Galilean moons
The first spacecraft to visit Jupiter were "Pioneer 10" in 1973, and "Pioneer 11" a year later, taking low-resolution images of the four Galilean moons and returning data on their atmospheres and radiation belts. The "Voyager 1" and "Voyager 2" probes visited Jupiter in 1979, discovering the volcanic activity on Io and the presence of water ice on the surface of Europa. The "Cassini" probe to Saturn flew by Jupiter in 2000 and collected data on interactions of the Galilean moons with Jupiter's extended atmosphere. The "New Horizons" spacecraft flew by Jupiter in 2007 and made improved measurements of its satellites' orbital parameters.