From Slate Magazine: 

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John Milton—poet, free speech advocate, civil servant, classics scholar—was arguably a forefather to Asimov, Bradbury, Delaney, and the rest. Their outlandish other worlds owe a debt to his visionary mode of storytelling; their romance—characters who go on quests, encounter adversaries at portals, channel the forces of light and dark—is his, too. 

 

Not to mention that the text of Paradise Lost is saturated in science. Milton met Galileo, for the first and only time, in a 1638 visit that Jonathan Rosen compared to “those comic book specials in which Superman meets Batman.” The “Tuscan artist” appears in Paradise Lost more than once. Book I compares Satan’s shield to the moon seen through a telescope. And the poem is studded with scientific details—“luminous inferior orbs” churning through outer space, descriptions of sunspots and seasons, creatures that evolve (according to divine plan, but still). Through it all, Milton, a storyteller, comes off as entranced by the laws governing the universe. (His mouthpiece in this regard is Adam, who cannot get enough of the angel Raphael’s disquisition on celestial motions in Book VIII.) There’s something very sci-fi about anyone who, while taking care to present his era’s astronomical theories as speculative, still likes to spin that speculation out into long descriptions of cosmic phenomena. Arthur C. Clarke would surely be proud.

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Also, Milton kinda sorta thought that extraterrestrial life might be possible. In Book III of Paradise Lost, Satan flies down from Heaven to Earth, passing distant stars that, on closer inspection, turn out to be “other Worlds.”