Gravity plane

An illithid nautiloid with its gravity plane and resulting air envelope

A Gravity plane is the plane of attraction that runs along the long axis of any non-spherical object in the Spelljammer campaign setting.

Overview

All objects in wildspace have their own gravity, which is why most celestial bodies, ships and creatures have their own personal air envelopes. For a large spherical object, like most planets, gravity is directed towards a point at the object's centre, meaning that creatures can stand anywhere on its surface, and falling objects will drop perpendicularly to the object's surface. In non-spherical objects, like spelljamming ships or certain asteroids, gravity does not act at a central point but rather along a plane which cuts horizontally through the object, along its longest axis.

All objects with an axis of at least 25 feet long will have a gravity plane powerful enough to attract other objects, meaning that a person could feasibly walk along the back of a 25 foot tall giant floating in space as if it were a planet. The gravity plane exerts a pull identical to that of a standard planet (defined as Toril, Krynn or Oerth-normal gravity). A body smaller than 25 feet still maintains its own personal envelope of air, but does not attract other solid objects.

Gravity is an all-or-nothing proposition. Either it's there, at full strength, or it's not there at all. Gravity always exerts the same force whether the object creating it is the size of a whale, the planet Toril, or a massive gas giant. The gravity of an object extends to the limit of its air envelope (i.e. to the edge of the object’s atmosphere). Within those bounds, gravity functions normally; however, outside those bounds, gravity ceases to function at all, and all objects are considered weightless.

Since gravity in a non-spherical object is represented by a plane running through its long axis, all such objects will have a "top" and a "bottom". A sailor can actually stand on the bottom of a ship’s hull and move around as easily as if they were walking on the deck. In this case "down" is actually "up", back towards the plane of gravity that cuts through the ship.

One of the stranger side effects of this is that an object falling off the side of a spelljamming ship 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. This trick is commonly used to amuse passengers new to space travel. More than one groundling has gotten into trouble for standing at the ship’s rail and tossing an endless stream of apples overboard just to watch them bob up and down.

A carefully thrown object can actually go into orbit around the ship. Such orbits do not last long; the object quickly collides with the ship’s rigging or oars, or slows down in the air envelope and crashes into the ship. A favourite prank of experienced sailors is to toss an apple or similar object off one side of a ship in such a way that it curves around and hits someone standing at the other rail.

Objects lying along a gravity plane are to all intents and purposes weightless, but are also subject to a mild force that inexorably pushes them to the edge of the gravity field. This means that anyone falling overboard would eventually come to rest at the ship’s plane of gravity, and then begin drifting away from the ship along the plane towards the edge of the air envelope. Upon reaching the edge of the gravity plane, they would then be pushed outside the air envelope and be left behind in wildspace as the ship moves away. Because objects along the gravity plane are weightless and undergo a small but definite push towards the edge, the gravity plane is often used to launch heavy missiles or prepare boarding parties prior to attacking another ship.

When gravity planes intersect (such as when two ships pass each other at close range and at different angles), 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 is closest. A creature could leap between two passing ships, altering their down direction as they cross the midpoint between the two.

When two ships come into direct contact (one rams the other or lands on the other), the gravity of the ship with higher tonnage is dominant and becomes the gravity for both ships. For example, an elven light scout has its own gravity, until it lands on its' parent ship, at which point it adopts the down direction of the larger vessel. In the case of a larger vessel ramming a smaller ship from directly above, the smaller ship's gravity plane would be overwhelmed by that of the larger vessel, causing everything on the smaller ship to tumble toward the larger ship’s plane of gravity, probably with disastrous results.

A weightless object entering the air envelope of a larger body is immediately affected by the pull of the gravity of that body. It will, in effect, fall from where it entered the air envelope to the surface of the larger body, or to the plane of gravity, whichever is closest. Normal falling damage applies if the object hits something solid at the end of the fall.

In cases where the drop is more than one mile, there is also danger of the falling object heating up and bursting into flames due to friction with the air (becoming a shooting star). Flight, levitation, or any sort of control over speed and descent can negate this effect. All weightless objects are subject to the normal laws of physics. Any force has an opposite force. A living creature drifting in a gravity plane can move slowly by throwing equipment in the opposite direction. Maximum movement in this fashion is 3 feet per round.

Combat in weightless conditions is considered an exotic combat environment for those non-natives and creatures who are unfamiliar with weightless conditions and fighting in three dimensions, and are penalized as such. Natives to wildspace or those who are proficient with fighting in weightless conditions can fight without penalty.

Missile fire functions normally within gravity fields. Once outside an air envelope, however, there is nothing to slow down the missile once it is in flight. Missiles will continue to travel in a straight line forever, with no limit, until they encounter something. All missile weapons in wildspace can be fired at extreme range, limited only by the persons line of sight. A missile travels the equivalent of twice its long range each round. Note this only applies to normal hand-held weapons; siege machines such as catapults and ballistae mounted on ships suffer no such limitation, as they are designed with such long distances in mind.

Ships and Gravity Planes

Since the gravity plane of a ship passes through its two horizontal axes, it is possible to stand on the bottom of the vessel, facing the opposite direction to the rest of the ship. In fact, it is quite feasible to build a ship with two decks, both parallel to the gravity plane. In practice this is almost never done because such a ship cannot land. However, some deep space craft and asteroid bases have structures on both the top and bottom.

The fact is that most sentient races evolved in an environment with a single up or down, and as such have a tendency to want all things right side up when working with them; and while certain spaceborn races deny any such similarity with groundlings, they are as creatures of habit as their ancestors were.

There is also a practical reason beyond difficulties with landing, and that has to do with what happens when two large bodies meet in space. Because the larger gravity plane overwhelms the smaller gravity plane, a ship with a standard "up" direction can more easily adjust to this new situation, while one with multiple decks will be caught up in a situation where one side will go spilling off the deck towards the newly established gravity plane.

This situation ensures that most ships have a definite "up". Whether they can land on a planet or asteroid or not, they will eventually find themselves in the gravity plane of a larger ship, and are designed accordingly. The neogi were confident that their huge deathspiders would not have to deal with this kind of situation, until they ran into a dwarven citadel that caused their umber hulk slaves to go splattering against the overhead dome that had suddenly become the floor. Older dwarven citadels, most of which are now abandoned, had a multidirectional nature, but encounters with even larger craft (such as the Spelljammer) caused the dwarves to readopt a standard facing.

In combat, a larger ship can take advantage of its ability to upset an opposing ship's gravity by passing close in a shearing or ramming attack, forcing the smaller ship to reorient itself (often pulling its weapons out of range) or risk crashing onto the larger ship. Dwarven ships, with their greater mass, often prefer this method to ramming.

It is relatively easy for a ship to reorient itself as long as its helm remains powered by a helmsman. However, a ship's helm is knocked out or otherwise taken off-line, their is chance the crew could fall off their ship, towards the gravity plane of the other ship. Most ships have nets strung above their decks to prevent the crew from falling off, and to provide a modicum of protection against debris falling from opposing ships above them.

The correct procedure for approaching a larger vessel or docking with an asteroid base, is to approach along the larger object's plane of gravity. This negates the need for corrective action once in the field, allowing communication between vessels, and gives both an advantage if they intend to attack each other. Show off's can can bring their ship in along the gravity plane but facing the opposite direction, but this is considered a breach of courtesy on diplomatic missions and can sour relationships with with other captains.

Crystal Spheres

Although they are, by definition, the largest objects in existence, a crystal sphere does not have gravity along either its external or internal surfaces (an exception to the rule that all large objects have gravity). Some sages have speculated that because a crystal shell is a perfect sphere, its' plane of gravity shrinks to a single point at its heart. This is of course unsubstantiated and does not explain why there is no gravity on a sphere's outer surface.

The Phlogiston

Gravity functions normally in the phlogiston, acting in the same way as it does in wildspace.

Appendix

Gallery

An Illithid nautiloid with its gravity plane and resulting air envelope

External Links

• Gravity plane article at the Forgotten Realms Wiki, a wiki for the Forgotten Realms campaign setting.