This study presents a brief review of Hybrid Power Sources (HPSs) for space applications to compare the results obtained for a HPS under unknown load containing pulses. The reliable technologies for energy sources and Energy Storage Systems (ESS) that can operate safety in extreme environments (very low temperature, intense radiation environments etc.) and under dynamic load demand (including load pulses) are compared based on the targets for power and energy density, efficiency, and lifetime. The pros and cons for HPS architectures and ESS topologies proposed in the literature are discussed in frame of optimization of the whole system. Two new optimization strategies were proposed to optimally operate the Fuel Cell (FC) system based on two control loops implemented based on the global optimization control of the boost DC-DC converter and the load-following control of the fuel flow rate or of the air flow rate. The comparative study performed (under constant load, dynamic load, and variable PV power) points out the advantages of one of the proposed optimization strategy in all performance indicators. For example, the gaps compared with the reference strategy are of 1.88%, 13.61 W/lpm, and 293 lpm for FC system efficiency, fuel consumption efficiency, and fuel economy, if the maximum load is considered. Also, different control methods are proposed at the ESS side to mitigate the load pulses (protecting the FC system) and regulate the DC voltage. The results obtained in this paper are discussed related to other ESS hybridizations and control solutions reported in the literature.