It was 120 years ago that H. G. Wells’s “The War of the Worlds,” about a Martian invasion of Earth, appeared as a magazine serial. (In the United States, the story was rolled out in Cosmopolitan.) Nearly 80 years ago, a radio dramatization of the novel caused “a wave of mass hysteria,” according to The New York Times, among some listeners who didn’t know the terrifying scenario was fiction. Now, Stephen Baxter has written a sequel, “The Massacre of Mankind,” authorized by Wells’s estate. (Baxter is the author of dozens of science-fiction books, and, even more pertinently, a vice president of the H.G. Wells Society.) His book was published in Britain earlier this year, where The Guardian said: “It’s Baxter’s best novel in a long time, and this may be because it reads like a genuine labor of love.” Below, Baxter talks about the evolution of Wells’s original story, his own childhood love of science fiction and more.

When did you first get the idea to write this book?

This is my second Wells sequel. “The Time Ships,” about 20 years ago, was a sequel to “The Time Machine,” which ends on a huge cliffhanger. I thought, as a kid, there must be a sequel by Wells, but no, he never did one.

It stuck in my mind, again, that “The War of the Worlds” finishes as a kind of cliffhanger, because clearly the Martians are going to come again and follow up on their initial invasion. So it goes back to my boyhood reading. I was 12 or 13. My secondary school had a good science-fiction collection, as it happened.

What’s the most surprising thing you learned while writing it?

The work that Wells put into the original; the development it went through. There are some surviving drafts, at the University of Illinois. What really surprised me was how the narrator evolved. In the initial drafts, he’s a much more competent character, much more purposeful. He loses his wife to the Martians; they destroy the town he lived in. He becomes enraged and wants revenge, so he falls in with the resistance, and he’s going to blow up the Martians, like a suicide bomber.