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"Animation of a fly-by of the Galilean moons of Jupiter, ending at the planet itself eclipsing the Sun. The moons are seen in order of their distance from the planet, starting with the outermost, Callisto, some 1.9 million kilometres from Jupiter. Next is Ganymede, the largest moon in the Solar System, which orbits 1.07 million kilometres from Jupiter. Ice-covered Europa is seen next, some 671, 000 kilometres from the planet, and within that volcanically active Io, just 421, 000 kilometres from Jupiter. Ganymede, Europa and Io orbit in a 4:2:1 resonance, meaning that Io completes four orbits for every two of Europa and one of Ganymede. This resonance, combined with tidal effects from massive Jupiter, heats their interiors, powering the vulcanism on Io. The Galilean moons, particularly Europa and Ganymede, are the targets of the European Space Agency's JUICE (Jupiter Icy Moons Explorer) spacecraft, due to launch in 2022."