The spacecraft structure, like any other structure, is there primarily to guarantee the integrity of the spacecraft under any loading, such as during handling, testing and launch. In addition, it must allow the spacecraft to serve as an optical bench for a telescope and therefore the structure must provide the necessary thermo-elastic stability in orbit. In the case of XMM-Newton, this led immediately to the selection of ultra-high-modulus carbon-fibre composites (low thermal expansion) for the main structural elements. Another advantage of this material is its very high modulus of elasticity, which limits structural mass for a structure like this, which is (also) designed for stiffness.