This paper discusses the sizing of the heat shield of a lifting-body spacecraft, protected by a rigid aeroshell, to minimize its mass for a future aerocapture mission to Neptune. Reducing the heat shield mass is a primary requirement for the mission design because the high expected heat loads can raise the value of its mass fraction to levels that would be unacceptable for the successful execution of the mission. The heat shield is divided into several regions, each of which is characterized by different levels of the entering heat flux. Its mass is minimized by identifying the most suitable materials to be used in the different zones and by determining their minimum thicknesses. To accomplish these tasks, a mapping is established a priori based on a common case treated in the literature. The analysis demonstrates that to minimize the mass for this vehicle, it is necessary to adopt a heat shield composed of different ablative materials that vary depending on the area to be protected. The front part of the spacecraft, near the stagnation point, should be protected exclusively by carbon phenolic, a high-density material, using substantial thicknesses, whereas thinner, lower-density ablative materials should be used to protect the ventral and dorsal regions. The frontal area alone constitutes approximately half of the entire mass of the heat shield while covering less than 10% the total surface.