The insertion device for the MUPUS penetrator on Phylae, the lander to be set by the Rosetta mission on the nucleus of Tchurumov-Gerasimenko comet, will have to work in extreme environmental condition. The electronic unit inside the device should be kept at the temperature greater than -55 degrees C (220 K) in order to work properly. The expected temperature of the nucleus is of about 130 K, while the cosmic background temperature is as low as 3 K. Therefore, the electronic box must be heated continuously to balance the heat loss by radiation. The experiments performed in a vacuum-thermal chamber on the flight model of MUPUS has shown that the temperature of the electronics increases, when heated with a power of 1.33 W, from 140 K by about 120-130 K after more than I h of heating. We present a numerical thermal (FEM) model that gives a very good fit to the measured temperature dependence on time. In the process of fitting, several important thermal parameters of the insertion device have been determined, for instance the emissivity of electronic boards and their specific heat. The numerical models agree qualitatively with the results of simple thermal analysis. The main conclusion concerning performance of the instrument on the comet is that it should safely operate above the lover limit of acceptable temperatures against expected variations of the external temperature in the day-night cycle of the comet. (c) 2005 Elsevier Ltd. All rights reserved.