Outer space is a vast, airless void, with planets, moons, and other celestial bodies.
This is a game meant solely for your enjoyment. But for those looking for a different feel, here's an optional approach. Gravity on a roughly spherical body is directed toward the centre of the body. To understand how this works, imagine the way that an object would float if it was tossed into a gigantic pool of water. The surface of the water would then show the exact location of the gravity plane in relation to the object.