The Orion Nebula.
As with urban fantasy, my instinct to define this on my own isn't too off the mark: hard sci-fi places strong emphasis on the SCIENCE in science fiction. I would go so far as to describe this genre as the fiction of impossibility's plausibility...that is, experimenting with what doesn't or cannot exist at this moment in time, and using scientific thought and theory (and the basic elements of fiction) to imagine not only "what if," but, "how" and "when."
As we learned in our genre discussion class with Dr. Arnzen during our January residency for the Writing Popular Fiction program, speculative fiction's goal is to create wonder by asking the what-if questions. But what about the how/when questions?
This leads to the idea of innovation in the genre. "Make no mistake: the most important part of a science fiction story is the story itself [. . .] That said, science fiction relies more on conceptual innovation than any other branch of literature" (Doctorow, Schroeder 60). Innovation is utilized in the answering of the "how" and "when."
Though innovation is key to all speculative fiction, the sci-fi author has a heavy task before them, because there are limitations to how innovative he or she can be. Ideas are to be imaginative and original, but only within the framework of hard science. This type of fiction must be "based mainly on accepted scientific principles and plausible technological analogies" (Hardsf.net). It is "characterized by rigorous attention to accurate detail in quantitative sciences, [. . .] or on accurately depicting worlds that more advanced technology may make possible" (Wikipedia).
As David Samuelson wrote for DePauw University's Science Fiction Studies journal, "Rhetorical features of science do help characterize hard SF, since it uses scientific findings and theories as measures of reality. Accurate but unobtrusive science may not define the subgenre, but neither does a rhetoric of hardness without scientific substance. In the best examples, the two interact positively, demanding reader sensitivity to both as indicators of quality" (Science Fiction Studies, Vol. 20).
Though there is a strong emphasis on the writer of hard sci-fi to exhibit sensitivity to its subject matter, as Samuelson suggested, the reader is also expected to be sensitive to the fine balance of rhetoric and accuracy. In fact, "readers of 'hard SF' often try to find inaccuracies in stories, a process which Gary Westfahl says writers call 'the game'" (Absolute Astronomy). In this way, science fiction requires its readers to be innovators and problem-solvers, too. This makes sci-fi a "living" genre and substantial in its contributions to literature, culture, and science.