Gleaned Details About Spelljammer 5E

 So, what is "space" in D&D?

Traditionally, Spelljammer in D&D has, despite being otherworldly and strange, still taken place entirely in the Prime Material Plane. The crystal spheres and phlogiston (the region between the spheres that contain the conventional settings and their solar system-like nearby planets) were basically the structure of the Prime Material Plane. Some settings, like Dark Sun's Athas, were sort of locked away - their crystal spheres impenetrable to preserve the bleak "trapped" feeling of such a rough world (otherwise one would assume most people from Athas would do whatever they could to just leave).

The language WotC has been using to describe Spelljammer with the upcoming box set, however, seems to imply that big changes are coming to the way that the setting works, and that a good portion of it will take place within the Astral Plane.

The Astral Plane is one of the "weird" planes in D&D. It's a "transitive plane." While the "Great Wheel" cosmology kind of treats the outer planes like a big cookie, with the Outlands at its core and the other outer planes forming the rim, they're also kind of afloat in the Astral Plane.

The Astral Plane is a realm beyond time - the Githyanki, who make it their home, have to send their young to the Prime Material just in order to let them mature to adulthood. It's also a place where gods go when they die - their corpses floating as decaying titans on the endless expanse.

Travel to the various Outer Planes is potentially easy in the Astral Plane, as there are portals adrift there.

Given this, Spelljammer is actually positioning itself to be an even broader setting than it previously was. As before, you can sail your spelljammer between prime material worlds like Toril or Krynn, but if you're sailing into the Astral Plane, that means you could technically take a portal and wind up flying around Mount Celestial or the Nine Hells.

This also seems to imply that travel between worlds of the Prime Material will also involve the Astral Plane, which sort of makes the Prime Material into less of one contiguous realm and more a kind of scattering of plane-particles. And that, honestly, feels like it could tie into the mythos of the "First World" mentioned in Fizban's Treasury of Dragons.

"Astral" of course means a connection to stars, so I sort of wonder if Spelljammer is going to present Wildspace as "nearby space" and the Astral Plane as the weird "outer space" of aliens and odd monsters. Certainly the Star Lancer shown in the Monstrous Compendium is an Astral Plane denizen, but I wonder if the kind of asteroid fields where we might encounter an Asteroid Spider would be in Wildspace or in the Astral Plane.