As a genre, science fiction never goes out of style. Novels by Arthur C. Clarke, Isaac Asimov, and Ray Bradbury have remained in print for decades. Most sci-fi novels are set in space, although plots may address time travel, dystopias, alternate dimensions, mind control, new technologies, and any number of other themes. Ghostwriting science fiction requires that an author be very literate in the genre and have a strong background in science, technology, and sociology.
Many sci-fi books also delve deeply into themes of cultural change, ethics, sexuality, justice, and morality. Some of the most popular science fiction titles of all time were written by authors wishing to address issues that had enraged them. Ray Bradbury's Fahrenheit 451 was inspired by censorship imposed by a repressive political state.
Science fiction can also be highly predictive of future changes in society and technology. Most of the gadgets we use today in the digital age were first seen as "props" on Star Trek.
I've been a lifelong fan of sci-fi, so drop me a line if you want to expand the boundaries of your readers' universe.
You may also be interested in reading my Ghostwriting a Series page.