Limits of Passiv Deceleration for Relativistic Lightsails

This paper investigates passive and semi-passive deceleration mechanisms for relativistic interstellar lightsails of the Breakthrough Starshot class. Four braking concepts are analyzed and compared: interstellar medium (ISM) ram drag, magnetic sail braking, stellar radiation pressure, and a hybrid combination of all three mechanisms. Analytical expressions and order-of-magnitude numerical estimates are derived for multiple mission scenarios using representative nanosail parameters. The study evaluates braking performance over variable inner-system deceleration distances ranging from 20 AU to 10,000 AU and discusses the physical limitations of each mechanism in both relativistic transit and post-arrival regimes. Particular attention is given to the validity of commonly assumed braking distances, the role of stellar wind suppression, and the scaling behavior of radiation-pressure braking near luminous stars. Alternative target systems including Sirius A/B, Tau Ceti, Procyon A/B, and Epsilon Eridani are compared with Alpha Centauri regarding theoretical orbital insertion feasibility. The paper concludes that no currently known passive mechanism alone is sufficient for reliable relativistic orbital insertion, but hybrid approaches may provide partial or conditional deceleration under optimistic assumptions. Several conceptual extensions for future mission architectures are also proposed.