Voidjammer: Fantasy Space Cosmology Pt I

Travel in Fantasy Space is full of weird and wonderful sights and encounters. From Space Dragons and Living Planets to the stars and planets themselves. In part, fantasy space is based on Ptolemaic Astronomy, Aristotelian Physics. But it is also inspired by space-going adventure tales from past decades such as Flash Gordon, John Carter of Mars and others. Its not so much about real-world astrophysics (which is far weirder), but about daring adventures against jaw-dropping vistas.
The multiverse of Voidjammer presented below uses the Planar Rules from pages 19-24 of the Blood and Treasure Treasurekeepers Tome. Whether or not planets and wildspace are actually other planes from the Prime Material, the planar rules are still excellent for portraying strange environment. As for my home campaign, other planets are other planes.


WILDSPACE (Starry Sea, the Void, Space)
"When ships to sail the void between the stars have been invented there will also be men who come forward to sail those ships." -Johannes Kepler
Wildspace is the "space" between celestial bodies (planets, suns, etc). It is considered coterminous with the Material Plane, the crystal shells and the phlogiston. Wildspace is also coexistent with the Ethereal Plane, The Astral Plane, and the Plane of Shadow. Wildspace doesn’t have an atmosphere of any kind and is a perfect vacuum. Conventional interplanetary journeys around a solar system take place within wildspace. It is the first obstacle that must be conquered by would-be space travelers.

Mind Eater Dreadnaught

Wildspace has no gravity, and is finite only within its own crystal sphere (though these are quite large). It is neutral in alignment, and has no elemental or energy traits. While, for the most part, magic works normally in wildspace there are some limits. Magical fire doesn’t need air to work properly but if the end result a spellcaster wants requires an atmosphere (i.e. oxygen) then that end result cannot occur. For example, a Fireball spell will work but it hasn’t any chance of lighting an object or person on fire.

Travel through Wildspace is managed with the use of specialized vehicles called Voidjammers. Ships not unlike sailing vessels in design but made (or altered) to fly through the depths of space. Through the use of magical devices called Helms, these ships can travel at tremendous speeds, and even contain their own atmosphere and gravity. 

Game Notes: Fantasy space is a dangerous place, and characters can die very quickly if they are thrown beyond an air envelope. A person can hold their breath for one round per two points of constitution if they are not moving, or for one round per three points of constitution if they are active (swimming, fighting, etc). Once they run out of breath, they suck water into their lungs and begin suffering 1d6 points of constitution damage per round until dead. Constructs and Undead are immune to this, for obvious reasons.

CELESTIAL BODIES 

Asteroids are great for those who don't like neighbors.

The celestial body that is most familiar to typical player characters is their home planet, be it Barsoom, Mongo, Eternia, or any one of countless others that populate the multiverse. Celestial bodies extend upward in size to that of the largest sun, and downward to the size of asteroids and planetoids. The tremendous variety that is possible (and proven) in celestial bodies mandates that the only accurate definition for the term is any significantly large conglomeration of matter that is wheeling about in wildspace.
In general, however, a celestial body is a plane in and of itself, generally corresponding to one or more of the Inner Planes. Most of these Celestial Bodies are single-biome worlds, with only those with diverse biospheres (like our world) being considered "Prime Material" planets. Treasure Keepers are encouraged to be creative when designing Celestial Bodies. Some common classifications: Asteroids, Cloud Planets (Gas Giants), Dark Planets, Death Planets, Desert Planet, Discworlds, Fire Bodies (aka Suns), Forest Planets, Ice Planets, Jungle Planets, Ocean Planets, Swamp Planets, Twilight Worlds, and Volcano Planets.
Celestial Bodies can have any gravity, elemental and energy traits, or magic rules, depending on the concept of the world in question. Alignment of Celestial Bodies are usually some variation on Mildly Neutral (Lawful Neutral, True Neutral, Chaotic Neutral), but some few are more strongly aligned.
Celestial bodies can have any shape, though the most common is spherical. Still, there are flat worlds, elliptical worlds, cubic worlds, amorphous worlds, ring-shaped worlds, hollow worlds. Some astronomers from have even theorized the existence of a mobius world.

Next: Crystal Spheres and The Phlogiston!