A charged spherical capacitor kept in air do not loose charge because air is a bad conductor and increase in charge results in Corona Discharge. Is it because the nucleus of air molecule repels the charge in sphere and after a limit the repulsion is less than the attraction by sphere leading to Corona Discharge. But if the same charged spherical capacitor is kept in vacuum, what would happen. Shouldn't charge repel each other and escape the spherical surface because now there is no nucleus of air molecule to repel those charges. Will the capacitance decrease or not.
If the charge of an object does not change, it means the net flux of charges to and from it is zero. It all depends on the environment and the object itself. It is normal that an object has some charge even in the near vacuum of space. Sunlit surfaces for instance become charged positively due to ultraviolet radiation knocking electrons out of the material. Surfaces in shadow become charged negatively due to the fact that the flux of plasma electrons in space is higher than that of protons (due their smaller mass and thus higher velocities). The material charges up to a value so that the net flux of charges is zero and thus a steady state is reached. See http://holbert.faculty.asu.edu/eee560/spc-chrg.html for more information.