacecraft like the Voyagers, Pioneers, and Vikings -- they are putting it to work.

The inner planets, largely solid bodies, might seem to be of the most value in unraveling Earth's own evolutionary history. Scientists trying to understand the early history of our own planet are learning much from the Red Planet, Mars, and Earth's "twin," Venus, and from cratered Mercury.

But visits to the colossal outer planets reveal that they, too, have much to teach. Studying their atmospheres is helping "tune" theories concerning our own.

What new probings are planned? Which long-range program should the US undertake? These are the issues that US officials and public will be deciding as scientists sift through the volumes of new data from Earth's fellow travelers around the Sun.