Wollheim's stories were published regularly from 1940; at the same time he was becoming an important editor. In the 1950s and 60s he wrote chiefly novels. He usually used pseudonyms for works aimed at grownups, and wrote children's novels under his own name. Notable and popular were the eight "Mike Mars" books for children, which explored different facets of the NASA space program. Also well-received were the "Secret" books for young readers: "The Secret of Saturn's Rings" (1954), "Secret of the Martian Moons" (1955), and "The Secret of the Ninth Planet" (1959). As Martin Pearson he published the "Ajax Calkins" series, which became the basis for his novel "Destiny's Orbit" (1962). A sequel, "Destination: Saturn" was published in 1967 in collaboration with Lin Carter. "The Universe Makers" (1971) is a discussion of themes and philosophy in science fiction.