Outer planets satellites

During the four years 1979–1982, studies of the satellites of the outer planets matured into a major branch of planetology, largely in consequence of seven spacecraft encounters with the Jovian and Saturnian systems. Previously these objects had been investigated only by a small group of planetary astronomers, and few planetologists recognized their potential contribution to the study of planetary origins and evolution. A good perspective on the accomplishments of these largely astronomical studies can be found in reviews by Morrison and Cruikshank (1974), Morrison and Burns (1976), Morrison et al. (1977), Johnson (1978), and Cruikshank (1979). The first direct studies of the outer solar system were accomplished by the Pioneer spacecraft, which flew through the Jovian system in 1973 and 1974 and achieved a first encounter with Saturn in September 1979. These appropriately‐named craft were the pathfinders for the much more sophisticated and intensive explorations of Voyager 1 and 2, which encountered the Jovian system in 1979 (March and June) and the Saturnian system in 1980 (November) and 1981 (August). More than a dozen objects, heretofore only faint points of light visible in the telescope, were revealed as complex worlds amenable to geological and geophysical investigation. As a result, a planetologist in 1982 can consider the comparative study of about triple the number of solid‐surface planets under investigation four years ago. This sudden and dramatic expansion in our data base promises to support a revolution in perspective comparable to that achieved earlier when the first geological information was returned by spacecraft from the Moon, Mars, and Mercury.