Spelljammer-Adventures In Realm Space
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 planet so that people can stand anywhere on the surface and dropped objects fall perpendicular to the surface. In smaller objects, like space craft, gravity is not a central point but rather a plane which cuts horizontally through the object
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 he was walking on deck. In this case, “down” 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. 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.
An example of this is on a viking longboat cruising through the fjords, suddenly thrown from its home into wildspace. Its crew can still stand normally on its deck. Down for them is toward the bottom of the boat. Similarly, an individual could stand on the bottom of the longship, and find that “down” for him was back toward the top of the boat. Someone who jumps off the deck of the boat would fall downward and pass through the boat’s plane of gravity, whereupon the direction of gravity would reverse for him and pull him back toward the underside of the hull.
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. One of the favorite pranks of experienced spellsailors is to toss an apple or similar light object off one side of a ship in such a way that it curves aroun dand strikes someone standing at the other rail.