Spelljammer is a fictional setting for Dungeons & Dragons, roleplaying game. Spelljammer introduced into the AD&D universe a comprehensive system of fantasy astrophysics, including the Ptolemaic concept of crystal spheres. Crystal spheres may contain 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, birds, fish or even more wildly fantastic shapes.