Knowing it when we see it
Hard science fiction, the most science-oriented sf, enjoys greater popularity with readers and writers than with critics. Less criticism is written on hard sf than on many other aspects of sf. What criticism exists tends to try to define it compactly, thus making it easier to study retrospectively. What critics usually exclude from the sub-genre either plays too loosely with the facts, or lacks the proper attitude. (The latter is usually considered the more serious transgression.)
Sf's community of writers, readers and editors resists defining genres and sub-genres. In 1999, long-time Analog editor Stanley Schmidt, heir to John W. Campbell's job, remarked:
Lately I've been saying I'd like the term 'Hard SF' to go away. Too many people use it to mean something much narrower than what I mean by it . . . science fiction is simply fiction in which some element of speculation plays such an essential and integral role that it can't be removed without making the story collapse, and in which the author has made a reasonable effort to make the speculative element as plausible as possible. Anything that doesn't meet those requirements is not science fiction at all, as far as I'm concerned, so there's no need for a separate term like 'Hard SF' to distinguish it from 'other' kinds of sf.