Perry Rhodan is the title of the most successful science fiction series ever written, having sold more than 2 billion weeklies since it’s introduction in September 1961. The series now approaches some 2,900 novella-sized installments in the old pulp magazine format. Despite such phenomenal success, it remains relatively unknown outside Germany, although not for lack of effort. As with most early sci-fi, the initial episodes were aimed at younger readers, and so when they were translated into English and published by Ace in the 70s, there was not widespread enthusiasm and the effort ceased after the first ~130 issues.
The story borrows heavily from the science fiction canon, including elements such as Asimov’s positronic brain, and is exactly what you would expect from any long-running serial — from comics to soap operas — with a now-complex continuity that includes many side characters, spinoffs, alternate universes, dramatic resurrections, and so on. The eponymous hero is an American astronaut and part of the first moon landing, which the series creators (in 1961) placed ten years in the future in 1971, rather than the actual date of 1969, and has since expanded to produce an entire universe, including its own theory of superintelligent life.
Plots are told in arcs spanning 25-100 issues, with “grand arcs” spanning several smaller ones. The writing staff has turned over, of course, but several authors worked on the series for decades, and some their whole careers.
In the introduction to the first English-language edition, Forrest J. Ackerman — don of mid-century science fiction (who I’ve written about before) — said: “In Germany, all serious SF buffs claim to hate Perry Rhodan, but somebody (in unprecedented numbers) is certainly reading him.”