Air

All bodies of any size carry with them an envelope of air whenever they leave the surface of a planet or other stellar object.  Unlike real-world astrophysics, this air envelope is not dispersed by the vacuum of space.

A typical human will drag enough fresh air with him/her to breathe for 2-20 minutes.  After that time runs out, the air will turn foul for a like period of time and then become deadly.  The amount of air a spelljamming ship carries with it is based on its size – the larger the vessel, the more air is dragged into its pocket (these bubbles of air provide breathable atmosphere for varying lengths of time, but 3 months is considered "standard").

Air is relatively easy to replenish.  Entering a larger air envelope like that of a planet or asteroid is one of the most popular and cheapest methods.  Green plants will refresh air; some vessels make great use of these for just this purpose.  Many asteroid colonies keep at least half of their surface area reserved for plants for this reason as well.

Air

Praetorian's Spelljammer The_Praetorian The_Praetorian