The Solar and Heliospheric Observatory (SOHO) is a spacecraft built by a European industrial consortium led by Matra Marconi Space (now Astrium) that was launched on a Lockheed Martin Atlas II AS launch vehicle on December 2, 1995, to study the Sun, and has discovered over 3000 comets. It began normal operations in May 1996. It is a joint project of international cooperation between the European Space Agency (ESA) and NASA. Originally planned as a two-year mission, SOHO continues to operate after over 20 years in space. In November 2016, a mission extension lasting until December 2018 was approved.
Much more info is available at the Solar and Heliospheric Observatory (SOHO) website here: https://sohowww.nascom.nasa.gov/home.html