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.