Blockchain has already reshaped how we approach security, traceability, and decentralization in traditional sectors likefinance and supply chain. But its application in space missions is still evolving — and facing a very different set of constraints. Whilethe potential is massive, the reality is that most existing consensus protocols like Proof of Work (PoW), Proof of Stake (PoS), andPBFT were never designed to handle high-latency, power-constrained, and intermittently connected environments like space.Thispaper presents a structured review of consensus algorithms from both terrestrial and aerospace research, with a focus on identifyingtheir suitability for space missions. Special attention is given to models like SAGIN, which introduced blockchain in Space-AirGround networks but still fall short under deep-space conditions. Through this review, we highlight critical gaps—like the absenceof delay-tolerant validation, adaptive trust scoring, and role-based node architecture—that directly led to the development of thePOAST framework. Our goal is to establish a strong literature-backed foundation that justifies why space missions need a purposebuilt, permissioned consensus protocol like POAST, instead of retrofitting existing terrestrial models.Keywords : - Blockchain in Space, Consensus Mechanisms, SAGIN, PBFT, PoW, PoS, DAG, Deep Space Communication, DelayTolerant Networks, Trust-Based Validation, Permissioned Blockchain, Autonomous Missions, POAST