Long after the flybys of Jupiter, continued analysis of Voyager images revealed another new satellite, Jupiter’s fifteenth. Initially designated 1979J2, the unexpected new satellite orbits the planet at 3.17 R_J, between Io and Amalthea. Stephen Synnott of the JPL Optical Navigation Team discovered the satellite on pictures taken during the Voyager 1 events on March 5, 1979, while searching for additional images of satellite 1979J1. It is about 75 kilometers in diameter, but nothing else is known about its physical properties. Together, the 15 satellites circling giant Jupiter form a mini-solar system. Perhaps the outer, irregular satellites were captured or resulted from the catastrophic collisions of one or more larger satellites with passing asteroids. The inner seven satellites constitute a coherent system, almost certainly formed together with Jupiter and sharing a common 4.5-billion-year history. They are fascinating as individual worlds, and also as brothers and sisters, and the study of their interrelationships undoubtedly will provide insights into the general problems of planetary formation and evolution. SIZES AND DENSITIES OF THE GALILEAN SATELLITES Name Diameter Density (grams per (kilometers) cubic centimeter) Io 3640 3.5 Europa 3130 3.0 Ganymede 5270 1.9 Callisto 4840 1.8 Callisto Callisto is the least active geologically of the Galilean satellites. Basically a dead world, it bears the scars of innumerable meteoric impacts, with virtually no sign of major internal activity. Callisto is a world of craters, and to understand it we must explore the role that cratering plays in molding planetary surfaces. The space between the planets is filled with debris, ranging from the larger asteroids, hundreds of kilometers in size, down to microscopic grains of dust. Inevitably, each planet collides with some of these fragments. The smaller particles do little damage; in the case of a planet with an atmosphere, like Earth, they burn up as meteors before reaching the surface, whereas on an airless planet, they erode the surface by sandblasting the exposed rock. The larger impacts are another matter, and the craters they produce can be the dominant features on the surface of a planet. [Illustration: Voyagers 1 and 2 photographed most of the surface of Callisto at resolutions of a few kilometers or better. Shown here is a preliminary shaded relief map. Additional measurements will improve the accuracy of the coordinate system. [260-672]]