Science fiction that prioritizes scientific accuracy and logical extrapolation from real physics, biology, or engineering.
Hard science fiction treats real science as a constraint, not a suggestion. The technology obeys known physical laws (or carefully theorized extensions of them), and the story's problems are solved through logic, engineering, and actual science. Getting the details right is part of the point. When hard SF readers spot a physics mistake, they notice.
Hard SF teaches you to think through your premises rigorously. Even if you're writing softer science fiction or fantasy, the discipline of asking 'would this actually work?' makes your worldbuilding stronger. And if you're genuinely writing hard SF, your readers will hold you to a high standard of accuracy.
Survival on Mars solved through chemistry, botany, and orbital mechanics, with the math shown on the page.
An alien spacecraft explored with scientific rigor, the story driven by the mysteries of its engineering.
An interstellar survival problem solved through real biochemistry and physics, wrapped in an unexpectedly warm story.
Science should create interesting problems, not lecture the reader. If your explanation stops the plot, you've gone too far.
Hard SF readers are often scientists themselves. If you're unsure about the physics, research it or soften your classification.
Take a real scientific paper from the past year (try arxiv.org) and extract one finding. Write a 500-word scene set 30 years from now where that finding has become a technology people use daily. Get the science right and make the story compelling.