Jupiter is large enough and bright enough that even early observers were able to learn a substantial amount about the planet.When Galileo first began viewing the sky with his newly made telescope, he almost immediately found the four largest moons, now called the Galilean satellites. He recorded their orbits around Jupiter; this was the first major support for the Copernican theory that planets orbit around the Sun (in this case, the evidence was simply that the moons orbited around Jupiter, rather than around the Earth, which everything was assumed to orbit). The Inquisition sentenced Galileo to house arrest for the heresy of a solar system not centered on the Earth, though this was a light sentence, since they would have been within their rights at the time to torture and execute him for his outrageous ideas.
Similarly, the Great Red Spot has been observed from the Earth for over 300 years. It may have been seen first by Giovanni Cassini, the great Italian astronomer, or by Robert Hooke, the English experimentalist. Though the large features of the planet are easily seen through even the smallest telescopes, it took space missions to discover Io's abundant volcanism, many of Jupiter's smaller moons, and the many details of its composition and atmosphere that are now known.
Following are brief descriptions of the seven space missions that have visited Jupiter to date. The excitement created by the first images sent by Pioneer 10 was intense, and though newer science by more recent missions was perhaps more exciting and revolutionary, people have become more inured to images of distant planets, thinking them more as commonplace pictures than as results of exceptional scientific and engineering endeavors.
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