SOHO

Launch data: Designation  23726 / 95065A Launch date  02 Dec 1995 Launch site  CC, LC-36B Launch vehicle  Atlas-II AS (AC-121) Mission  Sun observation Heliocentric orbit Perihelion / Aphelion   A.U. Eccentricity   Inclination   deg Period   min

Spacecraft data: Prime manufactor  Matra Marconi Space (now Astrium) Platform   Operator  NASA Mass at launch  1850 kg Dry Mass  610 kg Basic shape   Dimensions (m)  Approximately 4.3 × 2.7 × 3.7 m Solar array  2 solar array's span 9.5 m Stabilization  three-axis stabilised spacecraft that constantly faces the Sun. DC power   Design lifetime  Originally planned as a two-year mission, In October 2009, mission extension lasting until December 2012

Soho satellite before launch

Sun eruption take by SOHO on Jan 12, 2007

The SOHO satellite reveals the magnetic fields on the visible surface of the Sun.

• CDS (Coronal Diagnostic Spectrometer) from Rutherford Appleton Laboratory, United Kingdom.
• CELIAS (Charge, Element, and Isotope Analysis System) from the University of Bern, Switzerland.
• COSTEP (Comprehensive Suprathermal and Energetic Particle Analyser) from the University of Kiel, Germany.
• EIT (Extreme ultraviolet Imaging Telescope) from the Institut d'Astrophysique Spatiale, France.
• ERNE (Energetic and Relativistic Nuclei and Electron experiment) from the University of Turku, Finland.
• GOLF (Global Oscillations at Low Frequencies) from the Institut d'Astrophysique Spatiale, France.
• LASCO (Large Angle and Spectrometric Coronagraph) from the Naval Research Laboratory, United States.
• MDI (Michelson Doppler Imager) from Stanford University, United States.
• SUMER (Solar Ultraviolet Measurements of Emitted Radiation) from the Max-Planck-Institut für Aeronomie, Germany.
• SWAN (Solar Wind Anisotropies) from Service d'Aeronomie, France.
• UVCS (Ultraviolet Coronagraph Spectrometer) from Harvard-Smithsonian Center for Astrophysics, United States.
• VIRGO (Variability of Solar Irradiance and Gravity Oscillations) from PMO/WRC Davos, Switzerland.

Descriptions:

• The Solar and Heliospheric Observatory (SOHO) is a spacecraft launched on December 2, 1995 to study the Sun, and has discovered over 2200 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) 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.
• 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 flares, so electrical grids and satellites can be protected from their damaging effects (mainly, solar flares may produce geomagnetic storms, which in turn produce geomagnetically induced current creating black-outs, etc.).
• 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. However, ESA and NASA engineers managed to use SOHO's low gain antennas together with the larger 34 and 70 meter DSN ground stations and judicious use of SOHO's Solid State Recorder (SSR) to prevent total data loss, with only a slightly reduced data flow every three months.[

Near Loss of SOHO:

• The SOHO solar observatory, orbiting the Earth-Sun L1 point, has been lost due to an apparent gyroscope problem. No contact has been made with the satellite since Jun 25, 1998.
• The SOHO Mission Interruption sequence of events began on June 24, 1998, while the SOHO Team was conducting a series of spacecraft gyroscope calibrations and maneuvers. Operations proceeded until 23:16 UTC when SOHO lost lock 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 spinning, losing electrical power, and no longer pointing at the Sun.
• 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 August 3 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 hydrazine fuel tank using SOHO's thermal control heaters began on August 12. Thawing pipes and the thrusters was next, and SOHO was re-oriented towards the Sun on September 16. After nearly a week of spacecraft bus recovery activities and an orbital correction maneuver, the SOHO spacecraft (bus) returned to normal mode on September 25 at 19:52 UTC. Recovery of the instruments began on October 5 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 7 kg of fuel weekly, while ESA developed a new gyroless operations mode that was successfully implemented on February 1, 1999.

Mission now:

• The Solar and Heliospheric Observatory (SOHO) is stationed 1.5 million kilometres away from Earth. There, it constantly watches the Sun, returning spectacular pictures and data of the storms that rage across its surface. SOHO's studies range from the Sun's hot interior, through its visible surface and stormy atmosphere, and out to distant regions where the wind from the Sun battles with a breeze of atoms coming from among the stars.