A repost of the basic setting information from the Spelljammer game from last time I tried to run it, back in 2004. Anyone who's potentially participating (which should feel free to include anyone from the Massachusetts corridor) feel free to ask questions or whatever. Map to follow once I PhotoShop it up a little.
The concept of Spelljammer is, quite simply, "D&D in Space." As such, it's a setting with a presumed technological level of quasi-medieval, with numerous demi-human and monstrous species roaming around, magical items and strange phenomena everywhere, and spellcasting a common sight.
The physics are quite a bit different from "real" physics. Each object, no matter its size, has gravity -- a universal force of attraction. (There are a few weird exceptions.) All gravity has the same strength, too -- that we think of as "Earth Normal" gravity. Each object's gravity reaches out to about once again that object's radius on each side. When the gravities of two objects overlap, the bigger object wins. Thus, on a planet, everyone is stuck firmly to the planet, whatever large objects may float about. However, in space, one's ship is generally the largest object -- thus one's ability to walk about in space. The direction of gravity is determined by an object's shape -- a planet, being round, pulls evenly from all sides, while longer objects (like most ships) have a nearly flat plane of gravity. Objects can have other gravity fields, however -- people could walk just fine on either beam of a giant "L" shaped object, for example.
In addition to gravity, each object holds onto a field of air. The amount of air is roughly based on an object's volume -- the bigger it is, the more air it carries. As such, a person going into space can only hold about enough air to survive for a few minutes -- thus spelljamming ships. Ships, by virtue of being large solid objects, can hold a great deal more air than a person.
Planets also aren't quite like they are in reality. For one, there's no restriction that says planets have to be round -- flat, toroid, and even more weirdly shaped planets abound. For another thing, each planet typically has as its base one of the elements. Earth worlds are what we usually think of as planets -- big hunks of rock, with or without life, water, or other features on their surface. Fire worlds are often what we might think of as suns -- giant spheres of flame. Air worlds can be freestanding clouds of gas, or loose frameworks or structures filled and surrounded by air. And water worlds range from giant oceanic spheres to icy pillars. There are even living worlds -- everything from tremendous integrated networks of plants, to sentient worlds of flesh and blood.
Spacetravel is accomplished by what are called "helms." These are magical devices which serve both to drive and to maneuver ships flying through space. Typically, a pilot sits within, touches, or holds the device in order to direct the helm's magical energies and fly the ship. The helms most commonly used amongst most spacefaring races are "spelljammers" -- they are powered by arcane spell energy. As such, they require a magic-user to be present to "recharge" them when they run low on energy. However, other species -- especially those who don't have access to magical spells -- rely on helms with different power sources.
A little note on style here. In the published material for Spelljammer, the basic metaphor they use for how things work in space is circa-Age-of-Exploration seafaring -- several of the ships provided are basically flying galleons, journeys between solar systems are incredibly long and dangerous, and so on. There's also a related issue: since Spelljammer was originally designed as a method of journeying from one D&D campaign world to another, there are some concessions made: the spacefaring races don't originate from anywhere (since they appear in multiple campaign worlds), don't control territory in space, and don't really bother "groundlings" (who apparently often know about space travel but inexplicably fail to care.) Personally, I prefer to use a different metaphor for the space travel here: Star Trek. As such, everyone has a set origin, most species control some area of space (even if it's just their homeworld), and there's no "ground" society -- planets that are part of spacefaring societies are fully integrated therein.
This campaign takes place in an area of space that we'll just call "Astromundi." It's occupied by quite a few spacefaring "nations" -- areas of space ruled by a single race. In the center of all this lies Free Space -- a region of independent worlds which are ruled by no outside power, and in which people of many species freely interact. The hub of this interaction is the Rock of Bral -- a giant asteroid, once a pirate den, but now a place for those across the galaxy to meet and trade.
A rundown on some of the important races in Astromundi:
Humans: The humans are what is known as "Displaced" -- they have no homeworld, and are scattered throughout the galaxy. It has been so long since the humans lost their home that even the Elves cannot remember where it stood -- and the legends cannot recall how it was destroyed. Now, humanity clings to life in the Free Worlds. Despite the length of years, some still seek to discover why exile is their lot -- and just what happened to their long lost home.
Elves: The elves are the primary military and social power in Astromundi. Their Empire is ruled from the Elvish homeland of Arcadia, a sylvan paradise of unimaginable splendor. Its power comes from the Elvish Imperial Navy -- a fleet of graceful, powerful living ships unmatched in the galaxy. The power of this fleet was last seen in full several hundred years ago -- an unprovoked attack on Elvish ships in goblin-controlled space led to the Inhuman Wars, a massive conflict that only ended with the complete eradication of the Goblinoid nation. Unsurprisingly, the Elvish people are often seen as arrogant, haughty, and paternalistic by other races in Astromundi.
Gnomes: The elvish empire is not settled purely by the elves -- there are a number of races whose homeworlds fall within the empire's borders, and who have accepted a place under the elves' rule. The aarakocra, the avariel, and the sylvain fall into this category, but the most prominent are the gnomes. The gnomish people maintain their own homeworld government (under the supervision of the Elvish ruling councils) on their homeworld near the Free Space border; their people serve the Elves throughout the empire in exchange for the protection of the Elvish fleet -- a deal brokered after tensions rose between the gnomes and nearby settlements of kobolds. The Gnomes are a typically good-natured people, with a great love of nature and of craft. Gnomish gardeners and agriculturalists are praised throughout the galaxy as the greatest of their trades.
Dwarves: The Dwarves are diligent and withdrawn. Despite bordering Free Space, the Dwarvish people are typically isolated from other races, interacting with them rarely and briefly. Much of this has to do with the nature of Dwarvish society -- rather than occupying planets as most races do, the Dwarves live out their lives in the great Dwarvish Citadels -- massive asteroids carved into incredible forms and filled with the cavernous networks of Dwarven cities. The Dwarves drift through space, slowly driven by the massive Forge Helms built into the bedrock of their cities. They search out new asteroids, which dwarven crews go to mine for raw materials -- both to build from, and to fuel their Forges. Outside their Citadels, the Dwarves have almost no small ships -- they largely rely on trade caravans journeying to their citadels to exchange goods, and rarely leave to go anywhere themselves. Some Dwarves, however, are struck with wanderlust, and take leave of their people to serve on a ship crew and see the worlds.
Halflings: The Halflings are (excuse the pun) a fairly small power in space. They control just their single homeworld, a tiny planet within Free Space, and have little in the way of a Navy. They are generally a happy, free-spirited people with little desire for wealth or power.
Thri-Kreen: The Thri-Kreen are a race of mantid creatures with a sizeable area of space which borders upon Free Space. One of the more alien species to deal directly with the free worlds, the Thri-Kreen are often difficult for humanoid species to interact with due to the difficulty of reading their expressions and body language. This difficulty is not helped by the Thri-Kreen's tendency towards disaffection and deadpan statements -- often it is extremely difficult to determine when one is truly being serious. They tend to take a bemused and slightly patronizing viewpoint towards the achievements of others in direct proportion to those others' opinion of themselves -- thus they tend to get along quite well with the halflings, and hold little but contempt for the elves. Thri-kreen have incredible small motor skills and extremely precise memories -- as such, their skill in specialized tasks like medicine and engineering are renowned.
Giff: The Giff a race of hippopotamoids, are another Displaced people, although their plight is much more recent than that of the humans. Their homeworld, once in one corner of Free Space, was reduced to rubble in the matter of a day -- and no survivors lived to tell the tale of how. Now, the Giff are scattered, and few -- few enough that no significant communities of Giff seem to exist today. Since their species reproduces infrequently, and there are so few of them remaining, the Giff are overcome with a tremendous sense of melancholy -- few expect their species to live long now. Many have thrown themselves into difficult and dangerous work -- Giff mercenaries are well known and feared through much of the galaxy. Others do little but bemoan their fate and slowly waste away.
Lizardfolk: The Lizardfolk are relative newcomers to the spacefaring scene. The reptilians draw great nourishment from sunlight, and are greatly reverent towards any radiant worlds. They still hold onto a largely primitive way of life -- they live in small communities ruled by tribal chieftans and assisted by shamans. They tend to be somewhat thoughtless and quick to act, but are tremendously cunning when pressed.
Myconids: The fungus men are one of the stranger species to frequently appear in Free Space. Unsurprisingly given their origin, the Myconids are often seen as slightly "off" by other humanoid species. They are prone to wandering attention, strange non sequiturs, and unpredictable interests -- and they, in return, tend to be in a constant state of puzzlement while dealing with creatures of flesh and blood. Their society is largely hierarchical, although its organization is difficult for outsiders to grasp. The myconids are considered invaluable trading partners by many other species for their ability to produce incredible varieties of psychoactive and medicinal spores.
Minotaurs: The minotaurs are another minor but significant spacefaring nation. Their culture is heavily competitive and driven towards success -- minotaurs frequently compete physically and intellectually with one another in any field they can find to do so. They are often seen as gruff and unpleasant upon first meeting with other species -- however, minotaurs tend to be loyal and determined.
Avariel: The avariel at first glance resemble the elves -- until you notice the huge feathered wings protruding from their back. Closer examination reveals abnormally elongated limbs, large eyes, and delicate features which further distinguish them from the elves. The avariel are generally a quiet, withdrawn people -- most of them have little interest in leaving their homeworld for places unknown. However, a few are driven by a great curiosity, and have taken to exploring.
Aarakocra: The aarakocra are an avian race, originating on a dry, windy world. They are extremely claustrophobic -- enough so that most refuse to travel through space in enclosed vessels. Aarakocra tend to have a great curiosity about other species and their objects and ideas, but are easily spooked, commonly misreading actions as aggressive or negative. They are often eager to trade in Free Space, looking to acquire unusual items from far-off worlds.
Sylvain: The sylvain originated on a living jungle world within the elvish empire. They are thin, tall humanoids, with green and brown "skin" -- which further inspection reveals to be leaves and bark. The sylvain are fairly weak physically, lacking in strength and suffering injury fairly easily. The sylvain are greatly empathetic, typically relating well to individuals of any species. Sylvain have served as negotiators for the elves on several occasions. Many seek to travel in order to meet and come to understand a greater number of beings. Their culture has a strong tradition of history, which they maintain through songs and elaborate wood carvings.
I welcome suggestions as to campaign direction here. I think the obvious default is to assemble a group of various random people who embark on trade, exploration, piracy, or adventure. If people are interested in something else (like being in the Elvish Imperial Navy or something) just let me know.