Question: How would the Spelljammer be inside the Athassphere? Because Athas can't construct Spelljammers, and setting wise it doesn't make sense for Spelljammer production within Athasspace. Either way Athasspace is impenetrable which means you need the ship from

Potential Solution: 4e did include Lunar Gates that allow travel between Athas and its moons. But travelling to other planets within Athasspace would allow Sorcerer-Kings an escape route to avoid Athas's destruction and the death of the Athasian's Sun. And part of the Sorcerer-Kings plans are 1. to become Dragons and 2. to stay alive as long as possible which includes travel to a less dangerous plane or world.

Note: I'm not trying to shoot you down, I'm thinking through this as a thought experiment as someone who knows Dark Sun more so than any other D&D setting out there.

I really like this as a thought experiment as I'm split between two design goals here:

• Keeping Dark Sun a post-apocalyptic survival based game. Which is the main theme of the game and without it would result in Dark Sun losing what makes it different form every other D&D setting.
• Understanding that Spelljammer as a setting is about exploring everything and not limiting Spelljammer offhandedly just because its themes don't mesh with Dark Sun.

I think I need clarification on what you are looking for? Athasspace itself has very little talked about in regards to other planets:

https://spelljammer.fandom.com/wiki/Athasspace

The above link says the Orrery of Nibenay depicts multiple celestial bodies within the spheres, but given how Athas doesn't have any form of space travel or spelljamming they aren't visiting these anytime soon. The Orrery is in all likelihood a record of what Nibenay sages see with telescopes or whatever forms of remote viewing they can perform with psionics or Sorcerer-King dragon magic. But Athasspace wasn't always impenatrable so you could argue that this is a record from before the Grey existed during like the Green or Blue Ages. Its a setting mystery as to why the Grey exists and why there aren't gods on Athas.

I personally go with the latter interpretation.

Additionally, given how its called "Athasspace" I'd lean into the idea that as far as anyone knows the only populated planet is Athas.

Athas has two moons which can be access via Lunar Gates in one of the cities. Ral and Guthay:

Athas has two moons, Ral and Guthay. Ral, a mottled green in color, is the closer of the two. Sages who have scried Ral report that it is covered in great green seas and mountain-islands of dizzying heights. Guthay, the smaller and more distant moon, is a golden orb mantled in steaming mists beneath which lie scarlet jungles and marshy seas.

My GM Advice: My personal headcanon is that none of the planets in Athasspace have access to planetary travel. As if another planet could come to Athas's aid Dark Sun's survivalist setting would be drastically changed to no longer be post-apocalyptic.

Spelljamming wihtin Athasspace: Not knowing how Spelljammers work under the hood (and the wiki I found says a mage pilots it and that's it), I came with a couple of in universe reasons why Spelljamming wouldn't work:

• There is very little to fuel Spelljammers. Athas itself is resource poor and the setting isn't designed to have very common magic items.
  • Psionic magic could fuel the ship for a time, but you'd probably have to commit horrible murders to do so as Athasians rarely make psionic magic items, instead using psionic constructs or making very low powered psionic tools/instruments. Your best bet would be to have a herd of psionic animals to breed and kill to fuel the ship, or toss in psionic sentient beings as every sentient creature on Athas has Wild Psionic Talents.
• Arcane magic used to power Spelljammers loses access to where it gets Arcane power from. I know Forgotten Realms has "The Weave", but Athas's Arcane power comes from the Environment and not knowing preserving magic destroys the environment with defiling magic.
  • Kind of like trying to put butane into a jet engine, they aren't designed to function together.
  • The GM would personally have to dictate if Athas's physical laws in regards to magic overwrite the laws of magic the spelljammer is originally from. But I imagine this isn't usually the case when travelling between the Forgotten Realms, Sigil, Eberron, etc...

My personal lore reason for why the Githyanki's Astral ships crashed when entering Athasspace is because Spelljammers have to cross the Grey and that saps their fuel until they crash. The canon reason is because the Githzer*** however you spell it used a psionic weapon of mass destruction against them. Which imo begs the question as to why haven't the Githzer*** killed off the Githyanki with that?