Jupiter's satellites

Probes Voyager I and Voyager 2 provided the first close observations of Jupiter's four major satellites, revealing unique geological features like Io's active volcanoes and Europa's smooth surface. The mission also led to the discovery of three new satellites and a thin ring around Jupiter.
 (NASA) United States 1979 Jupiter's satellites

Probes Voyager I and Voyager 2 provided the first close observations of Jupiter's four major satellites, revealing unique geological features like Io's active volcanoes and Europa's smooth surface. The mission also led to the discovery of three new satellites and a thin ring around Jupiter.