Earth and planetary entry vehicles require heat shield materials whose transparency to radio frequency (rf) signals is unaffected by the ablation process. Teflon or fused silica are currently used for this purpose, but these materials are heavy and difficult to attach. For applications such as Mars entry vehicles or lifting bodies where low heating rates prevail, low-density, rf-transparent ablators are desired. To fill this need, Martin Marietta Corporation has developed a rf-transparent composition designated SLA-220. This ablator, a silica-filled elastomeric silicone, has a density of 0.25 g/cm3and forms a siliceous rather than a carbonaceous char. Pyrolysis of SLA-220 at slow heating yields a pure silica residue. As heating rate increases, the volatile pyrolysis products tend to undergo secondary decomposition and to deposit carbon while passing through the hot char. Therefore, rf signal attenuation increases with heating rate. Electrical properties of SLA-220 (dielectric constant, signal attenuation) have been measured before and after ablation. Ablation testing has been conducted at heating rates ranging from 6 to 60 Btu/ft2-sec. Thermal properties (conductivity, specific heat, expansion) and mechanical properties (strength, elastic modulus, elongation) have been determined. The properties of SLA-220 are compared with those of 0.50 and 0.70 g/cm3porous Teflon ablators to demonstrate the advantages of SLA-220 over other low-density rf-transparent ablative systems.