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The Solar and Heliospheric Observatory ('''SOHO''') is a Spacecraft that was launched on an Atlas IIAS launch vehicle on 2 December 1995 to study the Sun , and 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 ten years in Space . 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 Advanced Composition Explorer (''ACE''), ''SOHO'' is one of two spacecraft currently in the vicinity of the Earth - Sun L1 point, a point of gravitational balance located approximately 0.99 Astronomical Unit s (AU) 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 Wheel s 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 610 kg ''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. It is about 1.5 million kilometers from the Earth. Gravity from the Sun is 2% (118 µm/s&2) more than at the Earth (5.9 mm/s&2), while the reduction of required centripetal force is half of this (59 µm/s&2). The sum of both effects is balanced by the gravity of the Earth, which is here also 177 µm/s&2. Although sometimes described as being at L1, the ''SOHO'' Satellite 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 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. COMMUNICATION WITH EARTH In normal operation the spacecraft transmits a continuous 200 kbit/s data stream of photographs and other measurements via the NASA Deep Space Network of ground stations. ''SOHO'''s data about solar activity are used to predict Solar Flare s, so Electrical Grid s and Satellite s can be protected from their damaging effects. In 2003 ESA reported the failure of the antenna Y-axis Stepper Motor , necessary for pointing the High Gain Antenna and allowing the downlink of high rate data. At the time, it was thought that the antenna anomaly might cause two to three week data-blackouts every three months.1 However, ESA and NASA engineers managed to use ''SOHO'' NEAR LOSS OF SOHO The SOHO Mission Interruption sequence of events began on on the Sun, and entered an emergency Attitude Control mode called Emergency Sun Reacquisition (ESR). The SOHO Team attempted to recover the observatory, but SOHO entered the emergency mode again on June 25 02:35 UTC. Recovery efforts continued, but SOHO entered the emergency mode for the last time at 04:38 UTC. All contact with SOHO was lost, and the mission interruption had begun. SOHO was no longer pointing at the Sun, spinning, and losing electrical power. Expert ESA personnel were immediately dispatched from Europe to the United States to direct operations. Days passed without contact from SOHO. On July 23 , the Arecibo Observatory and DSN antennas were used to locate SOHO with Radar , and to determine its location and Attitude . SOHO was close to its predicted position, oriented with its side versus the usual front Optical Surface Reflector panel pointing toward the Sun, and was rotating at one RPM . Once SOHO was located, plans for contacting SOHO were formed. On 3 August a Carrier was detected from SOHO, the first signal since June 25 . After days of charging the Battery , a successful attempt was made to Modulate the carrier and downlink Telemetry on August 8 . After instrument temperatures were downlinked on August 9 , Data Analysis was performed, and planning for the SOHO recovery began in earnest. The SOHO Recovery Team began by allocating the limited electrical power. After this, SOHO's anomalous orientation in space was determined. Thawing the frozen with SUMER, and ended on October 24 1998 with CELIAS. Only one Gyro remained operational after this recovery, and on December 21 that gyro failed. Attitude control was accomplished with manual thruster firings that consumed 7kg of fuel weekly, while ESA developed a new Gyroless Operations mode that was successfully implemented on February 1 1999 . Additional References |
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