Problems of extremely long-duration missions in the inner solar system - Example Helios

The paper deals with the exploration of the region between the earth and the sun by means of the Helios I and II spacecraft, launched in December 1974 and January 1976, respectively. Initially intended for quiet-sun observations over a period of 18 months, the mission has been extended to cover the twenty first solar cycle for which an intense sun spot maximum is predicted. The spacecraft configuration, the onboard systems, the requirements of the scientific experiments, and the operational aspects of the mission are discussed.