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 IIAS launch vehicle on December 2, 1995 to study the Sun, and has discovered over 2300 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 currently continues to operate after over fifteen years in space. In October 2009, a mission extension lasting until December 2012 was approved.
In addition to its scientific mission, it is currently the main source of near-real time solar data for space weather prediction. Along with the GGS Wind and Advanced Composition Explorer (ACE), SOHO is one of three spacecraft currently in the vicinity of the Earth-Sun L1 point, a point of gravitational balance located approximately 0.99 astronomical unit (AU)s from the Sun and 0.01 AU from the Earth. In addition to its scientific contributions, SOHO is distinguished by being the first three-axis-stabilized spacecraft to use its reaction wheels as a kind of virtual gyroscope; the technique was adopted after an on-board emergency in 1998 that nearly resulted in the loss of the spacecraft.
Read more about Solar And Heliospheric Observatory: Orbit, Communication With Earth, Near Loss of SOHO, Scientific Objectives, Instruments
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