Aerospace radioisotope heater technology has been developed through analysis and testing of improved containment and ablation materials and configurations. To best meet the widest application range, an unvented 238Pu sphere/cube heater in two power sizes, 5 and 50 W(th), is recommended. The critical design constraint is helium pressure containment during reentry for up to 20 years. An ATJ-S graphite ablator combined with a heat-treated pyrolytic-graphite insulator wrapped around a T-111/Pt - 20 wt% Rh capsule is the optimum configuration using current technology. A HfB2 composite currently being developed would provide a major reduction in ablation rates. Computer-optimized geometry and layer thicknesses are described. The design approach was verified by fabrication and key development testing of prototype heater units.