[149] The highly successful missions of the Pioneer and Voyager 1 and 2 (Science, 1980, 1981, 1982; Morrison, 1982) spacecraft have tremendously expanded our knowledge about the planet Saturn, its rings, and its satellites. The system has been found to be an extremely complex group of at least 17 satellites and thousands of rings orbiting a huge planet (120000 km or 74 500 miles in diameter). The size of this system may be more fully appreciated if it is realized that the orbit of Earth's Moon would extend only to the outer limit of Saturn's ring system (Ring E) or that the largest moon, Titan, is almost half the diameter of Earth. Saturn itself is composed mostly of lighter gases such as hydrogen, deuterium, methane, ammonia, ethane, phosphene, and probably helium. Because of its rapid rotation (10 hr, 14 min), these gases are forced outward, resulting in an equatorial diameter 11 percent greater than the polar diameter. Because of its composition, the planet has a very low density; "average" Earth material weighs almost eight times as much as average Saturn material. Given a large enough body of water, we could float Saturn in it. Because of this low density and size, Saturn's gravity near the top of the atmosphere is only 0.9 times that of Earth; that is, anything that weighs 100 lb on Earth would weigh only 90 lb on Saturn. Temperatures in the outer atmosphere of Saturn range from -130° C (- 265° F) to-180° C (- 355° F). Ring particles and satellites appear to be frozen gases and water ice, and their densities indicate that the term "dirty snowball" is a reasonably accurate description for most of them. Ice at these low temperatures is as brittle as most rock, and eons of impacts have scarred the surfaces of the satellites with craters and massive fractures.

Saturn is the most distant planet known to the ancients, and its name is a Latin term for the Roman god of agriculture. From Earth, it is a bright object (magnitude-0.4 at greatest brilliancy) with a slightly orange color. More than 29 years (29.485 years to be exact) are required for this planet to complete one revolution about the Sun, and its path can be traced in our night sky as it slowly moves through the constellations of the ecliptic. In 1610, when Galileo first turned his crude telescope on Saturn, he was mystified by what he described as "companions" of Saturn. He drew Saturn as a planet with two handle-like protuberances. As telescopes were improved, the rings were resolved more clearly, and in 1655 Huygens finally perceived their geometry. In 1675, Cassini noted a "gap" in the rings, which today bears the name Cassini Division. Voyager found that this is not a gap at all, but a ring that appears from Earth to be darker than adjacent rings. The rings separated by this "division" were named Rings A and B by W. Struve. An additional division was found in Ring A by J. Encke in 1837 and is now called Encke's Division. A third ring inside Ring B was noted in 1838 by J. G. Galle and is known as the Crepe or Ring C. Other rings and ring features were suspected by observers, but were not resolved until spacecraft traversed the Saturnian system in 1980 and 1981.

The satellites of Saturn were discovered by painstaking observation over a period of 250 years. The moon Iapetus has been of great interest to astronomers because of its signficant variations of brightness (magnitude variation of 1.92) at different positions in its orbit. In 1944, G. Kuiper noted the presence of a methane atmosphere on Titan, further expanding the mysteries of the Saturnian system. The satellites are all fairly substantial....

....bodies, larger than the outer irregular satellites of Jupiter and five are larger than the largest asteroid, Ceres.

The sizes and other statistics regarding objects in the Saturnian system are listed in table B-1 and further illustrated in figures 3 (Introduction) and B-1. Comparison of the total known solid surface area in the Saturnian system with that in the whole solar system is illustrated in figure 2.

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