After astronauts set the Eagle down in the Sea of Tranquility, more than half a billion people heard Neil Armstrong’s voice transmitted from 240,000 miles away from Earth. “That’s one small step for man, one giant leap for mankind,” he said as he became 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,” Cernan said, “There has to be somebody…