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Regarding Mystaraspace and Spelljammer and Planescapeby Marco Dalmonte
Ok, whenever I see this thread I cannot refrain from jumping on the bandwagon, since I think I am the only person on this list to have actually run a two months long campaign in Mystaraspace (maybe Leroy did also, but I don't recall), so I can give u my first-hand input.
I assure u that converting Mystaraspace "Bruce version" to Spelljammer is easier than converting the D&D Outer Planes to Planescape cosmology.
The main points are:
Cold Void: Mystaraspace's temperatures are extremely low (some -30°C), in contrast to the warm temperatures other Crystal Spheres enjoy (but then, there are also some veeery freezing spheres out there also in Spelljammer)
Airless Void: Unlike the Known Spheres, skyships travelling through Mystaraspace do not retain an air envelope. This explains why the traffic in voidspace is so low in comparison to other spheres. A spelljamming ship entering Mystaraspace from the phlogiston simply loses its air in a number of days equal to its tonnage, period. The only way to travel and live in Mystaraspace is to build airtight vessels and cast Climate and Create Air inside them, or give the sailors Air Breathing Masks. Note however that some space monsters do not need air to live and are immune to cold (they are normal versions of Spelljammer creatures that have adapted to Mystaraspace features).
Gravity: no double sided gravity planes in Mystaraspace, another bad news for spelljamming ships. Gravity works always normally (one side's up and the reverse is down and gravity always pulls towards the "down side"), even in space (no absence of gravity like in RW)
Crystal Sphere and the Galaxy: Mystaraspace is unique in that it lies at the hub of two different dimensions, the Immortals' Multiverse properly, and the Planescape Multiverse. It exists in both realities without being doubled and as such it is both a Crystal Sphere (according to Spelljammer rules) and a "normal" solar system, part of the Glittering Way galaxy in our Prime universe. The veil that separates the two dimensions lies at the edge of Mystaraspace, and it is called the Golden Barrier (all IMC). It is a big magic field which contains the whole solar system. For spelljamming vessels it is the crystal sphere that harbours the system, for other vessels (not propelled via a spelljamming helm) it is simply a great space phenomenon.
If a ship powered by a spelljamming helm (Minor or Major Helm or similar) crosses it (no need to create a portal through it like it happens in the other crystal spheres!) it is immediately transported in the phlogiston, mainly because of its motive power which is related to that dimension and universe. However, if a vessel using another motive power (permanent fly, dynamos of flying, furnace, psi-helm etc..) crosses it, it simply travels ahead and leaves this solar system, sailing further throughout the galaxy of this universe.
Theoretically a Spelljamming powered ship could bypass the crystal sphere and sail into the rest of the galaxy, but it needs opening a portal to the other side of the Golden barrier, something which spelljamming ships don't do since they know they can enter the phlogiston without opening gates!
Some other stats: in order to reach the last planet of the solar system (Pluto, which I called Thanatos) u need 72 days taking off from Mystara. If u follow the rules, then the Golden Barrier should lie around 160 days of travel from the sun, but this would just be plain boring and impossible (no spelljamming ship would ever come to Mystara, since they finish their air capacity waaay before), so I placed it at about 80 days of travel from Mystara, about 85-90 days from the sun. (another weirdness of Mystaraspace).
I just finished the first part of a campaign that took my Master level players all across the solar system on a skyship, pursuing another flying vessel till the Golden Barrier, and I tell u it can really be funny and challenging if u arrange the encounters in a proper way... they never experienced wild magic zones on Mystara, and imagine their surprise when they were confined for three days on a moon of Uranus with wild magic effects, after crashing with their skyship and with strange beasts assaulting them.. eheheh :)
Well, hope this helps to solve the Mystaraspace D&D-AD&D dichotomy (it helped me a lot!). I am still trying to figure how to reconcile Planescape planes with OD&D outer planes, but that's a hell of a work so I'll probably give up and keep my OD&D planes (and it seems from some rumours I've heard that the new 3E version of the outer planes will be closer to the OD&D cosmology rather than to Planescape!)..
however I think I am making improvements in devising a theory that explains the presence of demons (tanar'ri/Immortal lesser fiends) and devils (baatezu/no D&D counterpart) in the D&D Multiverse... basically put: demons are lesser immortal and cannot reproduce but must corrupt other species to ascend to their status, while devils are not properly immortals (much like the fairy folk), don't live in the outer planes, but can reproduce and spawn other devils. The major difference between the two is that demons are CHAOTIC to the core, while devils are lawful, although extremely malignant and evil, and they are bent on reconquering their rightful place in the Multiverse...
Ok, I admit it's not much yet, but I'm working on it ;)