Scifi writer Wright has a stab at defining the genre, with some success:
Science fiction is the genre which introduces its sense of the fantastic through wonders made plausible by reference to the scientific world view. In other words, science fiction has a setting, props, or characters of extraterrestrial or futuristic origins rather than magical or supernatural. A yarn set on Mars is science fiction, set in Oz is a fantasy; jetpack is science fiction, flying carpet is fantasy; a monster is fantasy, but a Morlock is science fiction.
Lowgrade science fiction, space opera (my own genre) or sciffy (like STAR WARS) use the props and settings but not the essential feature of the science fiction genre: The essential feature of science fiction is speculation from what is known to be scientifically plausible to what is implausible, to treat the unrealistic element in the tale realistically, so that the reader is taken as if by surprise: “Ah! Well, of course that is what it would be like!”
I agree with his definition.
Read the whole thing, its a superb review of Out of the Silent Planet which should increase your appreciation of that beloved book.