 What is the relationship between Sattell and Pop Young?
His name was Sattell and he had reason not to talk. Pop Young alone knew the whole truth, and he kept his mouth shut, too. The shack and the job he filled were located in the medieval notion of the physical appearance of hell. 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. Where Pop Young's shack stood it was only a hundred yards, but the colony was a full mile down, in one wall. 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. But Sattell couldn't comfort himself so easily. It doesn't take too long for the low gravity to tear a man's nerves to shreds. Even now there were some who had to be carried, but there were some tougher ones who were able to walk to the rocketship if Pop put a tarpaulin over their heads so they didn't have to see the sky. In any case Pop was essential, either for carrying or guidance. Sattell got the shakes when he thought of Pop, and Pop rather probably knew it.
 Sattell is trying to escape Pop, who believes he killed his family
 Sattell uses methods to help Pop recover his memories  Sattell is trying to escape Pop, who believes he killed his family  Sattell was Pop's neighbor back on Earth  Sattell is Pop's son and the only witness who saw Pop murder his wife and other children