The objective of this study was to investigate the uncertainty in shock layer radiative heat predictions on the surface of a hypersonic inflatable aerodynamics decelerator during Mars and Titan entries at peak radiative heating conditions. Computational fluid dynamics simulations of planetary entry flows and radiative heat predictions possess a significant amount of uncertainty due to the complexity of the flow physics and the difficulty in obtaining accurate experimental results of molecular level phenomena. Sources of uncertainty considered include flow field chemical rate models, molecular band emission, and the excitation/deexcitation rates of molecules modeled with a non-Boltzmann approach. Due to the computational cost of the numerical models, uncertainty quantification was performed with a surrogate modeling approach based on a sparse approximation of the point-collocation nonintrusive polynomial chaos technique. Accurate results were obtained with only 500 samples of the computational model. Baseline results indicated that radiative heating during Titan entry was nearly 10 times greater than that of the predicted radiative heating during Mars entry. These results indicated that worst-case uncertainty intervals of surface radiative heating predictions were as wide as 30 W/cm2 during Mars entry and 150 W/cm2 during Titan entry. Global nonlinear sensitivity results show that the contribution of the uncertain parameters to output uncertainty measures changes across the surface during Mars entry, whereas Titan radiation uncertainty is dominated by flow field chemistry uncertainty throughout the flow field.