[Illustration: The four Galilean satellites of Jupiter are planet-like worlds, revealed by Voyager to be as diverse and fascinating as the terrestrial planets Mercury, Venus, Earth, and Mars. In this Voyager 1 composite, all four are shown in their correct relative size, as they would appear from a distance of about 1 million kilometers. Relative color and reflectivity are also approximately preserved, although it is not possible to show on a single print the full range of brightness from the dark rocky surface of Callisto to the brilliant white of Europa or orange of Io. [260-499C]] [Illustration: Io (longitude 140°).] [Illustration: Europa (longitude 300°).] [Illustration: Callisto (longitude 350°).] [Illustration: Ganymede (longitude 320°).] CHAPTER 9 FOUR NEW WORLDS Jupiter’s Satellite System In a sense, the Voyager Mission revealed a new planetary system. Astronomers had long been fascinated by the large Galilean satellites of Jupiter, but they had only looked from afar, watching the dancing points of light in their telescopes, and, occasionally, as the atmosphere steadied, seeing these points resolve themselves into tiny disks before dissolving again in the turbulence of the terrestrial atmosphere. Much had been learned from telescopic studies, but not until the Voyager flights had we truly seen the Galilean satellites. The historic hours as Voyager 1 cruised past each satellite on March 5 and 6, 1979, fundamentally altered our perspective. Four new worlds were revealed, as diverse and fascinating as the more familiar terrestrial planets. Although not yet household words, the names Io, Europa, Ganymede and Callisto have now been added to Mercury, Venus, Moon, and Mars in the lexicon of important “Earth-sized” bodies in the solar system. Jupiter has fifteen known satellites, counting the two new satellites discovered by Voyager. These moons vary greatly in size, composition, and orbit. The four outermost satellites, Sinope, Pasiphae, Carme, and Ananke, circle the planet in retrograde orbits of high inclination; their distances from Jupiter vary between 20 and 24 million kilometers (290 R_J to 333 R_J). These small bodies, none more than 50 kilometers in diameter, require nearly two years for each orbit of Jupiter. It is possible that they are captured asteroids, but so little is known about them that astronomers cannot tell if their surface properties resemble those of asteroids, or if these four satellites are even similar to each other.