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Solar and Heliospheric Observatory
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.〔(【引用サイトリンク】publisher=NASA ) ((2,703 discoveries as of 21 April 2014) )〕 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 19 years in space. In June 2013, a mission extension lasting until December 2016 was approved.〔(ESA science missions continue in overtime ), ESA, 20 June 2013〕
In addition to its scientific mission, it is the main source of near-real-time solar data for space weather prediction. Along with the GGS Wind, Advanced Composition Explorer (''ACE'') and DSCOVR, ''SOHO'' is one of four spacecraft 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.
==Orbit==
The ''SOHO'' spacecraft is in a halo orbit around the Sun–Earth L1 point, the point between the Earth and the Sun where the balance of the (larger) Sun's gravity and the (smaller) Earth's gravity is equal to the centripetal force needed for an object to have the same orbital period in its orbit around the Sun as the Earth, with the result that the object will stay in that relative position.
Although sometimes described as being at L1, the ''SOHO'' spacecraft is not exactly at L1 as this would make communication difficult due to radio interference generated by the Sun, and because this would not be a stable orbit. Rather it lies in the (constantly moving) plane which passes through L1 and is perpendicular to the line connecting the Sun and the Earth. It stays in this plane, tracing out an elliptical lissajous orbit centered about L1. It orbits L1 once every six months, while L1 itself orbits the Sun every 12 months as it is coupled with the motion of the Earth. This keeps ''SOHO'' at a good position for communication with Earth at all times.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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