SOMEWHERE OUT in space -- the scientists know exactly where, but that is no longer important -- Pioneer 11 is racing toward an unknown and unknowable fate. It has accomplished the tasks for which it was built. It is now headed toward nothing in particular. But it carries, on its side, a message designed to communicate that the earth and humankind exist if it should happen to encounter intelligent life forms elsewhere in the universe. It is a fitting end for a spacecraft that told us more about a part of our solar system during the past weekend than all the scientists in the world had been able to learn in all the years that went before.

Pioneer 11, like most of the spacecraft in the exploration program, was remarkably productive. It completed its original mission more than four years ago when it flew past Jupiter. With a few nudges from its earth-bound controllers, it then set out after Saturn. The photographs and data it sent back across almost a billion miles of space as it plunged through the planet's rings and after it looped around the far side will keep scientists working for years. Some of that information, no doubt, will confirm existing theories about Saturn and the nature of the solar system; some will produce new theories.

But, Saturn will never be viewed the same way. A few of the mysteries of the "mysterious silver beacon" with "cup handles," as Galileo described it in the 17th century, have been pierced. More will be when the two Voyagers now on their way to Saturn reach their destination. Saturn is no longer just a dot in the sky or a fuzzy image seen dimly through a telescope.

The inevitable question, when government finances such exploration, is whether the bits of knowledge that have been acquired are worth what they cost. The answer is that, standing alone, they are not. Little, if anything, has been learned from Pioneer 11 that will improve the quality or lengthen duration of life on earth. But no one knows what will emerge when this spacecraft's information is added to what has been and will continue to be accumulated. It is this search for knowledge about the earth, the solar system and, grandly, the universe that the Pioneer 11 flight is all about.