How Things Work in the Spelljammer Universe

Air

As an adventurer moves higher (whether climbing a mountain or on the back of a roc), the atmosphere becomes thinner and thinner until at last it becomes vacuum . The climber can still breathe, however, because as he moves upward, an envelope of air clings to him. When he reaches the point where the planet's air is no longer breathable, he is breathing his own air, held near him by his body's own gravity. This air envelope attaches to everything that passes through the atmosphere and allows normal survival in Wildspace, at least for a short time.

Larger objects, such as ships, are rated for their tonnage. Each ton represents 100 cubic yards of space, which brings along enough air to support one human-size crew member for three to six months.

The longer the ship goes without replenishing the air supply, the more the air quality degrades. It is probably one of the most important jobs of the quartermaster to monitor the current conditions of the air quality of the ship. The capacity of a ship’s air envelope can healthily support a ship’s listed crew and passenger capacity for about 3 months. After 3 months, the air becomes stale. It is a bad sign when air starts to stale, as it usually indicates that the ship is lost, or worse. Stale air smells bad, and while it’s enough for you to survive, it isn’t nearly as oxygen rich as it should be. Working in oxygen thin environments is fatiguing. The best way to track this fatigue, is to have characters make a Vigor roll at the start of any scene involving physical activity. If the roll fails, the characters have one level of Fatigue. Snake-eyes means they have two levels. If the characters haven’t had enough rest between episodes of hard work, the GM may decide that the Fatigue levels carry over to the next encounter, when another Vigor roll should be made at the start of the scene. On the six month, the air quality moves from stale to foul. This is bad news, as it means everyone is suffocating to death. Characters who make an effort to move as little as possible make a Vigor roll every 12 hours or gain a Fatigue level. Keep rolling after the character becomes incapacitated, a failure indicates the character has passed on. Characters who perform any activity makes a Vigor roll every minute. On the seventh month, everyone who needs to breathe oxygen to survive dies of suffocation.

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.

Every body in space has its own gravity. Gravity is an accommodating force in that its direction seems to be "that which is most convenient." In an object the size of a planet, gravity is directed toward a point at the center of the planet so that people can stand anywhere on the surface and dropped objects fall perpendicular to the surface. In smaller objects, like spacecraft at least 25’ long, gravity is not a central point but rather a plane which cuts horizontally through the object. This gravity plane of a ship is two-directional: it attracts from both top and bottom. A sailor can actually stand on the bottom of the ship 's hull and move around as easily as if he was walking on deck. In this case, "down " is actually "up ," back toward the plane of gravity that cuts through the ship. A smaller body (such as a floating person) has no gravity that affects other objects, but still maintains an envelope of air.

Every body with a plane of gravity (including asteroids, most standard ships, and some huge or larger monsters) exerts a pull identical to that on a standard planet ("Earth-normal" gravity). While this is also true of almost all planets, exceptions do exist, and some planets can have differing gravity (sometimes due to mass, sometimes not). A spelljammer crew is advised to consult local star charts for any exceptions.

When two ships come into direct contact (one rams the other or lands on the other), the gravity of the ship with the higher tonnage is dominant and becomes the gravity for both ships. (For this reason, as well as preserving the ability to land on a planet, ships are usually built with a standard “up” direction, even though the gravity plane is bidirectional.)

The “convenience” gravity of Wildspace, whether of a ship or a planet, extends only to the edge of its air envelope. Outside of that, objects exert the same gravity as a standard object of similar mass. This means that (in most Spelljammer systems) the orbits of planets, moons and asteroids are governed by standard gravity, and floating objects will generally follow the rules of inertia and Newtonian gravity (continuing in a straight line at a constant speed, unless acted upon by an outside force).

Exceptions, as always, do exist. One system has stars and planets hanging like fruit from a giant tree; others have planets carried around their primary by great starbeasts.

Drifting

Along the gravity plane of a ship, objects are weightless but are slowly pushed towards the edge of the gravity plane. Therefore, a man overboard would eventually come to rest at the ship's plane of gravity, then begin drifting away from the ship along that plane toward the edge of the air envelope. On reaching the end of the gravity plane he is pushed outside the air envelope and then left behind as the ship moves away.

Aside from this slight push, there is no relative motion of a ship within its air envelope, aside from turning. Objects in a ship's air envelope do not turn with the ship when it turns, but they do not drift toward the rear of the ship simply because the ship is moving forward.

Temperature

As part of the convenient physics of Wildspace, temperature is not usually 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. Extremely close to a sun, temperatures get far warmem. In the outermost parts of a system, temperatures can drop to freezing or below.

Some systems, however, have surprisingly higher or lower temperatures, and those should be noted on any star chart worth reading.

Timekeeping

Local time often varies from planet to planet and sphere to sphere, depending on rotational periods, custom, and a host of other factors . 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).

Detail Articles

• Celestial Bodies
• Spelljamming Ships and Helms
• Phlogiston and Crystal Shells
• Creating Star Systems 

This Article

• Air
• Gravity
• Drifting
• Temperature
• Timekeeping