HEOS-1 (Highly Eccentric Orbit Satellite-1) was an important satellite mission conducted by the European Space Research Organisation (ESRO), which later became part of the European Space Agency (ESA). The mission focused on studying the Earth's magnetosphere, interplanetary space, and the solar wind. HEOS-1 continued to send valuable data back to Earth until 1975, long after its primary mission objectives had been achieved. The satellite eventually became inactive and remains in a highly eccentric orbit around the Earth.