 Why does the red-headed man come to the moon?
Some people said that no normal man could do it, and mentioned the scar of a ghastly head-wound to explain his ability. 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. He handed over the product of the mine, to be forwarded to Earth. 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 one stretch where it is a yawning gulf a full half-mile wide and unguessably deep. 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. It doesn't take too long for the low gravity to tear a man's nerves to shreds. In any case Pop was essential, either for carrying or guidance.
 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.