There is a logic in this branch: to develop any space system we  start by a global conception (1A), then we need a skeleton (1B). Power is always necessary (1C), then Equipments with their connections (1D) and management of Thermal problems (1E). Many times we need Propulsion (1F) and, if we want to go to a particular place we need Guiding (1G). When we have a guided spacecraft we need to Communicate with it (1H) and many times we introduce Sensors and Instruments (1I). Our mission can be Unmanned (automatic, robotized) (1J) or Manned (1K). And we need to control it from Ground (1L). Nowadays we are specially concerned with Space Environment (1M). Downstream sector is based on space acquired data (1N). New Space paradigm is taking a lot of attention (1P)