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 Airbus Defence and Space) that was launched on a Lockheed Martin Atlas II AS launch vehicle on December 2, 1995 to study the Sun. SOHO has also discovered over 3,000 comets.[2] 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 25 years in space: the mission is extended until the end of 2020 with a likely extension until 2022.[3]

Solar and Heliospheric Observatory (SOHO)
Artist's concept of SOHO
Mission typeSolar observation
OperatorESA / NASA
COSPAR ID1995-065A
SATCAT no.23726
Websitesohowww.nascom.nasa.gov
Mission duration2 years planned
24 years, 4 months and 22 days elapsed
Spacecraft properties
ManufacturerMatra Marconi Space
Launch mass1,850 kg (4,080 lb)[1]
Payload mass610 kg (1,340 lb)[1]
Dimensions4.3 m × 2.7 m × 3.7 m (14.1 ft × 8.9 ft × 12.1 ft)[1]
Power1500 watts[1]
Start of mission
Launch dateDecember 2, 1995 (UTC) (1995-12-02T08Z)
RocketAtlas IIAS AC-121
Launch siteCape Canaveral LC-36B
Orbital parameters
Reference systemSun–Earth L1
RegimeHalo orbit
Periapsis altitude206,448 km (128,281 mi)
Apoapsis altitude668,672 km (415,494 mi)
Epochplanned

ESA solar system insignia for SOHO
Horizon 2000
 

In addition to its scientific mission, it is a main source of near-real-time solar data for space weather prediction. Along with Wind, 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.

Scientific objectives

The three main scientific objectives of SOHO are:

  • Investigation of the outer layer of the Sun, which consists of the chromosphere, transition region, and the corona. CDS, EIT, LASCO, SUMER, SWAN, and UVCS are used for this solar atmosphere remote sensing.
  • Making observations of solar wind and associated phenomena in the vicinity of L1. CELIAS and COSTEP are used for "in situ" solar wind observations.
  • Probing the interior structure of the Sun. GOLF, MDI, and VIRGO are used for helioseismology.

Orbit
Animation of SOHO's trajectory
Polar view
Equatorial view
   Earth ·    SOHO

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.

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 coronal mass ejection (CME) arrival times at earth, so electrical grids and satellites can be protected from their damaging effects. CMEs directed toward the earth may produce geomagnetic storms, which in turn produce geomagnetically induced currents, in the most extreme cases 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.[4] 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.[5]

Near loss of SOHO

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 at 4:43 UTC, and the mission interruption had begun. SOHO was spinning, losing electrical power, and no longer pointing at the Sun.[6]

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 Goldstone Solar System Radar combined 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 revolutions every 53 seconds. 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.[7]

The 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.[8]

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 the ESA developed a new gyroless operations mode that was successfully implemented on February 1, 1999.[8]

Instruments
Scale model of the Solar and Heliospheric Observatory (SOHO) spacecraft at the Euro Space Center in Belgium

The SOHO Payload Module (PLM) consists of twelve instruments, each capable of independent or coordinated observation of the Sun or parts of the Sun, and some spacecraft components. The instruments are:[9][10]

  • Coronal Diagnostic Spectrometer (CDS) which measures density, temperature and flows in the corona.
  • Charge Element and Isotope Analysis System (CELIAS) which studies the ion composition of the solar wind.
  • Comprehensive SupraThermal and Energetic Particle analyser collaboration (COSTEP) which studies the ion and electron composition of the solar wind. COSTEP and ERNE are sometimes referred to together as the COSTEP-ERNE Particle Analyzer Collaboration (CEPAC).
  • Extreme ultraviolet Imaging Telescope (EIT) which studies the low coronal structure and activity.
  • Energetic and Relativistic Nuclei and Electron experiment (ERNE) which studies the ion and electron composition of the solar wind. (See note above in COSTEP entry.)
  • Global Oscillations at Low Frequencies (GOLF) which measures velocity variations of the whole solar disk to explore the core of the Sun.
  • Large Angle and Spectrometric Coronagraph (LASCO) which studies the structure and evolution of the corona by creating an artificial solar eclipse.
  • Michelson Doppler Imager (MDI) which measures velocity and magnetic fields in the photosphere to learn about the convection zone which forms the outer layer of the interior of the Sun and about the magnetic fields which control the structure of the corona. The MDI was the biggest producer of data on SOHO. Two of SOHO's virtual channels are named for MDI; VC2 (MDI-M) carries MDI magnetogram data, and VC3 (MDI-H) carries MDI Helioseismology data. MDI has not been used for scientific observation since 2011 when it was superseded by the Solar Dynamics Observatory's Helioseismic and Magnetic Imager.[11]
  • Solar Ultraviolet Measurement of Emitted Radiation (SUMER) which measures plasma flows, temperature and density in the corona.
  • Solar Wind Anisotropies (SWAN) which uses telescopes sensitive to a characteristic wavelength of hydrogen to measure the solar wind mass flux, map the density of the heliosphere, and observe the large-scale structure of the solar wind streams.
  • UltraViolet Coronagraph Spectrometer (UVCS) which measures density and temperature in the corona.
  • Variability of solar IRradiance and Gravity Oscillations (VIRGO) which measures oscillations and solar constant both of the whole solar disk and at low resolution, again exploring the core of the Sun.

Public availability of images

Observations from some of the instruments can be formatted as images, most of which are also readily available on the internet for either public or research use (see the official website). Others such as spectra and measurements of particles in the solar wind do not lend themselves so readily to this. These images range in wavelength or frequency from optical (Hα) to extreme ultraviolet (UV). Images taken partly or exclusively with non-visible wavelengths are shown on the SOHO page and elsewhere in false color.

Unlike many space-based and ground telescopes, there is no time formally allocated by the SOHO program for observing proposals on individual instruments: interested parties can contact the instrument teams directly via e-mail and the SOHO web site to request time via that instrument team's internal processes (some of which are quite informal, provided that the ongoing reference observations are not disturbed). A formal process (the "JOP" program) does exist for using multiple SOHO instruments collaboratively on a single observation. JOP proposals are reviewed at the quarterly Science Working Team ("SWT") meetings, and JOP time is allocated at monthly meetings of the Science Planning Working Group. First results have been presented in Solar Physics, volumes 170 and 175 (1997), edited by B. Fleck and Z. Švestka.

Comet discovery
Comet discoveries[12][13]
Year#
2013213
2012222
2011216
2010209

As a consequence of its observing the Sun, SOHO (specifically the LASCO instrument) has inadvertently allowed the discovery of comets by blocking out the Sun's glare. Approximately one-half of all known comets have been spotted by SOHO, discovered over the last 15 years by over 70 people representing 18 different countries searching through the publicly available SOHO images online. Michał Kusiak of the Polish Jagiellonian University (Uniwersytet Jagielloński) discovered SOHO's 1999th and 2000th comets on December 26, 2010.[14] SOHO has discovered over 2700 comets April 2014,[2][15] with an average discovery rate of every 2.59 days.[16] In September 2015, SOHO discovered its 3000th comet.[17]

Amateur astronomer Mike Oates' discovery of over 140 comets in the SOHO data[18] resulted in the minor planet "68948 Mikeoates" being named after him; this was used by lexicographer Erin McKean in her TED talk as an example of how Internet users can contribute to collections.[19]

SOHO 2198 is a sungrazing comet discovered by Indian amateur astronomer Salil Mulye and Polish astronomer Szymon Liwo[20] by analyzing data from the Solar and Heliospheric Observatory. Large Angle and Spectrometric Coronagraph aboard SOHO is used to capture digital images of the Sun. One such sungrazing comet, SOHO 2198, was discovered using LASCO images. This sungrazer belongs to a family called Kreutz Sungrazers which usually disintegrate after discovery.[21] With this discovery on December 13, 2011, Mulye became the second Indian to discover a sungrazing comet.[22]

Instrument contributors

The Max Planck Institute for Solar System Research contributed to SUMER, LASCO and CELIAS instruments. The Smithsonian Astrophysical Observatory built the UVCS instrument. The Lockheed Martin Solar and Astrophysics Laboratory (LMSAL) built the MDI instrument in collaboration with the solar group at Stanford University. The Institut d'Astrophysique Spatiale is the principal investigator of GOLF and EIT, with a strong contribution to SUMER. A complete list of all the instruments, with links to their home institutions, is available at the SOHO Website.

See also
  • Heliophysics
  • Transition Region and Coronal Explorer (TRACE), launched 1998, decommissioned 2010.
  • Ulysses (spacecraft), launched 1990, decommissioned 2009.
  • Wind (spacecraft), launched 1994, still operational.
  • Solar Orbiter, launched 2020, still operational.
  • Parker Solar Probe, launched 2018, still operational.
  • Advanced Composition Explorer, launched 1997, still operational.
  • STEREO (Solar TErrestrial RElations Observatory), launched 2006, still operational.
  • Solar Dynamics Observatory (SDO), launched 2010, still operational.
  • High Resolution Coronal Imager (Hi-C), launched 2012, sub-orbital telescope.
  • Deep Space Climate Observatory (DSCOVR), launched 2015, orbiting in L1

References
  1. "SOHO (Solar and Heliospheric Observatory)". ESA eoPortal. Retrieved April 12, 2016.
  2. "3,000th Comet Spotted by Solar and Heliospheric Observatory (SOHO)". NASA. Retrieved September 15, 2015. (2,703 discoveries as of 21 April 2014)
  3. Green light for continued operations of ESA science missions
  4. "Antenna anomaly may affect SOHO scientific data transmission". ESA news. Retrieved March 14, 2005.
  5. "SOHO's antenna anomaly: things are much better than expected". ESA news. Retrieved March 14, 2005.
  6. SOHO "Mission Interruption Joint NASA/ESA Investigation Board Final Report." NASA. Retrieved: March 12, 2018.
  7. David, Leonard (May 1999). "Saving SOHO" (PDF). Aerospace America.
  8. "SOHO's Recovery: An Unprecedented Success Story" (PDF). European Space Agency. Retrieved March 12, 2018.
  9. Domingo, V., Fleck, B., Poland, A. I., Solar Physics 162, 1--37 (1995)
  10. Fleck B (1997). "First Results from SOHO". Rev Modern Astron. 10: 273–96. Bibcode:1997RvMA...10..273F.
  11. "MDI Web Page". soi.stanford.edu. Retrieved January 16, 2019.
  12. Karl Battams [@SungrazerComets] (April 16, 2014). "These are SOHO discovery counts in the past few years: 2013: 213, 2012: 222, 2011: 216, 2010: 209 ... consistent!" (Tweet) – via Twitter.
  13. Karl Battams [@SungrazerComets] (January 2, 2013). "The SOHO comet discovery rate has been remarkably consistent over past 3yrs: 2010: 222 comets, 2011: 213, 2012: 219" (Tweet) – via Twitter.
  14. SOHO's 2000th Comet Spotted By Student, SOHO Hotshots, December 28, 2010
  15. Karl Battams [@SungrazerComets] (April 21, 2014). "As of Apr 21, 2014, the @ESA/@NASA SOHO satellite comet discovery count stands at 2,703! #Sungrazers" (Tweet) – via Twitter.
  16. Karl Battams [@SungrazerComets] (October 19, 2012). "Since the @ESA/@NASA SOHO mission launched in 1995, it has discovered a new comet every 2.59-days on average!" (Tweet) – via Twitter.
  17. Mike well, space.com (September 16, 2015). "Whoa! Sun-Watching Spacecraft Finds 3,000th Comet". Retrieved September 16, 2015.
  18. Mike's SOHO Comet Hunt
  19. http://www.ted.com/talks/erin_mckean_redefines_the_dictionary.html video time 12:36-13:06
  20. "SOHO Comets 2011".
  21. "Spacecraft Discovers Thousands of Doomed Comets - NASA Science". science.nasa.gov. Retrieved October 26, 2015.
  22. . "Salil Mulye : 2nd Indian Discoverer of SOHO comet". Khagol Mandal.
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