<Tr> <Td> Cassini </Td> <Td> December 30, 2000 </Td> <Td> 10,000,000 km </Td> </Tr> <Tr> <Td> New Horizons </Td> <Td> February 28, 2007 </Td> <Td> 2,304,535 km </Td> </Tr> <P> Beginning in 1973, several spacecraft have performed planetary flyby maneuvers that brought them within observation range of Jupiter . The Pioneer missions obtained the first close - up images of Jupiter's atmosphere and several of its moons . They discovered that the radiation fields near the planet were much stronger than expected, but both spacecraft managed to survive in that environment . The trajectories of these spacecraft were used to refine the mass estimates of the Jovian system . Radio occultations by the planet resulted in better measurements of Jupiter's diameter and the amount of polar flattening . </P> <P> Six years later, the Voyager missions vastly improved the understanding of the Galilean moons and discovered Jupiter's rings . They also confirmed that the Great Red Spot was anticyclonic . Comparison of images showed that the Red Spot had changed hue since the Pioneer missions, turning from orange to dark brown . A torus of ionized atoms was discovered along Io's orbital path, and volcanoes were found on the moon's surface, some in the process of erupting . As the spacecraft passed behind the planet, it observed flashes of lightning in the night side atmosphere . </P>

Jupiter has the highest average density of the planets