THE VISIT VOYAGER 2 is now making to Saturn completes two decades of exploration of the planetary system. No more information about those fascinating objects in the sky will pour in from American spacecraft for at least four years, once Voyager's equipment is powered down. And the next data from outer space may be from Voyager itself if it and its equipment survive the journey to a planned rendevous with Uranus early in 1986.

In one sense, this halt in the acquisition of knowledge will be useful. More has been learned about the other planets that circle our sun in the years since Mariner 2 flew past Venus in 1962 than had been learned in all prior human history. That knowledge--some of it illuminating, some of it mystifying--does not all fit together neatly, and a pause for reflection, theory-building and planning can help define the questions to which a new generation of spacecraft can seek answers.

The flights of the unmanned spacecraft--from the rudimentary Mariners through the Vikings, Pioneers and now Voyagers--have been technological marvels. The idea that human beings could design, build and send through millions of miles of space instruments that could send home pictures, temperature and radiation readings and much other scientific data was still a dream whem most of the Americans alive today were born.

The people of ancient times knew about Mars, Venus, Mercury, Jupiter and Saturn long before the telescope was invented. In their imagination, such wandering stars became the homes of other civilizations. We now know civilization does not exist upon them; they and their moons are home to such strange things as oceans of methane and rains of sulphuric acid as well as ice and rock. There is no other Earth out there, and each planet is unique and strikingly different from its neighbors.

The first era of planetary exploration has produced the data for a revolution in the way scientists think about the solar system and the Earth's role in it. The next era--which must not be too long delayed by the budgetary pressures that have brought about this pause--may change it all again. The planetary system and the universe are as grand as any ancient observer ever dreamed they were, and craft like the Voyagers are just beginning to provide a glimpse of what they are really like.