Deep-Space Photos: Hubble's Greatest Hits

Jupiter's Moon
In 1610, the Italian astronomer Galileo Galilei observed Jupiter and four of its largest moons. The one called Io, (center) is the most geologically active in our Solar System, due to its many volcanoes. Although it appears to be skimming the planet's cloud tops, Io is actually 310,000 miles from the surface of Jupiter. The black dot to right center is the moon's shadow. It takes Io just 42.5 hours to revolve once around Jupiter.

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