Future missions to Mars for the 1998 launch opportunity and beyond will require advanced thermal control for electronics to minimize enclosure mass, power and volume. An additional requirement is that radioactive heating units (RHU) will not be available for future Mars missions. These strict requirements can be accomplished by integrating phase change material (PCM) panels with aerogel insulation in a structural/thermal enclosure for electronics and instruments. The aerogel insulation has extremely low thermal conductivity, and the PCM panels provide thermal capacitance. The advanced PCM panels consist of a sandwich panel design with an interlocking carbon fiber core which is filled with a suitable phase change material. The fibers provide structural stiffness, and prevent the PCM from forming voids or migration of voids by capillary action. With this design, a PCM mass fraction of 70% has been achieved. Included in this design is a diode heat pipe to recover thermal energy from radiators or other heat sources. This concept has been validated as a Warm Electronic Enclosure (WEE) in simulated Mars environmental conditions. The WEE used Dodecane as the phase change material (solid/liquid transition at -10 °C). The WEE consisted of a thermal insulation enclosure with six PCM panels on the interior. One of the PCM panel had an integrated diode heat pipe. The Mars 8 torr vacuum environment, and the diurnal temperature variation from 0° to -80 °C were simulated. The WEE was able to maintain interior temperatures within +/- 20 °C for a simulated 8 watt peak electronics power, and from 0° to -30 °C for a simulated 6.4 watt peak electronics power during sunlight operations.