What is Spelljammer?

Given the announcement that WotC is working on three new campaign setting books that are all based on classic D&D settings, I figured I'd go and do an overview of the various settings I think are most likely (or the ones that I'd most like) to see printed.

The big caveat here is that I am coming to this pretty green myself. Case in point: my only experience with the Spelljammer setting was a single one-shot in which I and a group of Adventurer's League players, having finished Descent into Avernus, decided to play in an all-Warforged (though one player missed this and brought a human) party - I resolved to make my character as non-AL legal as possible, making a Warforged Artificer with the Izzet Engineer background.

Anyway, that's beside the point. What is Spelljammer?

Spelljammer is a campaign setting for D&D that is built around pseudo-science-fantasy, primarily taking place on magical spaceships, of which the Spelljammer is the most legendary example (which is sort of the Flying Dutchman equivalent.) This, like Planescape, is something of a "meta-setting," which links other existing D&D worlds, so you can travel, say, from Toril of the Forgotten Realms setting to Krynn of the Dragonlance setting.

While the setting uses a lot of sci-fi imagery, the manner in which space truly works is quite different, and is actually based on medieval theories about the nature of the cosmos. In Spelljammer, each world exists within its own Crystal Sphere. These spheres are named after the various settings, so Toril is within Realmspace and Oerth, the world of the Greyhawk setting, is in Greyspace. The space within these spheres is referred to broadly as Wildspace, and looks a lot like ordinary outer space, except that gravity sometimes works a little differently - large flat objects will often have gravity simply pull things to its surface (so if you want a flat world, you can do that.)

The Spheres themselves separate Wildspace from the Phlogiston, an ocean of some sort of fifth element that can only exist outside of the crystal sphere, and it's along the currents in this vast space-ocean that Spelljammers can travel to other crystal spheres, and thus other worlds.

Even within the spheres, there are planets and asteroids and such beyond the main world, and so even with a ship that is unable to journey through the phlogiston, you can still do some planet-hopping.

Tonally, Spelljammer is famous or infamous for its silliness. For example, Gnomish ships in the setting are generally powered by Giant Space Hamsters, powering the ship by running on wheels within the ship's engines. While you have alien threats like the Neogi - a type of aberration with no concept of understanding others as anything but a slave or a master - or, of course, the Mind Flayers (whose Nautiloid ships can be seen in the Baldur's Gate III trailer,) the overall vibe of the setting is a parodic send-up of sci-fi.

Spelljammer didn't actually sell that well after it was first released in 1989, and so the setting hasn't been revisited since the 90s, apart from the occasional monster stat block.

Despite, or perhaps because of this, a lot of D&D fans have clamored for it to make a return. Personally, given that the Forgotten Realms already provides a pretty perfect standard-fantasy setting where most fantasy tropes are fair game, I'm much more interested in different takes on the genre, and space-faring science-fantasy would 100% be a different take.