SOHO, first cornerstone, with CLUSTER, of the ESA Horizon 2000 program, is part of the European contribution to international scientific programs STSP and ISTP studying the Sun-Earth relations. The ESA Scientific Program Committee (SPC) approuved the 5 years SOHO mission extension after its nominal lifetime, which means from May 1998 to April 2003, then at the beginning of 2002, a new extension until March 2007. This eleven years lifetime (instead of the 2 initially decided) will enable to cover the totality of a solar cycle.

SOHO (Solar and Heliospheric Observatory) is a joint mission of the European Spatial Agency and the NASA. Launched in 1995, the SOHO probe is on a halo orbit around the Lagrange point L1 at 1,5 million km on the sunward side of the Earth. From this localisation, SOHO observe the Sun 24 hours a day.

France participated greatly to the realization of 5 out of the 12 instruments intended to study several aspects of the Sun : the heliosismology, the electromagnetic radiation, the plasma and the solar wind.

The SOHO satellite was built in Europe by an industrial consortium lead by Matra, while the scientific instruments were provided by European and American scientists and funded by their national institutions. Nine of the instruments Pricipal Investigators (PIs) are European and three are Americans. The NASA was responsible of the launch and of the mission operations. The NASA DSN network is used to control the satellite and receive the data. The mission control is done from the Goddard Space Flight Center in Maryland.