What Every Groundling Should Know

All fantasy space shares certain, universal capabilities and properties. This allows ships to fly between planets and to voyage beyond the crystal spheres. This section covers these universal laws. In an overview, celestial bodies (like planets, suns, and comets) are suspended in wildspace. Wildspace is the standard void that exists between planets and stars within a solar system.
Wildspace, and the celestial bodies it holds, is contained within a crystal sphere. Many, many crystal spheres exist each with unique contents and all float upon the Phlogiston. This turbulent river of explosive, multicolored gas flows under its own currents; emanating from the primordial planes and flowing down to the astral and upper planes. With the right ship and proper planning, you can see it all.

Celestial Bodies

The most immediately noticeable celestial body is the ground under your feet; whether this is Krynn, Oearth, Toril, or one of the many other planets.
Celestial bodies extend upward in size up to suns, and downwards to asteroids and planetoids. Given the nature of wildspace, there are even more types of objects that fall into this category. Roughly speaking, any large, non-living conglomeration of matter counts as it wheels about the void. Most often, these objects are spherical but cubes, elliptical shapes, amorphous forms, rings, and hollow forms exist out there. Crazed old sailors even speak of seeing complex, Mobius forms.
Most of the large bodies have an atmosphere. However, this is not a hard rule and many voyages have failed for thinking so. Any celestial body may have an atmosphere of gas or they may not. These atmospheres could be safe, poisonous, thick, thin, windy, or still. A sun in wildspace is anything that provides sufficient light and heat to its crystal sphere. These celestial bodies can be aflame due to internal reactions, openings to the plane of fire, or some other reason.

Wildspace

All celestial bodies within a crystal sphere float in the airless void called wildspace. Interplanetary journeys within the crystal sphere journey predominantly in wildspace; the first obstacle to would-be travelers.
As an adventurer moves higher and higher, the air grows thinner and thinner until they reach the vacuum. Luckily, air is clingy in Spelljammer and collects around masses in amounts proportional to the object’s size. A traveler who enters wildspace can still breathe for a limited time. This is why ships, having much more mass to carry air, are the chosen method of wildspace travel.
Within an air envelope, fire behaves naturally. Be warned, as it will consume breathable air at an alarming rate. Within the vacuum of the wildspace, however, fires will not burn and are extinguished immediately. Magical fire will work in the vacuum of wildspace as the heat is fueled by magic, not air. Objects that should be set aflame by magical fire do not catch in a void unless some kind of magic sustains it.  

Crystal Spheres

All wildspace is bound by crystal spheres. Inside the crystal sphere is the vacuum of wildspace, then the planets, and stars. Outside these shells exists the Phlogiston and other spheres. Beyond those, the elemental, astral, and upper planes mark the ends of the Phlogiston sea.
The size of the crystal sphere is determined by the celestial bodies contained within. In most cases, the sphere has a radius twice the distance of the center to the outmost celestial body. Because of their great size, the shell appears virtually flat when approached.
The spheres consist of a dark, ceramic-like material that is impervious to all attempts to break it. Some legends state (and most theologians agree) the spheres were placed by the gods to protect their creations from the Phlogiston, which is held to be the prime matter of the universe. Less charitable philosophers maintain that the shells were placed to confine gods and men. In any case, all spheres are uniform excluding size.
The spheres are definitely solid but exert no gravity. No magic or means have been found to alter or damage the spheres. Even spells that allow the opening of portals are thought to be accelerating a natural phenomenon rather than altering the sphere. At this point, only 5 methods are known to penetrate the barrier:
1. The barrier can be passed using a Dimension Door or Teleportation spell.   2. Spells or magic items that force a portal to appear on the surface like Create Portal.
3. Naturally occurring portals appear on the shell at random intervals. Finding one can be a time-consuming task.
4. In some systems, the local star is part of the shell and a portal itself. Passing through this type can only occur by diving straight into the heart of the star. It is recommended to check locally before accidentally incinerating yourself.
5. The legendary Spelljammer and space dragons seem to have an innate ability to open portals that close slowly over time. A daring ship could sneak in behind them. While described as portals, these only make part of the shell ethereal for a limited time and do not transport matter to another plane. Indeed, magic that relies on other planes or dimensions are notoriously fickle near the sphere shells.
Crystal spheres also outline the definite border of a god’s or other dimensional creature’s power and influence. Magic that summons or draws power from one of these beings does not function outside of a sphere with their influence. Clerics and Paladins do not naturally regain spells slots above 2nd level when in the Phlogiston or in a sphere where their deity, or an aspect of their deity in another pantheon, is not recognized.

The Phlogiston

Outside and between the spheres is a turbulent, rainbow ocean of flammable ether called the Phlogiston. Within this ether floats the many crystal spheres that make up the spelljamming universe. Phlogiston refers to both the ether and the area it fills.
The Phlogiston has varying thicknesses in space and forms dense rivers between spheres. Travelers find that they sail faster when following these concentrations; rising out of and diving deeper to control their movement. This relation is completely dependent on the surface area exposed and the reason why spacefaring ships keep their groundling sails.
Phlogiston is none of the known matters in the world. It is comprised of neither air, water, nor earth. It cannot be reproduced or brought into a crystal sphere or another plane. Any attempt to take it into a sphere or plane sees the material dissipate; even in sealed containers.
Finally, the Phlogiston is dangerously flammable. Any flame brought into it or ignited within the ether immediately explodes and subsequently extinguishes. The explosion increases both the radius of the effect and the damage by 3 times.
Candle: 1d6 fire damage; 4’’ radius Lantern: 3d6 fire damage; 1’ radius Oil Flask: 3d6 fire damage; 3’ radius Cooking Fire: 4d6 fire damage; 10’ radius Match: 1 fire damage; 2’’ radius Gunfire: 2d6 fire damage; 2’ radius; shot misfires
This effect occurs immediately. For instance, trying to light a candle is impossible as the match explodes as soon as it is struck. Spells like Fireball explode as soon as the fire appears on the wizard’s hands to full effect, centered on the wizard.
Ships fly dark in the Phlogiston; extinguishing all flames before entering. The Phlogiston is radiant and lights all of the outer decks of a ship in it. Below deck, or in rare (and haunted) Dark Regions, cold lights like glowing moths, mushrooms, or fireflies or magical lights are used. Unless, of course, the crew can naturally see in the dark. The flow of phlogiston between spheres typically has currents in both directions. However, it is possible for one-way flows to exist; making travel upstream impossible.
Crystal spheres bob in the Phlogiston like corks in the ocean (mind-boggling, enormous corks, but they do bob). They naturally move around but do not change the flow of the Phlogiston. The Phlogiston always thickens to prevent collisions between spheres so they keep their relative positions. Navigation is therefore possible in this strange ocean, albeit more of an art than a science.

Gravity

The reason matter pulls air through wildspace is because of gravity. Gravity is the reason people can stand on the deck of a ship without floating off and on a spherical planet without falling. Every collection of mass (planets, ships, creatures, and the cup you dropped over the ship’s size) all have gravity. In Spelljammer, gravity is a generous force in that its direction always seems to be “that which is most convenient”. On large celestial bodies, gravity always pulls toward the object’s center. This way, anyone on the object’s surface is pulled down to the ground no matter where they stand on it. On smaller objects, like spacecraft, gravity is not a point but a plane that bisects the object horizontally through the point of most mass.
This plane is two-directional; it allows attraction from both the top and bottom. A sailor can actually stand on the bottom of a ship’s hull and move around just as easily as if he was on the deck. Objects thrown off the side of a spelljamming ship oscillate back and forth across the plane. First, it falls down towards the plane, passes it, then reverses and falls “up” in the other direction. To a person, on deck, it looks as if the objects falls down, then up, then down, and so on in a bobbing manner. This trick is commonly used to amuse passengers new to space travel. More than one groundling has gotten in trouble for standing at the ship’s rail and tossing apples to see them bob. Objects caught on this gravity plane and not connected to the mass itself will slowly drift away from the center. A sailor who falls overboard will eventually leave a ship’s atmosphere and gravity plane.
A carefully thrown or shot object can actually begin to orbit the ship. Such orbits tend not to last long; the object usually collides with a mast, rigging, or oar. One of the favorite pranks of seasoned sailors is to throw an object from one side to hit someone standing on the opposite side of the ship.
When two gravity planes exist in the same area and both remain intact, objects within their ranges adhere to whichever object’s mass is closer. Therefore, a sailor could jump from ship to ship with their “down” changing mid-flight. Once the two objects collide, the gravity plane of the larger of the two objects becomes the main gravity. 
Combat and function in zero gravity is difficult. All attack rolls and ability checks are performed at disadvantage. Additionally, creatures in zero gravity cannot control their movement. However, missiles shot across wildspace continue traveling. A trained archer can still fire through the vacuum but arrows and bullets fired into space are usually lost for good.

Temperature

Due to planetary activity, and various gates to the plane of elemental fire, temperature in wildspace is not an issue. The ambient temperature in space is about the same as a moderate summer day. Some spheres may vary from this temperature and planets very likely will. There are no seasons in space but local temperatures vary. Being close to a sun could incinerate a ship while other phenomenon could result in frozen chunks of matter floating about. Time Local time on planets varies by location, rotation, culture, customs, and other factors. As such, Spelljammers tend to use an agreed-on standard day. One standard day is 24 hours divided into three 8 hour shifts; first, second, and night. A standard week is 7 standard days and a month is four standard weeks for a 28-day total.
Spelljammers have no need or care for measurements greater than months. Seasons are just as irrelevant.