The primary scientific aims of the Solar and Heliospheric Observatory mission (SOHO) are to investigate (1) the physical processes that form and heat the Sun's corona, maintain it and give rise to the expanding solar wind, and (2) the interior structure of the Sun. The spacecraft was launched aboard an Atlas 2-AS rocket on December 12, 1995, from Cape Canaveral, Florida.
Imaging and collection of spectroscopic plasma diagnostics of the solar chromosphere, transition region and corona, as well as in-situ solar-wind measurements are used to study the corona and solar wind. Solar interior structure is investigated by helioseismological means and by the observation of variations in solar irradiance. SOHO is part of the International Solar-Terrestrial Physics Program (ISTP).
The SOHO spacecraft is three-axis stabilized and points towards the Sun with an accuracy of +/- 10 arcsec per 15 min. It consists of a Payload Module to accommodate the instruments and a Service Module carrying the spacecraft subsystems and solar arrays. SOHO occupies a halo orbit at the Earth-Sun L1 Lagrangian point to obtain uninterrupted sunlight. The design life is two years, but onboard consumables are sufficient for an extra four years of operations. Twelve instruments comprise the payload, producing a continuous stream of data at 40 kbs, except when the solar oscillations imager Michelson Doppler Imager (MDI) is operated in high-bit-rate mode, producing 160 kbs. The high-bit-rate mode is used during scheduled daily eight-hour periods or during dedicated campaigns. Magnetic tape stores data between telemetry contacts with the Experiment Operations Facility, located at NASA Goddard Space Flight Center.