Some readers may realize that this story, first published in 1956, has been overtaken by events. In 1965, astronomers discovered that Mercury does not keep one side always to the Sun, but has a period of rotation of about fifty-four days, so that all parts of it are exposed to the sunlight at one time or another.Well, what can I do except say that I wish astronomers would get things right to begin with?And I certainly refuse to change the story to suit their whims.