Hard science fiction is driven more by ideas than characterization. Plausible science and technology are central to the plot. If your story is set on a lunar colony, for example, issues of technology may be of greater concern than a character's personal life. To write effectively in this subgenre, an author must generally have a good grasp of the scientific principles involved. Much classic science fiction, including the earlier works of Asimov and Heinlein, fall into this category.