The novel “The Seedling Stars” by James Blish was published for the first time in 1955. It’s the fix-up of previously published stories.
Donald Sweeney is an Adapted Man, born with physiological modifications that allow him to survive in environmental conditions that would kill a normal human being very quickly. He’s a product of pantropy, a controversial experiment that aims to adapt humans to environmental conditions they might find on other planets in order to colonize them.
A group of Adapted Men took refuge on Ganymede thanks to adaptations that allow the members of the group to be comfortable in the conditions existing on Jupiter’s moon. Sweeney is sent to Ganymede by the Earth authorities with the aim of infiltrating the group of fugitives to help get them back to Earth, especially their main scientist. In return, he was promised to be modified to be able to live in the Earth’s conditions.