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. 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 sphere so that people can stand anywhere on the surface, and dropped objects fall perpendicular to the surface. In smaller objects, like spacecraft, gravity is not a central point but rather a plane which cuts horizontally through the object to the end of the air envelope. Gravity itself is conveniently an all-or-nothing proposition. Either it is there at full strength or it is not there at all (though there are exceptions to every rule, per DM discretion).

Significantly, this gravity plane 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 she was walking on deck. In this case, what was "down" on the deck is actually "up", back toward the plane of gravity that cuts through the ship. One of the stranger side effects of all this is that an object falling off the side of a Spelljammer can oscillate back and forth across the plane of gravity, falling first in one direction until it crosses the plane, then reversing direction and falling back across the plane again, and so on until something causes it to stop. To a person standing on the deck, the object appears to fall down, then up, then down, then up, and so forth.

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 an endless stream of apples overboard just to watch them bob.

Along the plane an object is weightless, but it is slowly pushed out toward the edge of the gravity field. Therefore, a creature that falls overboard that couldn't find a hold or isn't tied down, would eventually come to rest at the ship's plane of gravity, and would 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 (at the very edge of the air envelope) she is pushed out and left behind as the ship moves away. This movement takes place at a rate of 5 feet per round. Aside from this slight push, there is no relative motion of a ship within its air envelope, aside from turning. A ships air envelop does not turn with the ship when it turns, but objects in the ships air envelope do not drift toward the rear of the ship simply because the ship is moving forward.

When gravity planes intersect (such as when two ships pass each other, or when a ship passes a planetoid), the gravities of both ships remain in effect, regardless of size, up to the point where they physically intersect. An object is under the influence of whichever gravity plane it is closest to. A character could leap between two passing ships, altering her down direction as she crosses the midpoint between the two.

When two ships come into direct contact, the gravity of the ship with the higher tonnage is dominant and becomes the gravity for both ships. A large mind flayer vessel could ram a smaller ship from directly above and spin the smaller ship's gravity plane by 90 degrees, causing everything on the rammed ship to tumble toward the large ship's plane of gravity, probably with disasterous results.

A weightless character who enters the air envelope of a larger body is immediately affected by the pull of gravity on that body. She effectively falls the distance from where she entered to the surface of the body or to the gravity plane, whichever is closer. Normal falling damage is applied, as well as massive damage rules. When the drop is more than one mile, there is also danger of the subject heating up and igniting from friction with the air. This happens after one mile of uncontorlled descent. The falling object catches fire and takes normal damage from fire as well as falling damage. Any sort of control over speed and descent (flight, levitation, feather fall, etc) negates this effect.

Characters who are weightless can move under familiar laws of physics - For every action, there is an equal and opposite reaction. A drifting fighter may move by throwing her equpiment in the opposite direction. Max movement is half-speed when acting this way. Combat in a weightless environment is difficult and foreign to creatures used to fighting in normal environments, and any attack roll or saving throw using STR, DEX, or CON is made at disadvantage.