The two Voyager encounters with Jupiter were periods unparalleled in degree and diversity of discovery. We had, or course, expected a number of discoveries because we had never before been able to study in detail the atmospheric motions on a planet that is a giant spinning sphere of hydrogen and helium, nor had we ever observed planet-sized objects such as the Jovian satellites Ganymedes and Callisto, which are half water-ice. We had never been so close to a Moon-sized satellite such as lo, which was known to be dispersing sodium throughout its Jovian neighborhood and was thought to be...