<div class="htmlview paragraph">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).</div> <div class="htmlview paragraph">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.</div> <div class="htmlview paragraph">The Envisat PLM supports a complement of 10 earth observation instruments and was successfully tested in July/August 1999 at the Large Space Simulator (LSS), at ESTEC, The Netherlands. The objectives of the thermal balance test were to provide correlation data for the thermal mathematical models and to demonstrate the Thermal Control Subsystem (TCS) performance. Test data, taken from the test, is compared with the test predictions and a correlation performed. The resultant thermal model will be utilised to provide the final flight predictions.</div> <div class="htmlview paragraph">The paper outlines the rationale behind the selection and definition of the test, discusses technical aspects of the test set-up and shows the achieved, thermal mathematical model, correlation.</div>