The heart of the "hard science fiction" designation is the relationship of the science content and attitude to the rest of the narrative, and (for some readers, at least) the "hardness" or rigor of the science itself. One requirement for hard SF is procedural or intentional: a story should try to be accurate, logical, credible and rigorous in its use of current scientific and technical knowledge about which technology, phenomena, scenarios and situations that are practically or theoretically possible. For example, the development of concrete proposals for spaceships, space stations, space missions, and a
US space program in the 1950s and 1960s influenced a widespread proliferation of "hard" space stories. Academic Jessica Imbach defines hard science fiction as that which "adheres generally to the known physical laws of the universe and abstains from the use of magic, although this does not necessarily mean that its futuristic scenarios are more 'realistic' than other forms of science fiction and fantasy." Later discoveries do not necessarily invalidate the label of hard SF, as evidenced by
P. Schuyler Miller, who called
Arthur C. Clarke's 1961 novel
A Fall of Moondust hard SF, Hard science fiction authors only include more controversial devices when the ideas draw from well-known scientific and mathematical principles. In contrast, authors writing
softer SF use such devices without a scientific basis (sometimes referred to as "enabling devices", since they allow the story to take place). Readers of "hard SF" often try to find inaccuracies in stories. For example, a group at
MIT concluded that the planet
Mesklin in
Hal Clement's 1953 novel
Mission of Gravity would have had a sharp edge at the equator, and a Florida high school class calculated that in
Larry Niven's 1970 novel
Ringworld the topsoil would have slid into the seas in a few thousand years. Niven fixed these errors in his sequel
The Ringworld Engineers, and noted them in the
foreword. ==Representative works==