Isn't Spelljammer like D&D wizards in space?
Yeah, in the setting they craft magical artifacts called a "helm" (from memory), to levitate an actual sea-faring ship, then a magic user or priest mans the helm and their magic power directly powers the ship.
The physics / cosmology is pretty cool too, each solar system is a crystal sphere (like the old ptolomaic model), the stars are weak points on the sphere, and outside it is bright but chaotic area called the phlogiston. Then, you need to navigate to the next crystal sphere, and sphere exist for all the major worlds of D&D at the time, so it had rules for travelling from Krynn to the Forgotten Realms and a few others. So ... yeah, it isn't "space" as we know it.
Also, gravity works differently. In SpellJammer, small objects retain atmosphere, so asteroids or ships carry a bubble of air around with them, and they have their own planar gravitational field, so if you fall off a ship, you bob along at a tangent to the plane of the ship. So effectively, there's air in space in this game, so they don't have to ruin the flavor with space suits and stuff. You have stuff such as being attacked and boarded by skeleton space pirate galleons and stuff.