All science fiction is fantasy.
I'm not suggesting there aren't two distinct genres. I'm not referring to ambidextrous authors who write well in both genres. I don't like it when second-hand bookshops to lump it all together under the umbrella heading 'Sci-Fi/Fantasy'.
I guess what I am saying is that there is really no such thing as hard science fiction.
Hard science fiction is the term used to describe science fiction that pays attention to scientific detail. Science fiction that cares about scientific plausibility. I've sometimes heard the term used in almost an elitist sense, hard sci-fi is capital 'S' Science fiction. As if space opera or cyberpunk, etc. were lower life forms.
But when it comes down to it science fiction isn't just science it's science fiction. As fiction it moves beyond the boundaries of science. Science fiction is about taking the world as we know it and extrapolating into a alternative reality or a possible future. Except that in all science fiction that created world, that extrapolated future is actually not scientifically possible.
At our present level of understanding wormholes are not controllable. Time travel is impossible. Intersolar trade is completely uneconomical, interstellar war unpractical. Human consciousness cannot be understood, let alone digitalised. And there is no reliable evidence for extraterrestrial life. All of these elements of science fiction are fantasy.
The difference between science fiction and fantasy is not scientific credibility. It is a matter of themes and customs. Fantasy has common themes like magic, mythical races and creatures. Science fiction has different themes, technology and extra terrestrial races and creatures. Each genre intersects our mundane world at a different point. Fantasy leaves our world in dark forests or in the mists of time and legend. Science fiction launches into a different reality from the launch pads of a space port.
But both leave the reality of our mundane world.
What distinguishes science fiction from fantasy is the additional requisite that science fiction must seem scientifically plausible. Good fantasy must have a created world that is consistent and believable. A magic system must be logical and workable. In Science Fiction the world must also be consistent and believable but science fiction must also seem scientifically plausible. But this scientific plausibility is an illusion. At some point every science fiction writer fudges the science. Deliberately blurs the detail or skates over the limit of our current knowledge or overemphasises the strength of that force or the extent of our mastery of that technology.
They do that because we let them, we want them to.The reader and the writer have an agreement. If you have a good crack at it, if you make it half way believable, I'll suspend my disbelief. I'll come along for the ride.
We make that agreement because it allows us to explore human nature. To mentally explore the universe while we can't yet physically do so. It allows us to discuss the ramifications of science and it's effects on our societies. But perhaps mostly, we write and read science fiction because it's fun.
But when it comes down to it, science fiction is a fantasy, a fiction. A useful fiction, an enjoyable fiction, but a fiction nonetheless.