ESA’s Huygens probe had only one attempt to successfully enter Titan’s atmosphere. Due to large uncertainties, it was crucial to ensure the mission’s robustness during the development phase. Common methods like Monte-Carlo simulations are computationally expensive and provide only a lower bound for worst case values. Therefore, this paper presents a robustness analysis method that determines the upper bound to complement Monte-Carlo simulation results and to increase the confidence in the mission’s robustness. The developed method is based on linear time-varying systems and quadratic constraints to describe deviations from the nominal trajectory resulting from a set of uncertain initial conditions.