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The XMM-NEWTON satellite is the ESA X-ray spaceborne observatory covering the soft X-ray portion of the electromagnetic spectrum. XMM-NEWTON has been put in orbit on December, 10th 1999 by an Ariane 5 single launch. The spacecraft has a conventional thermal design that takes full advantage of the stable environment provided by its high altitude/long period orbit and by the limited variation of solar attitude angles in order to provide a stable platform for the telescope system. The precise geometry and alignment of the telescope system impose strict temperature requirements so that not only temperature gradients have to be kept small but also, and more importantly, time-variations of the gradients have to be minimised. In the paper, the thermal behaviour of the spacecraft as verified by its thermal test programme is compared with the early in-orbit temperature measurements.

The XMM (X-ray Multi-Mirror) spacecraft is a space-borne observatory for soft X-ray astronomy. It is developed and built under an ESA contract by an industrial consortium led by Dornier Satellitensysteme GmbH. Due to the large dimensions of the whole spacecraft measuring more than 10 m in height and more than 4 m in diameter, it is split into an upper and a lower module for integration and testing. These modules were separately vacuum tested in the Large Space Simulator at ESTEC, allowing for different spacecraft attitudes with reference to the solar simulator beam. This paper reports on the thermal vacuum test of the XMM lower module (LM), which was conducted in January 1999. The successful completion of this test was a further decisive cornerstone in passing the XMM flight acceptance review in late 1999.