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On January 7, 1610, Galileo Galilei pointed his "spyglass" to the heavens and stared up at Jupiter, one of the brightest lights in the evening sky, and noted what he at first assumed to be three bright stars near the planet. But over the following nights, he realized that those three bright bodies weren't fixed in the heavens like stars, but rather seemed to dance around Jupiter along with a fourth, smaller body. Galileo triumphantly announced his discovery of four "planets" that revolved around Jupiter in his March treatise, Starry Messenger [pdf]. Thinking of his pocketbook, he dutifully proposed naming them the Medicean Stars in honor of his patron, Cosimo de Medici. But the name didn't stick, and today we honor the scientist rather than the patron by calling Jupiter's four largest satellites the Galilean moons. The discovery dealt a death blow to the Ptolemaic understanding of the universe, in which all planets and stars were believed to orbit the Earth. For, as Galileo wrote in his treatise, "our own eyes show us four stars which wander around Jupiter as does the moon around the earth." In the 400 years that have passed since Galileo first laid eyes on them, we've learned a great deal about the moons Io, Europa, Ganymede, and Callisto (all named after the mythological paramours of Jupiter). If all goes according to plan we'll soon get to know them much more intimately--NASA and the European Space Agency are currently planning missions to closely observe three of the moons. Click though this gallery to view NASA's most stunning photos of the four satellites, and to find out what we've discovered in the four centuries since Galileo began the work. (For more on Galileo's discovery and what it meant to science, check out this post from DISCOVER's Phil Plait.) Image: NASA/JPL/DLR

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Io, the innermost of the four moons, is a tempestuous place. It's covered with more than 400 active volcanoes, which spew out sulfur compounds that may be responsible for its mottled, pizza-like appearance. We got our first good look at Io when the Voyager probes flew by in 1979 on their way to the solar system's edge; one of the probes took this portrait of the fiery moon. Image: NASA

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Io's volcanic activity is driven by intense tidal forces. On one side, massive Jupiter tugs at it, while the other three Galilean moons pull at it from the other direction. The tidal forces stretch and squeeze the moon's interior, which causes heat and pressure that are released in colossal volcanoes. This image, taken by the Galileo probe that explored the Jupiter system in the 1990s, shows two volcanoes erupting. At the top, a plume of gas rises 87 miles from the moon's surface, while in the middle of the image the ring-shaped Prometheus plume is visible. That plume has been visible in every image of the region taken by our probes and space telescopes, suggesting that the Prometheus volcano has been erupting for decades. Image: NASA/JPL/Galileo Project

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Europa is one of the most interesting places in the solar system--which explains why NASA hopes to send a probe there in the 2020s. Slightly smaller than the Earth's moon, Europa looks like a well-used skating rink with an ice-covered surface marred by dark cracks and ridges. It has an ice layer that's miles thick, but underneath that ice scientists think a liquid ocean is sloshing around. And where there is water, there might be life. Scientists speculate that the cracks and ridges spangled across Europa may be caused by tidal forces from Jupiter and the other Galilean satellites; the moon's surface may have fractured in the past as internal material was forced up. Image: NASA/Goddard Space Flight Center

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NASA's Europa probe is currently scheduled to launch in 2020 as part of an international mission that would also include a European Space Agency orbiter that would study Ganymede and Callisto. The Europa probe's mission will be to measure the thickness of the ice sheet that covers the moon, and to determine whether there really is a liquid ocean beneath the ice. Much of the probe's $3 billion budget will be spent on shielding to protect it from the fierce radiation belts produced by Jupiter's magnetic field. That won't leave any money for a Europa lander this trip, so there won't be any drilling into the ice or exploration by submarine. The thickness of the ice is a crucial question, and may determine whether or not there is life on the moon. If the ice is too thick (one estimate suggested a 15-mile-thick barrier), rudimentary life forms wouldn't be able to use solar energy and photosynthesis to sustain themselves. However, a thick ice shell would also protect any life forms from Jupiter's punishing radiation, and if the right chemical components are present in the water organisms could live on chemical energy, just like marine creatures here on Earth that live deep in the ocean near thermal vents. Image: NASA/JPL

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Ganymede, named after the beautiful boy who became Jupiter's cup-bearer in Olympus, is the largest moon in the solar system--in fact, it's larger than the planet Mercury. It's composed primarily of rock and water ice, but researchers believe it may have a deeply buried ocean sandwiched between layers of ice. The European Space Agency wants to send its probe to orbit Ganymede to study its surface, which is a mixture of older regions pockmarked by asteroid impacts and younger, smoother regions that boast streaks similar to those seen on Europa. The orbiter would also look for evidence of that buried ocean, which may lie about 100 miles beneath the surface. Finally, it would examine the oddball moon's magnetic field--Ganymede is the only moon in our solar system known to have a magnetosphere of its very own, and researchers still aren't sure how it's generated. Image: NASA/JPL/Galileo Project

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The final Galilean moon, Callisto, will also be observed by the European probe in a series of fly-bys. Like its neighbor Ganymede, it too is composed mostly of rock and ice, and it's also suspected to have a deeply buried ocean layer. It's the most heavily cratered object in the solar system, and its surface appears to have changed little in the last 4 billion years. But Callisto has a unique attraction. As the Galilean moon farthest from Jupiter, it's exposed to far less radiation from Jupiter's intense magnetosphere--a person standing on the surface of Callisto would be exposed to about .01 rems of radiation per day, while someone on the surface of Europa would receive 540 rems daily. That's why some advocates of manned exploration of the solar system suggest that Callisto would be a great spot for a human settlement [pdf]; from that base, astronauts could control robot scouts throughout the Jupiter system. Who knows, maybe in another 400 years it will come to pass! Image: NASA/JPL/Galileo Project

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