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This study investigates round-trip Earth–Mars–Earth missions during the 2031 opposition, applying a trajectory design framework derived from the early orbital configuration of asteroid 2001 CA21. Using Lambert-based analysis and JPL Horizons ephemerides, two optimized and dynamically consistent mission architectures were identified: a rapid scenario featuring a 33-day outbound transfer, a 30-day surface stay, and a 90-day return (total ≈ 153 days), and a feasible scenario combining a 56-day outbound transfer, a 35-day surface stay, and a 135-day return (total ≈ 226 days). Both trajectories were validated through full ephemeris computation, confirming heliocentric coherence within the CA21-anchored orbital plane and physically realistic departure and arrival velocities. The 2031 alignment minimizes plane-change penalties and yields energetically balanced outbound and inbound arcs. These findings demonstrate that short-duration, reversible Earth–Mars missions can be designed from early asteroid-derived orbital templates, establishing a predictive framework for identifying future high-velocity transfer opportunities.