Spelljammer

Spelljammer is a campaign setting that was created by Jeff Grubb and was published by TSR, Inc., 1989 for 2nd edition Dungeons & Dragons. It features a fantastical (as opposed to scientific) outer space environment - a system of fantasy astrophysics, including the Ptolemaic concept of crystal spheres. Crystal spheres are like a bubble that contains multiple worlds and are navigable using ships equipped with "spelljamming helms". Ships powered by spelljamming helms are capable of flying into not only the sky but into space. With their own fields of gravity and atmosphere, the ships have open decks and tend not to resemble the spaceships of science fiction, but instead look more like galleons, animals, or even more wildly fantastic shapes. Players can travel between the Forgotten Realms, Dragonsphere, and Greyhawk and many other settings without invalidating the lore of any of the existing worlds.