A Pioneer spacecraft that left Earth 6 1/2 years ago was headed today to a historic encounter with the ringed planet Saturn. Scientists said the craft stands no better than a 50-50 chance of surviving the rendezvous.

Traveling at more than 50,000 mph, the 570-pound Pioneer 11 will end a voyage of more than 2 billion miles Saturday morning when it passes under Saturn's three dazzling rings.

Flying no farther than 21,000 miles from the edge of the outermost ring, Pioneer is to skim under the rings at distances ranging from 1,200 to 6,200 miles, and then fly a curving path that is to take it over the rings and as close as 13,000 miles to the second largest planet in the solar system.

At any time in its two-hour passage by the rings of Saturn, Pioneer 11 could be struck by debris that has wandered out of the rings. Even if it survives passage of the rings, the spacecraft could be destroyed by radiation, which has been hidden from earthbound instruments because of the enormous distance from Earth.

"If the ringed particles are big, we will probably get through because there won't be that many of them," Dr. Larry Esposito of the University of Colorado said today at the Ames Research Center, where the flight of Pioneer 11 is being directed.

"If they're small and there are a lot of them, we probably won't make it through. The big question right now is how small does one of the ringed particles have to be to destroy our spacecraft," he said.

Not only is Pioneer 11 the first spacecraft to reach Saturn, it is also acting as pathfinder for two Voyager spacecraft that earlier this year flew by Jupiter on their way to encounters with Saturn in 1980 and 1981.

Equipped with more and better cameras and instruments, the Voyagers are expected to get measurements and photographs of Saturn 100 times better than those to be taken by Pioneer in the next three days.

Pioneer's safe passage by Saturn is crucial to whether Voyager 2 will fly on to Uranus when it reaches Saturn in August 1981. Voyager 2 must fly along the same path as Pioneer 11, close to the rings of Saturn, if it is to be targeted for Uranus.

If Pioneer is destroyed by the rings Saturday, it almost surely means the Uranus mission of Voyager 2 will be dropped.

"It will be a tough decision to make, but if Pioneer does not survive on Saturday I don't see how we can target Voyager along the same route," Voyager Project scientist Edward C. Stone said today. "There would be just too much at stake still going in our mission to Saturn."

Even though Pioneer's telescopic camera is not a match for Voyager's two big cameras, the three pictures of Saturn and its rings it took today were described by scientists as better than any ever taken from Earth. By the time Pioneer leaves Saturn Monday, it will have taken pictures of the planet and its rings that are expected to be 20 times better than any Earth-based photographs.

Saturn is circled by at least three and as many as four enormous rings, all of them spinning around the planet's equator at different speeds. The innermost ring is moving around the planet at a faster speed than the planet is rotating; the outermost ring is traveling at a slower rotation than is Saturn.

The rings are believed to be made up of snowballs and of rocks, covered by frost, no bigger than automobiles and possibly as small as grapefruit. The second ring (the so-called B ring) is the brightest of the rings because, it is believed, it contains the most objects.

Two moons of Saturn, Mimas and Titan, are believed to exert a pull on the rings, strongly influencing the concentration of objects in each ring and therefore their appearance to observers on Earth.

As these two moons pass close to the outer rings, they are believed to tug at the particles in the rings in ways that move the smaller particles away from the planet and the larger particles toward the planet.

This exchange of particles could also mean that there is hidden debris circling Saturn, beyond the rings, that could strike Pioneer with such force Saturday that they could destroy it.