Prototype of the Apogee engine for the Sirio satellite

This is the Apogee engine of the Italian satellite Sirio for Telecommunications, launched in 1977. The apogee engine is installed on the satellite, which is placed by the carrier rocket on an elliptical orbit that has a point closest to Earth (perigee) and one furthest away, 36,000 km from the Earth. When the telecommunication satellite reaches this latter point, the apogee engine is switched on to circularize the satellite’s orbitat an altitude of 36,000 km. Here the satellite rotates in sync with the Earth, always appearing in the same place from an Earth station, and thus being ready to be constantly in use.