Spelljammer

Spelljammer is a campaign setting for the Dungeons & Dragons role-playing game. It introduces into the D&D lexicon the Ptolemaic concept of crystal spheres (each of which is roughly the size of a solar system), which may contain mutliple worlds and are navigable using "spelljamming ships"--magical airships which are operated by the power of magic and are capable of fying into not only the sky but into space. With their own magical fields of gravity and atmosphere, the "ships" involved tend not to resemble the spaceships of science fiction, but galleons, animals, or even more wildly fantastic shapes. Both alien space ("wildspace") and planets are inhabited by strange and dangerous creatures unknown to "groundlings", and the alien races engage in spelljamming and space warfare as well.

The Spelljammer setting is designed to allow some fantastic space adventures to take place within the traditional sword and sorcery mileu of Dungeons and Dragons; it also provides as a vehicle for crossovers between other D&D settings, in much the same fashion as the Planescape setting. Though the cosmology is derived largely from the Ptolemaic system of astronomy, many of the ideas owe much to the works of Jules Verne and his contemporaries, and to related games and fiction with a steampunk flavor.