An overview of the XMM observatory system

XMM is the largest scientific observatory developed by ESA and dedicated to exploring the Universe in the soft-X-ray portion of the electromagnetic sp ectrum. This article presents XMM as a complete system, composed by the spa ce segment (the satellite and its launch vehicle) and the ground segment ta ll of ground-based infrastructure needed to control the satellite and gathe r the scientific data). The definition of the XMM orbit, which is also desc ribed, is a key element in meeting the primary mission objectives and satis fying the project programmatic requirements and constraints.