Which planet's exploration by Pioneer and Voyager spacecraft is considered a major scientific achievement of the 20th century?

The exploration of Saturn by the Pioneer and Voyager spacecraft is indeed considered a significant achievement in the history of space exploration. During the late 20th century, both the Pioneer 11 and Voyager spacecraft provided humanity with unprecedented insights into Saturn and its complex ring system, as well as its numerous moons.

Pioneer 11 was the first spacecraft to fly by Saturn in 1979, sending back valuable data that helped scientists understand the planet's atmospheric conditions, magnetic field, and rings. Following that, Voyager 1 and Voyager 2 continued the exploration of the outer solar system, with Voyager 1 flying past Saturn in 1980 and capturing detailed images and measurements of its moons and rings. These missions greatly expanded our understanding of the planet, revealing the beauty and intricacy of its ring structure and the dynamic environments of its moons, such as Titan, which is particularly notable for its thick atmosphere and surface lakes of methane and ethane.

Overall, the missions to Saturn contributed to our comprehension of planetary science and the formation of planets in our solar system, marking substantial progress in the realm of astronomical research during the 20th century.