The ISO satellite had a height of 5.3m, a width of 3.6m, a depth of 2.8m and a launch mass of 2498kg (see Figure 1.1). It was conceived as two largely independent modules: the Payload Module and the Service Module (see Figure 3.1). The Payload Module was essentially a large cryostat containing superfluid helium which maintained the telescope and the scientific instruments at temperatures between 2 and 8 K for a lifetime of more than 28 months (the upper cylindrical part in Figure 3.1). The Service Module housed the warm electronics of the scientific instruments, the hydrazine propellant tank, and all the other classical spacecraft subsystems for the provision of the basic functions. The sun-shield, with its covering of solar cells, always faced the Sun to provide electrical power whilst at the same time protecting the Payload Module from direct insolation.
