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MIXS (Mercury Imaging X-ray Spectrometer) is an instrument in the remote sensing suite of the Mercury Planetary Orbiter (MPO), part of the Bepi-Colombo mission to Mercury. The optics for the MIXS instrument is funded by the European Space Agency. The development of the MIXS instrument is undertaken by an international consortium, led by the University of Leicester, UK. This is funded by the UK's Science & Technology Facilities Council. The instrument is intended to address scientific issues including: the origin of Mercury, the evolution of Mercury, the nature of surface modifications on Mercury (e.g. cratering, volcanism), There are two adjacent X-ray optics, each with different layouts, for the imaging telescope and an adjacent flux-collector (collimator). Accurate thermal and geometrical mathematical models were needed to model the optics, taking into account the extreme thermal environment around Mercury.

The Envisat spacecraft has a launch mass greater than 8.0 tonnes and external dimensions of 10.0 metres x 2.8 metres x 2.1 metres. Due to it’s large size, it was necessary to perform the thermal balance and thermal vacuum testing in two stages. Firstly, there was the testing of the Service Module and, secondly, the testing of the Payload Module (PLM). This paper discusses the thermal balance testing of the PLM. The PLM, itself, is 7.5 metres tall; too large to fit into a test facility solar beam. Originally, it was intended to conduct two solar beam tests; one for the upper part and the second for the lower part. Following a revision and re-scheduling within the programme, it was decided to perform a single, non-solar beam, thermal balance test. The thermal balance test would be performed using test specific, electrical, heaters and test facility shroud control.