Half a century ago, the astronauts of Apollo 17 spent three days on that pockmarked orb, whose gravitational pull tugs not just on our oceans but our imaginations. For 75 hours, the astronauts moonwalked in their spacesuits and rode in a lunar rover, with humanity watching on television sets 240,000 miles away. The Apollo program was shuttered after they splashed back down to the Pacific Ocean in December 1972, and since then, the moon has hung, uncharted and empty, a siren in the sky.

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