Re: (ReacTor) Defining Our Terms: What Do We Mean by "Hard SF"?

I've always felt that the definition of "Hard SF" is a simple one. It
refers to science fiction that doesn't involve anything we have no
reason to believe is possible; so a story about a journey to Alpha
Centauri with an FTL drive isn't hard SF, but one about a journey to
Alpha Centauri in a generation ship might be.
So a story about the first manned landing on Mars is likely to qualify
has hard SF.
Of course, with hard SF being solidly based on present-day science, what
differentiates it between the techno-thriller... or even plain old
fiction?
While hard SF needs to be justifiable in terms of presently-known
science, it can include anticipated feats of engineering based on
existing science that have not yet been realized.
And the term has nothing to do with how the science is handled; one
would expect a soft-SF story, using things like FTL or time travel, to
have a character explain to the audience how these things are supposed
to work. The interested reader can always look up real science in a
textbook.
John Savard