When the Galileo mission’s probe entered the Jovian atmosphere in December 1995, it experienced temperatures twice as hot as the surface of the sun, and required carbon phenolic shields to protect its onboard payload from the intense heat. Since that mission, NASA has not flown a spacecraft that required protection from such extreme heat. Recently, however, the NRC Planetary Science Decadal Survey has recommended that NASA consider in situ science missions to Venus and Saturn as a high priority in the New Frontiers competed mission set. To reach the surface of these planets, missions will require heat shields that are capable of withstanding very extreme entry environments, but are not as heavy as the previously used carbon phenolic heat shields.