Two well known "celestial hieroglyphs," or semi-magical, semi-scientific mainstays of science fiction, are time travel and travel that exceeds the speed of light. The latter is considered a sheer impossibility by physicists, the former a hopeless paradox by philosophers. But no telling what those black holes will do! "Science fiction writers don't admit magic, they don't admit UFOs even, but they accept as given these two magical properties, so that, in a sense, even their science fiction is built on fantasy," Sawyer comments.'How fantasy took over science fiction.' Contrary to the theme of the article, I don't think it makes much sense to talk about anything being "taken over"; rather, think of the tension between "fantasy" and "science fiction" as both a defining characteristic of genre fiction and one of its most productive creative engines. Finding the device (what Darko Suvin calls the novum) to justify and motivate the fantasy at the story's core is a huge part of what science fiction is all about.
In that sense, the two categories are essentially inseparable.