Half a century ago, Uranus, Neptune, and Pluto marked the ultimate in planetary mystery. Even as men set foot on the Moon and spacecraft sped past Venus and Mars, the outermost worlds seemed to yield their secrets as slowly as they revolve around the Sun: Uranus, twice as remote as the Ringed Planet, completing an orbit once every 84 years; Neptune, a billion miles beyond, taking a leisurely 165 years to do the same; and Pluto, nearly a billion miles beyond that, its year 248 times longer than our own.
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