Earth-sized planetary body. Gravity in the Spelljammer universe is also an exceptionally convenient force, and almost always works such that "down" orients itself in a manner most humanoids would find sensible. All bodies of any size carry with them an envelope of air whenever they leave the surface of a planet or other stellar object. Unlike real-world astrophysics, this air envelope is not dispersed by the vacuum of space. These bubbles of air provide breathable atmosphere for varying lengths of time, but 3 months is considered "standard". Crystal spheres A crystal sphere (also known as a crystal shell) is a gigantic spherical shell which contains an entire planetary system. Each sphere varies in size but typically they are twice the diameter of the orbit of the planet that is farthest from the sun or planet at the center of the sphere (the system's primary). The surface of the sphere is called the "sphere wall" and separates the void of "wildspace" (within the sphere) from the "phlogiston" (that surrounds and flows outside the sphere). The sphere wall has no gravity and appears to be impossible to damage by any normal or magical means. Openings in the sphere wall called "portals" allow spelljamming ships or wildspace creatures to pass through and enter or exit from a crystal sphere. Portals can spontaneously open and close anywhere on the sphere wall. Magical spells (or magical items that reproduce their effects) can allow a portal to be located. Other magic can open a new portal or collapse an existing one. Ships or creatures passing through a portal when it closes may be cut in two. Note that unlike the Ptolemaic system, the crystal spheres are not nested within each other. Wildspace Wildspace is similar to the outer space of science fiction, with planets, asteroids and stars, but with different physics. Gravity is either none or the same as that of Earth, and is directed towards the center of planet-sized bodies; on large objects in space like spacecraft and enormous creatures gravity is directed towards a flat plane running through the object's long axis, allowing characters to stand on the decks of ships. The Phlogiston The phlogiston is essentially a big ocean of a unique element that is neither air, fire, water, or earth. The phlogiston (also known as "the Flow") is a bright, extremely combustible gas-like medium that exists between the Crystal Spheres. A signature