THE SOHO MISSION - AN OVERVIEW

The Solar and Heliospheric Observatory (SOHO) is a space mission that forms part of the Solar-Terrestrial Science Program (STSP), developed in a collaborative effort by the European Space Agency (ESA) and the N ational Aeronautics and Space Administration (NASA). The STSP constitu tes the first ''cornerstone'' of ESA's long-term programme known as '' Space Science - Horizon 2000''. The principal scientific objectives of the SOHO mission are a) to reach a better understanding of the struct ure and dynamics of the solar interior using techniques of helioseismo logy, and b) to gain better insight into the physical processes that f orm and heat the Sun's corona, maintain it and give rise to its accele ration into the solar wind. To achieve these goals, SOHO carries a pay load consisting of 12 sets of complementary instruments. SOHO is a thr ee-axis stabilized spacecraft with a total mass of 1850 kg; 1150 W of power will be provided by the solar panels. The payload weighs about 6 40 kg and will consume 450 W in orbit. SOHO will be launched by an ATL AS II-AS and will be placed in a halo orbit around the Sun-Earth L1 La grangian point where it will be continuously pointing to Sun centre wi th an accuracy of 10 arcsec. Pointing stability will be better than 1 arcsec over 15 min intervals. The SOHO payload produces a continuous s cience data stream of 40 kbits/s which will be increased by 160 kbits/ s whenever the solar oscillations imaging instrument is operated in it s high-rate mode. Telemetry will be received by NASA's Deep Space Netw ork (DSN). Planning, coordination and operation of the spacecraft and the scientific payload will be conducted from the Experiment Operation s Facility (EOF) at NASA's Goddard Space Flight Center (GSFC).