A propellant depot (40, 150) includes a utility box (42, 42′) that has space flight equipment. A propellant cartridge adaptor (95) is coupled to the utility box (42, 42′) and to an exchangeable propellant cartridge system (41). The propellant depot (40, 150) also includes a docking adaptor (44) for coupling to an approaching spacecraft (24). A controller (66) controls the transfer of propellant from within the exchangeable propellant cartridge system (41) to the spacecraft (24). A method of providing propellant to a spacecraft in space includes launching an orbital propellant depot (40, 150) into space. The spacecraft is docked to the orbital propellant depot (40, 150) in space. Propellant is transferred to the spacecraft. The spacecraft is separated from the orbital propellant depot (40, 150).