SPACECRAFT OVERVIEW
Gaia has two main modules:
-The payload module, containing the telescopes, the focal plane and the related electronics.
-The service module, containing all the mechanical, structural, electrical and thermal elements that support the instruments. This includes the micro-propulsion system and the sunshield, thermal tent and solar arrays, and support functions for spacecraft pointing, electrical power control, central data management, and radio communications with Earth. In addition, the service module hosts the scientific video processing units and data storage memory for the instruments in the payload module.
Source: ESA European Space Agency