My 3 players of 18,19 and 20 level might explore the moon and other uninhabited planets of their solar system as a part of an ending act in my campaign. I couldn'f find any rules for differing gravity, space radiation or vacuum, which in my opinion are crucial when anything comes to space exploration. In the universe in which we are playing, it's more like the regular space, than the Spelljammer-ish one.
Because we are playing 3.5 Edition of Dungeons & Dragons I am exactly interested in its materials, along with Dragon Magazine and Dungeon Adventures, however if there are no such rules or they are quite poorly made, materials from other compatible systems may be used.
I am interested in the rules themselves. Please include summaries to give an idea of what they are like.
Best Answer
Consider d20 Future
The Dungeon Master's Guide says, "The d20 Modern Roleplaying Game, a D&D-compatible role-playing game for present-day adventures, contains a much more extensive treatment of firearms and other high-tech gear" (146), so it's not that much of a stretch also to employ an expansion of Wizards of the Coast's D&D 3.5-compatible d20 Modern rules, d20 Future, for those times when PCs venture beyond their sphere and into a space. While I've not had the opportunity to use these rules in any D&D 3.5 campaigns, I wouldn't hesitate to take them out if my PCs were foolish enough to, on a clear night, greater teleport to the campaign's equivalent of Mars or wherever. The d20 Future rules appear both playable and suitably lethal for the unprepared.
Be aware that you'll likely be at a bit of a loss if relying exclusively on Dungeons and Dragons 3.5 material. The Dungeon Master's Guide has rules for gravity (147-8), but for space radiation and vacuum, you're on your own. The DMG seems to assume a bottle of air (250) makes the user safe in a vacuum, for instance, and in a vacuum—because this is everyone's first question—blindsight doesn't work (290), but that's almost all you'll get. But Elder Evils provides rules for the effects of an ersatz black hole (142-3), so, at least, there's that.