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One startling realization that's come from NASA's explorations of the satellites of Jupiter and Saturn is that the so-called “habitable zone” around our Sun may not be restricted to Earth's vicinity. The Galileo mission found conditions that might support life on two Jovian moons-Europa & Callisto. This raises the possibility of habitable zones elsewhere near the outer planets. Consideration of human missions beyond Mars, likely to occur sometime beyond the year 2040, exceeds the horizon of even the most advanced planning activities within NASA. During the next 25 to 30 years, robotic spacecraft are envisioned to explore several moons of outer planets, especially Europa and Titan. Since Callisto lies well outside Jupiter's radiation belt, and there is evidence of water ice there is a compelling rationale to send human explorers to that Jovian moon.

The exploration of Mars is a major thrust of NASA. Some of the important goals of this exploration are the search for life; understanding the planet's evolution by in-situ and remote scientific measurements; developing an inventory of useful resources, including accessible water; and sample return as a precursor to human exploration. One of the key challenges of Mars's exploration hard-ware--- rovers, landers, probes, and science instruments -- is to be able to survive the planet's harsh environment on and below surface. This paper discusses the thermal challenges posed by relatively large temperature variations, analyses and experimental work done at JPL to address these challenges.

The Mars Exploration Technology (MET) Program Thermal Control Task at JPL provided for the design, development, fabrication, and demonstration of an insulated Warm Electronics Enclosure (WEE). The thermal control technique consisted of Phase Change Material (PCM) for efficient thermal storage, and heat switches to provide diode action to control the heat path between the solar array and the PCM enclosure. For a simulated Mars's thermal environment, the WEE maintained its interior temperature within the temperature range of -27 °C to +23 °C. Included in this paper is a description of the WEE design, the thermal vacuum tests and correlation with thermal model analyses as well as discussion of the analytical results.

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.