In 1909, Maurice Renard wrote in Le Spectateur an analysis of the emerging scientific-marvelous stories. Tracing the birth of scientific-marvelous from Edgar Allan Poe, Renard noticed that the scientific-marvelous had two defining characteristics: a scientific principle and the extrapolation of that principle into the unknown. Taking sight on Jules Verne, Renard said:
It's all about, for example, having the idea of a time machine to explore time, and not about a fictional protagonist who has managed to construct a submarine at a time when real engineers are hot on the trail of such an invention. And I strongly assert that this, in essence, is what differentiates Wells from Jules Verne--two writers so frequently lumped together. Jules Verne never wrote a single sentence of scientific-marvelous. In his time, science was pregnant with many impending discoveries; Verne simply supposed them already born before they actually were. He only barely extrapolated on discoveries that were already on their way to seeing the light of day.Verne lacked the extrapolation of the unknown, even though his stories were full of scientific accuracy born from endless research. His stories were never among the scientific-marvelous. Verne even denied that his works were ever supposed to be read as scientific. And while Renard held H. G. Wells up as the man who "[fleshed] out the themes of the scientific-marvelous," he also pointed out that many of Wells' scientific romances, like Verne's, fell short of inclusion into the genre of the scientific-marvelous.
Using Renard's definition in the present day allows us to make the following determinations: The Martian by Andy Weir is not hard SF, for hard SF in the traditions pre-pulp was “all about extending science fully into the unknown, and not simply imagining that science has finally accomplished such and such a feat currently in the process of coming to be.” The Martian has significant scientific accuracy, but it is all emergent engineering and not the extending of science fully into the unknown. John C. Wright’s Count to the Eschaton sequence is far more fanciful than The Martian, but it is true hard SF as it takes the vast distances between worlds and postulates what might be needed to create a polity between them. In fact, it is possible for scientifically implausible "soft" science fiction to still be hard, if, among its flights of fancy, it establishes one scientific principle and runs straight into the unknown with its implications.
Not all that crunches is hard.