Hard SF?

"This story is 'hard' science fiction, meaning that it assumes nothing a present-day scientist would consider physically impossible. True, in it humans have reached a distant star, but they did not necessarily travel faster than light. Perhaps their ship got close to that speed, giving them the benefit of time dilation, or perhaps they passed a voyage of centuries in some kind of suspended animation..."

(Poul Anderson, Introduction to "Strangers," All One Universe, New York, 1997, p. 2)

And I remember recently reading an Introduction to "The Queen of Air and Darkness" where Anderson said something similar, like this can be hard sf because its interstellar travel is slower than light, but I cannot remember which collection this was in.

(Incidentally, I once attended a talk at the London Planetarium where the speaker thought that, when sf authors wrote about traveling to another star, they meant literally onto the surface of the star, not to a planet of that star.)

I always thought that Larry Niven's Known Space and Anderson's Technic History were hard sf even though they involved FTL. These authors accept that FTL through relativistic space is physically impossible which is why they postulate hyperspace which, in Anderson's case, is a series of instantaneous quantum jumps. A story in which nothing was done that had not already been done would be scientifically based fiction but not "science fiction." An sf premise can be either that people have applied existing scientific knowledge to do something that has not been done yet, like flying to Mars, or that they have made a new scientific discovery, like hyperspace, enabling them to fly FTL to Proxima Centauri.