Everything you know about space is wrong. Stars as flaming spheres of plasma; movement through space as a balance of thrust and acceleration; scientific fact backing up natural phenomena… Forget all that. It’s wrong. We live in a magical universe.
– Khelben the Younger
Welcome to Spelljammer. You can get out of the atmosphere on the back of a roc; fly between the planets with your own personal gravity; sail between the spheres of wildspace that surround the inhabited worlds on a river of magical energy. This is a universe postulated on magical, not scientific, laws. This arcame space is governed by the laws of Mordenkainen, Elminster, and Fistandantilus rather than Galileo, Newton, and Einstein.
Arcane Space can be divided into two categories: Wildspace Systems and the Astral Sea that they inhabit.
Wildspace
Wildspace is what comes to mind when we think of “space.” Simply, Wildspace is where the Astral Plane overlaps with the Material Plane. It includes the vast, empty, airless space that lies between the planets and the stars. Wildspace is not truly a void, even though it is often referred to that way. The simple fact that there is “space” rules out its being a true void.
Astral Sea
The Astral Sea encompasses all of the space between the planes of existence. When a character moves through an interplanar portal, steps through a dimension door, or projects her spirit to a different plane of existence, then she travels through the Astral Plane. The Astral Plane is a great, endless sphere of clear silvery sky, both above and below. There are the occasional bits of solid matter here, but most of the Astral Plane is an endless, open domain where the laws of reality are much different than the planets and moons of Wildspace.
All fantasy space shares certain, universal basic properties and capabilities.
Air Envelopes
All bodies take an envelope of air with them, and this provides a breathable atmosphere. Unfortunately, this envelope is rarely large enough to support life for very long. Larger bodies, such as ships and asteroids bring enough air for a single human size creature to breathe for months (or years).
Gravity
The reason everything drags its own atmosphere around through space is gravity. This is also the reason why people can stand on a space-sailing ship without falling off its deck and can stand on a spherical planet without falling off the bottom side.
Helms
Ships travel through wildspace by the helm–a magical device that converts mystical energy into motive force, the push that moves the spelljammer. This allows rapid movement from planet to planet. Some of the specifics vary from race to race. In all its configurations, the helm is designed to pull magical energies from the user and convert them into full-fledged spelljammer power to move and maneuver space ships.
Spelljamming
Spelljamming is the conversion of magical energy directly into motive force for ships in space. Spelljamming can be accomplished either by spells or through a magical device known as the spelljammer helm. Clerics, sorcerers, warlocks, and wizards can spelljam. Other classes gain spelljamming abilities at the point that they may cast spells, but cannot spelljam until that time.
Temperature
Due to the activity of the planets, the spheres, and various gates to the plane of elemental fire, temperature in wildspace is not a problem for adventurers. The ambient temperature in most space is about the same as a moderate summer day in the temperate regions of most worlds.
Time
Spelljammers often rely on what is considered the standard day for time-keeping aboard ship. A standard day is 24 hours, and is broken into three watches of eight hours each: first, second, and night (also called graveyard) watch. A standard week is seven standard days, a standard month is four standard weeks (28 days).