The basic safety requirement imposed on a radioisotope heat source is containment of the fuel under all normal and accident environments. A two-layer heat shield was designed to protect the radioisotope capsule during atmospheric reentry. Using two- and three-dimensional thermal models, parametric analyses were performed to compare the effectiveness of various insulative materials under simulated reentry conditions. It was found that a material able to insulate the capsule and at the same time distribute heat quickly in lateral directions will provide maximum thermal barrier capabilities. Such a material is the anisotropic pyrolytic graphite. The outer heat shield was designed to survive ablation and thermal stress resulting from steep abort reentries. Numerous graphite materials were considered and compared on the basis of ability to withstand thermal stress, ability to be nondestructively tested, availability, and cost. AXF-5Q Poco graphite was superior in three of the selection categories and was selected. Design curves for ultimate tensile strength, strain to failure, modulus of elasticity, thermal expansion, and thermal conductivity were established and verified by destructive testing of samples of billets used. Nondestructive testing of the billets was performed to ascertain soundness. Ultrasonic pulse echo “c” scans and sound velocity traverses were performed and used to locate actual heat source components within the billets to contain the minimum number of defects possible.