Jupiter has four main moons, which were discovered by Galileo, as well as many smaller ones (47Jovian moons are currently known). Ganymede, seen here photographed by Voyager 1, is Jupiter's largest moon and the largest known planetary satellite in the solar system. It is larger than the planets Mercury and Pluto, and is an icy world with some regions scarred by impact craters and others with parallel ridges and grooves formed by unknown tectonic proceses. Ganymede has its own magnetic field, and is believed to have a molten iron core. Two Voyager spacecraft were launched in 1977 to explore the planets in the outer solar system. Voyager 1 flew past Jupiter at 278,000 kilometres in March 1979 before flying on to Saturn in November 1980.
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