 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. 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. 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. 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. The sun rose very, very slowly in what by convention was called the east. It took nearly two hours to urge its disk above the horizon, and it burned terribly in emptiness for fourteen times twenty-four hours before sunset. He'd come to the Moon in the first place because Sattell was here. At the same time, liquid air from another tank evaporated to maintain the proper air pressure in the shack. Writing-material was scarce, but he spent most of the time between two particular rocket-landings getting down on paper exactly how a child had looked while sleeping, some fifteen years before. The sun rose, and baked the abomination of desolation which was the moonscape. The occupation made it absurdly easy to live on the surface of the far side of the Moon, whether anybody else could do it or not.
 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.