After astronauts set the Eagle down in the Sea of Tranquility, Neil Armstrong said, “That’s one small step for man, one giant leap for mankind.” He was the first human to walk on the surface of the moon. Other space travelers followed, including the commander of the last Apollo mission, Gene Cernan. “There I was, and there you are, the Earth—dynamic, overwhelming, and I felt . . . it was just too beautiful to happen by accident...