It's difficult to define, because Sci-Fi is such a broad term.
When I Think Of Sci-Fi I think:
Dr Who, BladeRunner(and by extension Vangelis), Star Wars, Star Trek, War Of the Worlds, Thunder Birds,Journey to the Center of the Earth, Avatar, Metroid, Metal Gear Solid.
While containing a range of disperate musical styles, A common theme between the lot of them is a sense of the exotic. Whether It be through a classical medium or more bleeps bloops and synths, sci-fi's power is in taking you to another world, and thus it's music must reflect that.
In terms of tropes, again i think it's quite broad, so I'll stick to terms that I'd associate with the style. some won't fit with all the above ideas, but then again, Bach never wrote a Fugue.
Theremin, Synthesiser, Sliding Synth sound, echo.