Olaf Petersen came back from planet leave on Callisto with a self-assembly android version of Marilyn Monroe. He never put it together, but Dave Lister later assembled the droid as a present for Kryten. ("The Last Day", Series III)
Lister's ex-girlfriend Hayley Summers left him when she was offered a "dream job" on Callisto. Shortly after she arrived there, she discovered that she was pregnant. Three million years later, Lister found out from a much-delayed mail pod the child was not his, but "Roy"'s. ("Dear Dave", Series X)
The terraforming of Callisto was apparently not entirely successful, as Arnold Rimmer mentioned that he had witnessed the destruction of the colony on Callisto or a colony there due to intense acid rain. (Novel Better Than Life)
In an interview with Talkie Toaster on the official Red Dwarf website, Frank Hollister revealed some details on his earlier years, although he made up some tall tales before admitting the truth. After he was fired from working in a diner in Kentucky, USA, Hollister sold doughnuts on Callisto as "Dennis the Doughnut Boy". From there, Hollister moved onto Phobos, where he got his first posting as a Third Technician aboard a JMC spaceship, the McGovern.