The term hard sf (or hard science fiction) is used a lot but it seems to mean different things to different people. To me, it is a story in which the author sticks within the confines of how the universe acts based on science, with as small a number of extrapolations as possible. Some stories like this stay entirely in the solar system and adhere very closely to current scientific understanding. To me, The Martian by Andy Weir was a great example. To get outside the solar system, the most obvious constraint is the speed of light and the theory of relativity. If you accept this constraint, you are locked into generation ships or interstellar ramjets, either Bussard’s original proposal or the catalytic variant that was published in the 70’s. Many people will accept as hard sf a reasonable postulate for a way to circumvent this limit or the distance involved (wormholes, hyperspace, Alcubierre drives). Cherryh’s Alliance-Union universe (at least the Company Wars part), McDevitt’s Priscilla Hutchins series, and Scalzi’s Collapsing Empire (a particularly innovative way of approaching it) are good examples. What doesn’t work so well anymore are books like the Foundation trilogy. There is more to sticking to the science than relativistic physics, though. How could suspended animation work? If the star your planet circles is red, would the plants be green? (Probably not.) For good or ill, I tried to wrestle with some of these in Starman’s Saga. It’s fun but then there is always the risk that the science starts to dominate the story. Curious what others think. Let me know.
Close to completion on the audiobook! Can’t wait to see (hear) how that finally comes out.
Stay well and stay sate.