EARLY YESTERDAY morning, after a three-year journey of well over a billion miles, the first of two Voyager satellites passed within a mere 2,500 miles of what is thought to be the largest moon in our solar system, Titan, a satellite of the planet Saturn. Last evening, accelerating under the planet's gravitational pull, Voyager skimmed by Saturn's south pole, just 77,000 miles from its furface, and up through the rings. Its route will by then have provided mankind's closest look at this mysterious planet and its most interesting moon. But the time you read this, Voyager will already be one million kilometers beyond Saturn, headed on an endless journey out of our solar system.

The magnitude and precision of this extraordinary mission, and the wealth of information it has already provided, are cause for excitement -- and also pride in human scientific achievement. Last year, Voyager explored Jupiter, discovering a previously unknown ring around the planet, two additional moons in its system, huge volcanoes, unexpected chemical activity and a wealth of other knowledge.

Because of the two-satellite design of the mission, Voyager 2 -- traveling several months behind Voyager 1 -- either can be reprogrammed to take a closer look at Voyager 1's discoveries, or can be kept on its slightly different trajectory, providing additional information on key features. If all goes well in the first encounter with Saturn, Voyager 2 will be kept on its present course. Then, in 1986 it will be able to explore Uranus, and three years further on, Neptune.

It will take years for scientists to assimilate all the information Voyager sends back. Already, though, it has discovered three new moons circling Saturn and has provided a completely new picture of the planet's strange rings. There now appear to be rings within rings, and ringlets within them, and possibly a complex structure within the ring system. Voyager is also providing intriguing new information about Saturn's and Titan's atmospheres, which should help to understand how the Earth, and possibly the universe, was formed.

Voyager's discoveries, like the tremendous success of the earlier Viking missions to Mars that ended last summer, provide further evidence of the enormous gains in knowledge that unmanned space missions can yield. The only cloud in yesterday's close encounter with Saturn was the dearth of such missions now on NASA's drawing boards.