In my last post I talked about how the Astral Plane takes different forms depending on how you enter it: on an astral ship you sail the Astral Sea; from a Manual of the Planes you explore the Astral Library; etc. But however you experience it, it’s all the same place.
I forgot one major incarnation of the Astral Plane: space. (Duh. “Astral” does mean “starry”, after all.)
What is the Astral Plane? It’s a big bunch of nothing connecting a bunch of interesting nodes (planar gates, or astral islands in 4e). It maps pretty well onto space travel. Now we know another way to enter the Astral Plane: launch ourselves into orbit. Spelljammer’s Wildspace is just another manifestation of the Astral Plane.
When your D&D characters steal a spaceship, or hop on the back of a cosmic dragon, or teleport to the Warden colony ship, what can they expect to find out there where no adventurer has gone before? The usual sci-fi fare. A water world. A world ruled by gods. A dead world poisoned by a mind flayer empire. Just the same sort of places you could find by plane-hopping.