ESA’s activities fall into two categories – ‘mandatory’ and ‘optional’. Programmes carried out under the General Budget and the Space Science programme budget are ‘mandatory’; they include the agency’s basic activities (studies on future projects, technology research, shared technical investments, information systems and training programmes).

All Member States contribute to these programmes on a scale based on their Gross National Product (GNP). The other programmes, known as ‘optional’, are only of interest to some Member States, who are free to decide on their level of involvement.

Optional programmes cover areas such as Earth observation, telecommunications, satellite navigation and space transportation. Similarly, the International Space Station and microgravity research are financed by optional contributions.