Adding phase change material (PCM) to a mission payload can maintain its temperature above the cold survival limit, without power, for several hours in space. For the International Space Station, PCM is melted by heaters just prior to the payload translation to the worksite when power is available. When power is cut off during the six-hour translation, the PCM releases its latent heat to make up the heat loss from the radiator(s) to space. For the interplanetary Probe, PCM is melted by heaters just prior to separation from the orbiter when power is available from the orbiter power system. After the Probe separates from the orbiter, the PCM releases its latent heat to make up the heat loss from the Probe exterior to space.
Paraffin wax is a good PCM candidate. It has a high solid-to-liquid enthalpy, which is about 225 kJ/kg, and a range of melting points. For example, C18H38 has a melting point of 28 °C, which is well within the payload temperature limits. At the time of this reporting, paraffin wax PCM had a TRL (technology readiness level) of 7.
This work was done by Michael Choi of Goddard Space Flight Center. GSC-16539-1