Solar and Heliospheric Observatory

Space probe built by the European Space Agency and launched by NASA on 2 December 1995 to study the solar wind of atomic particles streaming towards the Earth from the Sun. It also observes the Sun in ultraviolet and visible light, and measures slight oscillations on the Sun's surface that can reveal details of the structure of the Sun's interior.

The US$1.2 billion probe is positioned towards the Sun, 1.5 million km/938,000 mi from Earth. SOHO's hydrazine fuel froze in June 1998 causing contact with it to be lost. Ground control at the Goddard Space Flight Center finally regained command of SOHO in September 1998, when SOHO was turned so that its solar power arrays faced the Sun. SOHO has been in continuous use since that time. It broadcasts daily images of the Sun, which are accessible to the public via the Internet. On 28 October 2003, it observed a spectacular solar flare that required two of its instruments to be temporarily shut down to avoid damage.

SOHO carries equipment for 11 separate experiments, including the study of the Sun's corona and measurement of its magnetic field and of solar winds. The coronal diagnostic spectrometer (CDS) detects radiation at extreme ultraviolet wavelengths and allows the study of the Sun's atmosphere. The Michelson Doppler imager (MDI) measures Doppler shifts in light wavelengths and can detect winds caused by convection beneath the Sun's surface. The extreme-ultraviolet imaging telescope (EIT) investigates the mechanisms that heat the Sun's corona. The large-angle spectroscopic coronagraph (LASCO) images the corona by detecting sunlight scattered by the coronal gases.