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The reason that objects in space drag their own atmosphere is due to gravity. Gravity is the reason that people can stand on space ships without falling off the deck, or stand on a spherical planet without drifting away.

Every body in space has its own gravity, an accommodating force that pulls objects in the direction that “would be most convenient.” For objects the size of a planet, gravity points towards the center so people can stand anywhere on the surface. A dropped object on planet would fall perpendicular to the surface. For smaller objects, such as spelljamming ships, gravity is not a central point but rather a plane which cuts horizontally through the object.

Gravity Planes

Gravity planes are two-directional; it attracts both from top and bottom. A sailor could actually stand on the bottom of the ship’s hull and move around as easily as if he was standing on the deck. One effect of gravity’s function is that an object which falls off a spelljammer can oscillate back and forth across the plane of gravity, reversing direction and falling back across the plane over and over until something causes it to stop. A carefully-thrown object can actually go into orbit around a ship if it rotates evenly around the gravity plane.

A plane of gravity always runs through the long axis of any large body. Structures or creatures with an axis at least 25 feet long have gravity planes sufficient to attract other objects. A person could walk on the back of a 25-foot long dragon floating in space as easily as they could the deck of a spelljammer.

Every body with a plane of gravity exerts the same pull, identical to that on a standard planet (“Earth-normal” gravity). Objects too small to exert a plane of gravity still pull envelops of air, but do not attract other solid objects. Further, gravity is an all-or-nothing proposition. Either it is there are full strength, or it is not there at all.

The gravity that a body extends to is the same distance as its air envelope. For celestial bodies, this is the same distance as their atmosphere. Outside of these bounds, regions are weightless.


Unrestrained objects resting on a plane of gravity of another, larger object gradually drift. The plane of an object is weightless, but over time will slowly be pushed towards the edge of the gravity field. Thus, a man overboard a spelljammer may oscillate up and down for a time, but come to rest on the plane of gravity, after which he will begin to drift away from the ship towards the edge of the air envelope. Upon reaching the end of the gravity plane, he would be pushed outside the air envelope and left behind as the ship moves away.

Aside from this slight push, there is no relative motion of a ship within its air envelop, aside from that caused by turning. A ship’s air envelop does not turn with the ship when it turns, but objects in the ship’s air envelop do not drift toward the rear of the ship simply because the ship is moving forward.

Overlapping Gravity

When gravity planes intersect, such as when two ships pass each other at close range, the gravities of both ships remain in effect regardless of the size. Objects remain under the effect of whatever gravity plane they are closest to. A sailor could leap between two ships, altering her “down direction” as she crosses the midpoint between the two.

When two ships come into direct contact, such as when one rams or lands on the other, the gravity of the ship with the higher tonnage is dominant and becomes the gravity for both ships. If a larger spelljammer rams a smaller vessel, it can force the direction of the gravity plane of the crew on board to alter.


A weightless character who enters the air envelop of a larger body is immediately affected by the pull of gravity of the body. She will begin to fall the distance from where she entered the plane of gravity to the surface of the body, or to the plane of gravity, whichever is nearer.

While a character is weightless, they can still move according to the basic laws of physics. Any force has an opposite force. A drifting sailor can move (slowly) by throwing equipment in the opposite direction.


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