8:36 P.M. EST, December 12, 1972: Apollo 17 astronauts Gene Cernan and Jack Schmitt braked to a stop alongside Nansen Crater, keenly aware that they were far, far from home. They had flown nearly a quarter-million miles to the man in the moon’s left eye, landed at its edge, and then driven five miles into this desolate, boulder-strewn landscape. As they gathered samples, they strode at the outermost edge of mankind’s travels. This place, this moment, marked the extreme of exploration for a species born to wander.