Make people believe in your story first and foremost
The Colombian author Gabriel Garcia Marquez received the Nobel Prize for Literature in 1982. The celebrated author of novels such as Cien años de soledad (translated as A Hundred Years of Solitude) was also a journalist. When asked about the difference between journalism and writing fiction, Marquez answered thus:
‘In journalism just one fact that is false prejudices the entire work. In contrast, in fiction one single fact that is true gives legitimacy to the entire work. That’s the only difference, and it lies in the commitment of the writer. A novelist can do anything he wants so long as he makes people believe in it.’
This last sentence is key: It doesn’t matter if you write realist fiction set in contemporary London or futuristic sci-fi set in a Mars colony. Create believable characters who have credible motivations and flaws, immersive settings and realistic tensions and conflicts and your fiction will feel believable.