 Why did Pop go to Lunar City?
Once in two weeks Earth-time a rocketship came around the horizon from Lunar City with stores for the colony deep underground. 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. 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. There was a warning-bell in the shack, and when a rocketship from Lunar City got above the horizon and could send a tight beam, the gong clanged loudly, and Pop got into a vacuum-suit and went out the air lock.
 Pop went to Lunar City because Sattell went to Lunar City.
 Pop went to Lunar City because the Earth is overcrowded.  Pop went to Lunar City because Sattell went to Lunar City.  Pop went to Lunar City because his family was murdered, and he couldn't stand to be on Earth any longer.  Pop went to Lunar City to mine diamonds.