A moon that seemed to vanish every few weeks puzzled one of the greatest astronomers of his age.

On October 25, 1671, Giovanni Domenico Cassini discovered Iapetus, Saturn's third-largest moon. He soon noticed something strange: he could see it on one side of Saturn but not the other. Cassini correctly guessed that one hemisphere is much darker than the other.

More than three centuries later, NASA's Cassini spacecraft, named in his honor, confirmed the moon's striking two-tone surface.

If you could send a probe anywhere in the solar system, where would it go?

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