The SAGE (Stratospheric Aerosol & Gas Experiment) spacecraft was the second of the Applications Explorer Missions (AEM). The small, versatile, low-cost spacecraft was made of two distinct parts:

- the SAGE instrument module containing the detectors and the associated hardware, and
- the base module containing the necessary data handling, power, communications, command, and attitude control subsystem to support the instrument mode.

The objective of the SAGE mission was to obtain stratospheric aerosol and ozone data on a global scale for a better understanding of the earth's environmental quality and radiation budget. The spacecraft was designed for a 1-year life in orbit. The spacecraft experienced power problems after 15 May 1979. Spacecraft operations continued until 19 November 1981. The signal from the spacecraft was last received on 7 January 1982, when the battery failed.