From the Chaosium website:
Science-Fiction Rules and Adventures for Call of Cthulhu
This work provides science fiction rules for Call of Cthulhu as well as four adventures set in the future. Naturally enough, writing a science fiction supplement for a universe based on H.P. Lovecraft’s Cthulhu Mythos proved a challenging endeavor.
This lack of detail about the future was both a boon and a bane when it came to writing this work. On the negative side, the lack of detail means that the future had to be created almost whole cloth and with few guides as to what Lovecraft might have intended or envisioned. On the positive side, the lack of details allowed a broad field in which to operate. Since I have been consistent with the few available details and the spirit of Lovecraft’s stories, it would be difficult for a critic to plausibly say “that is not what Lovecraft would have intended.”
In this work, I do not even pretend to try to guess as to what Lovecraft truly had in mind in regards to the future of man. My main goal has been to present a future consistent with Lovecraft’s stories and the spirit of his works.