The Solar and Heliospheric Observatory (SOHO) was launched on 2 December 1995. It was built in Europe by prime contractor Matra Marconi Space, which is now EADS Astrium. The spacecraft is operated jointly by the ESA and NASA and it studies the Sun in depth, all the way from its deep core to its outer corona and the solar wind.
SOHO is made of two modules, the Service Module and the Payload Module. The former provides SOHO with power, while the latter houses all of the instruments on the spacecraft. There are 12 instruments on board SOHO, nine of which are run by Europe and three from the US. They are used to study everything from the Sun’s interior to its outer corona and solar wind.
The observatory is located near to Lagrangian point 1, which is a point between the Earth and the Sun about 930,000 miles (1.5 million km) from our planet. It is the point where the gravitational attraction of the Sun and the Earth cancel out, so a telescope such as SOHO can remain in a relatively stable orbit to observe the Sun. SOHO is one of a handful of telescopes that can detect incoming solar flares that could be potentially hazardous to Earth satellites and other electronics.
Of the 12 instruments on board one of the most interesting is the Large Angle and Spectrometric Coronagraph (LASCO), which studies the Sun’s corona by creating an artificial solar eclipse. The LASCO instrument has been largely responsible for inadvertently discovering many comets near the Sun, with over 2,000 found to date, and recently it has been used to observe the sungrazing path of Comet ISON.
SOHO has three primary objectives that it has been carrying out since its launch. The first was to investigate the outer regions of the Sun, specifically the corona. At the moment it is still unknown why the corona is hotter than the photosphere and chromosphere (both more inner regions) of the Sun, so it is hoped that SOHO might provide the answer in the future. SOHO has also been used to observe the solar wind, and also to study the interior structure of the Sun through a process known as helioseismology.
Image courtesy of ESA