The primary scientific aims of the Solar and Heliospheric Observatory mission (SOHO) were 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. 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 was part of the International Solar-Terrestrial Physics Program (ISTP).

The SOHO spacecraft was three-axis stabilized and pointed towards the Sun with an accuracy of +/- 10 arcsec per 15 min. It consisted of a Payload Module to accommodate the instruments and a Service Module carrying the spacecraft subsystems and solar arrays. SOHO occupied a halo orbit at the Earth-Sun L1 Lagrangian point to obtain uninterrupted sunlight. The design life was two years, but on-board consumables were sufficient for an extra four years of operations.

Twelve instruments comprised the payload, producing a continuous stream of data at 40 kbs, except whenever the solar oscillations imager Michelson Doppler Imager (MDI) was operated in high-bit-rate mode, producing 160 kbs. The high-bit-rate mode was used during scheduled daily eight-hour periods or during dedicated campaigns. Magnetic tape stored data between telemetry contacts with the Experiment Operations Facility, located at NASA-GSFC. For more information, see B. Fleck, V. Domingo, and A. I. Poland, eds., Solar Physics, V. 162, 1995.

SOHO was a joint mission of the European Space Agency and NASA. It was launched aboard an Atlas IIAS rocket from Cape Canaveral Air Station, FL, on Dec. 2, 1995, and mission operations were directed from NASA's Goddard Space Flight Center, Greenbelt, MD.

In April 1998, SOHO successfully completed its nominal two-year mission to study the Sun's atmosphere, surface and interior. Major science highlights include the detection of rivers of plasma beneath the surface of the Sun; the discovery of a magnetic "carpet" on the solar surface that seems to account for a substantial part of the energy that is needed to cause the very high temperature of the corona, the Sun's outermost layer; the first detection of flare-induced solar quakes; the discovery of more than 50 sungrazing comets; the most detailed view to date of the solar atmosphere; and spectacular images and movies of coronal mass ejections, which were being used to improve the ability to forecast space weather.

Contact with SOHO was lost at 23:16 GMT on June 24, 1998 during maintenance operations. The spacecraft went into emergency sun reacquisition mode, which is activated when an anomaly occurs and the spacecraft loses its orientation toward the Sun. When this happened, the spacecraft automatically tried to point itself toward the Sun again by firing its attitude control thrusters under the guidance of an onboard Sun sensor. Efforts to re-establish contact with SOHO did not succeed and telemetry was lost, not to be reestablished for several weeks.

Attitude analysis led to the conclusion that SOHO went into a spin such that the solar panels were nearly edge-on towards the Sun, and thus did not generate any power. Because the spin axis is fixed in space, as the spacecraft progressed in its orbit the orientation of the panels with respect to the Sun gradually changed, resulting in increased solar illumination of the solar arrays as time progressed. On August 3, contact was re-established with SOHO following six weeks of silence. After a lengthy recovery process, on September 16 the spacecraft again locked onto the Sun, and experiment heaters were switched on. After recommissioning of various subsystems, and an orbit correction maneuver, SOHO was finally brought back to normal mode on 25 September at 19:52:58 GMT. Instrument switch-on started October 5, 1998, and by November 4 all instruments were back to normal.