The joint NASA-ESA Solar & Heliospheric Observatory mission -- SOHO -- was designed to study the Sun inside out, from its internal structure, to the extensive outer atmosphere, to the solar wind that it blows across the solar system.
Launched in December 1995, SOHO was meant to operate until 1998, but it has been so successful that ESA and NASA have endorsed several mission extensions over the past two decades, allowing it to cover multiple solar cycles -- 11-year periods of solar activity.
Over those two decades in space, SOHO has made many new discoveries, adding to scientists' understanding of our closest star. SOHO has revealed the first images ever of the Sun’s convection zone -- its turbulent upper layer of the interior -- and of the structure of sunspots below the surface. It has also provided detailed measurements of the interior and the slow and fast solar wind, event identifying source regions and acceleration mechanisms. The mission has also discovered over 3,000 new comets and new dynamic solar phenomena such as coronal waves and solar tornadoes.
SOHO is equipped with 12 instruments developed by 29 different institutions across 15 countries. The diverse instruments allow the wide range of science SOHO conducts from taking images of the Sun and surrounding atmosphere, to looking at the individual wavelengths and composition of that light, to measuring energetic particles passing the spacecraft.
