As space missions become increasingly autonomous and data-intensive, existing terrestrial blockchain consensus mechanismsstruggle to meet the unique demands of interplanetary networks. Traditional models such as PoW, PoS, and PBFT fail to operate reliablyin environments characterized by high latency, intermittent connectivity, and strict energy limitations. This paper introduces POAST (Proofof Authenticated Space-Time), a lightweight, permissioned consensus protocol tailored specifically for space communication systems.POAST employs a hierarchical node structure, trust-based validator selection, and epoch-based synchronization to achieve secure andefficient block validation, even in disconnected and delay-prone scenarios. Unlike public blockchains, POAST restricts participation toverified nodes such as ground stations, relay satellites, and deep-space probes—ensuring mission-critical control and resilience againstcyber threats. This paper outlines the core design of POAST, its key innovations, and how it addresses the core challenges fa ced in spacemission blockchain deployment. The proposed protocol offers a practical, low-overhead, and scalable solution aligned with the operationalneeds of space agencies such as ISRO, NASA, and ESA.Keywords: POAST, Space Blockchain, Permissioned Consensus, Delay-Tolerant Networks,Identity-Based Trust, Epoch-Based Validation,Interplanetary Communication, Autonomous Satellite Operations, Smart Contracts in Space, Byzantine Fault Tolerance