In an interstellar cryptocurrency system, double-spending attacks face a critical challenge: light-speed delay. Blockchain consensus relies on rapid transaction validation across nodes, but interstellar distances introduce significant latency due to the finite speed of light. This delay enables malicious actors to exploit timing discrepancies, executing double-spending by broadcasting conflicting transactions before consensus is reached. Proof-of-work and proof-of-stake mechanisms struggle to synchronize across vast distances, as propagation times exceed confirmation periods. Even with advanced cryptographic techniques, the physical limit of light-speed renders real-time consensus unattainable, making double-spending attacks theoretically unsolvable in a decentralized interstellar network. Centralized solutions compromise trustlessness, undermining cryptocurrency principles. Thus, light-speed delay poses an intractable barrier to secure interstellar transactions.