 Why does the red-headed man come to the moon?
The shack and the job he filled were located in the medieval notion of the physical appearance of hell. By night—lunar night, of course, and lunar day—it was frigidity and horror. Once in two weeks Earth-time a rocketship came around the horizon from Lunar City with stores for the colony deep underground. The Crack, of course, was that gaping rocky fault which stretches nine hundred miles, jaggedly, over the side of the Moon that Earth never sees. There is nothing like it on Earth, of course. When it was first found, scientists descended into it to examine the exposed rock-strata and learn the history of the Moon before its craters were made. Pop lived in it all alone, and in his spare time he worked industriously at recovering some missing portions of his life that Sattell had managed to take away from him. There were air-tight bulkheads for safety, and a hydroponic garden to keep the air fresh, and all sorts of things to make life possible for men under if not on the Moon. In the Moon's slight gravity, a man is really adjusted to existence when he has a well-developed case of agoraphobia. With such an aid, a man can get into a tiny, coffinlike cubbyhole, and feel solidity above and below and around him, and happily tell himself that it feels delicious. He'd shipped out, whimpering, to the Moon to get far away from Pop, and Pop was just about a mile overhead and there was no way to get around him. In any case Pop was essential, either for carrying or guidance. Pop had come back to consciousness in a hospital with a great wound in his head and no memory of anything that had happened before that moment. When he was stronger, the doctors told him who he was, and as gently as possible what had happened to his wife and children. All of which happened back on Earth and a long time ago. He usually reached the moondozer about the time the ship began to brake for landing, and he watched it come in. The pilot killed all forward motion just above the field and came steadily and smoothly down to land between the silvery triangles that marked the landing place. When Sattell fled to another continent, Pop followed because he had some distinct memories of his wife—and the way he'd felt about her—and some fugitive mental images of his children. When Sattell frenziedly tried to deny knowledge of the murder in Tangier, Pop had come to remember both his children and some of the happiness of his married life.
 To rescue Sattell, the diamonds are his payment.
 To kill Pop.  To steal the diamonds.  To rescue Sattell, the diamonds are his payment.  To destroy the lunar colony.