Solar and Heliospheric Observatory#Instruments

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. The instruments 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.

{{multiple image|align=left|direction=vertical|width=

| header = Animation of SOHO's trajectory

| image1 = Animation of Solar and Heliospheric Observatory trajectory - Polar view.gif

| caption1 = Polar view

| image2 = Animation of Solar and Heliospheric Observatory trajectory - Equatorial view.gif

| caption2 = Equatorial view

| footer = {{legend2| RoyalBlue| Earth}}{{·}}{{legend2|Magenta| SOHO}}

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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 halo 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 in a good position for communication with Earth at all times.

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.{{cite web|publisher=ESA|title=Antenna anomaly may affect SOHO scientific data transmission |url=https://www.esa.int/Newsroom/Press_Releases/Antenna_anomaly_may_affect_SOHO_scientific_data_transmission|date=June 24, 2003|access-date=March 14, 2005}} However, ESA and NASA engineers managed to use SOHO's low-gain antennas together with the larger {{cvt|34|m}} and {{cvt|70|m}} NASA Deep Space Network 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.{{cite web|url=http://www.esa.int/export/esaCP/SEM0VZWO4HD_index_0.html|title=SOHO{{'}}s antenna anomaly: things are much better than expected|publisher=ESA|date=July 2, 2003|access-date=March 14, 2005}}

The SOHO Mission Interruption sequence of events began on 24 June 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 25 June 1998, at 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 04:43 UTC, and the mission interruption had begun. SOHO was spinning, losing electrical power, and no longer pointing at the Sun.{{cite web |url=https://umbra.nascom.nasa.gov/soho/SOHO_final_report.html|title=SOHO "Mission Interruption Joint NASA/ESA Investigation Board Final Report"|publisher=NASA|date=|access-date=12 March 2018}} {{PD-notice}}

Expert European Space Agency (ESA) personnel were immediately dispatched from Europe to the United States to direct operations.{{cite web|title=SOHO's Recovery: An Unprecedented Success Story|url=https://www.esa.int/esapub/bulletin/bullet97/vandenbu.pdf|publisher=European Space Agency |access-date=March 12, 2018}} Days passed without contact from SOHO. On 23 July 1998, 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 revolution every 53 seconds. Once SOHO was located, plans for contacting SOHO were formed. On 3 August, a carrier was detected from SOHO, the first signal since 25 June 1998. After days of charging the battery, a successful attempt was made to modulate the carrier and downlink telemetry on 8 August. After instrument temperatures were downlinked on 9 August 1998, data analysis was performed, and planning for the SOHO recovery began in earnest.{{Cite magazine |last=David |first=Leonard |date=May 1999 |title=Saving SOHO |url=http://soho.nascom.nasa.gov/operations/Recovery/aerospace.pdf |magazine=Aerospace America |pages=60–67}} {{PD-notice}}

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 12 August 1998. Thawing pipes and the thrusters was next, and SOHO was re-oriented towards the Sun on 16 September 1998. After nearly a week of spacecraft bus recovery activities and an orbital correction maneuver, the SOHO spacecraft bus returned to normal mode on 25 September 1998 at 19:52 UTC. Recovery of the instruments began on 5 October 1998 with SUMER, and ended on 24 October 1998, with CELIAS.

Only one gyroscope remained operational after this recovery, and on 21 December 1998, that gyroscope failed. Attitude control was accomplished with manual thruster firings that consumed {{cvt|7|kg}} of fuel weekly, while the ESA developed a new gyroless operations mode that was successfully implemented on 1 February 1999.

File:Solar_and_Heliospheric_Observatory.jpg 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:Domingo, V.; Fleck, B.; Poland, A. I.; Solar Physics 162, 1--37 (1995){{cite journal|author=Fleck B.|title=First Results from SOHO|journal=Rev Modern Astron|year=1997|volume=10 |pages=273–296|bibcode=1997RvMA...10..273F}}

• Coronal Diagnostic Spectrometer ([http://solar.bnsc.rl.ac.uk/ CDS] {{Webarchive|url=https://web.archive.org/web/20201016234744/http://solar.bnsc.rl.ac.uk/ |date=16 October 2020 }}), which measures density, temperature and flows in the corona.
• Charge Element and Isotope Analysis System ([http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=30956 CELIAS]), which studies the ion composition of the solar wind.
• Comprehensive SupraThermal and Energetic Particle analyser collaboration ([http://www.ieap.uni-kiel.de/et/ag-heber/costep/ COSTEP] {{Webarchive|url=https://web.archive.org/web/20201018231156/http://www.ieap.uni-kiel.de/et/ag-heber/costep/ |date=18 October 2020 }}), 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 ([http://www.srl.utu.fi/projects/cepac/main_english.html CEPAC] {{Webarchive|url=https://web.archive.org/web/20060907093531/http://www.srl.utu.fi/projects/cepac/main_english.html |date=7 September 2006 }}).
• Extreme ultraviolet Imaging Telescope (EIT), which studies the low coronal structure and activity.
• Energetic and Relativistic Nuclei and Electron experiment ([https://srl.utu.fi/projects/erne/ ERNE] {{Webarchive|url=https://web.archive.org/web/20200807055456/https://srl.utu.fi/projects/erne/ |date=7 August 2020 }}), which studies the ion and electron composition of the solar wind. (See note above in COSTEP entry.)
• Global Oscillations at Low Frequencies ([https://web.archive.org/web/19991009161459/http://golfwww.medoc-ias.u-psud.fr/ 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 ([http://soi.stanford.edu/science/obs_prog.html 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.{{Cite web|url=http://soi.stanford.edu/|title=MDI Web Page|website=soi.stanford.edu|access-date=January 16, 2019}}
• Solar Ultraviolet Measurement of Emitted Radiation ([https://web.archive.org/web/20060615104607/http://www.mps.mpg.de/en/projekte/soho/sumer/ SUMER]), which measures plasma flows, temperature, and density in the corona.
• Solar Wind Anisotropies ([http://sohowww.nascom.nasa.gov/data/summary/swan/ 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 ([http://cfa-www.harvard.edu/uvcs/ UVCS]), which measures density and temperature in the corona.
• Variability of solar IRradiance and Gravity Oscillations ([http://www.ias.u-psud.fr/virgo/ VIRGO]), which measures oscillations and solar constant both of the whole solar disk and at low resolution, again exploring the core of the Sun.

Observations from some of the instruments can be formatted as images, most of which are readily available on the internet for either public or research use (see [http://sohowww.nascom.nasa.gov/data/realtime-images.html 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 (EUV). 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 via e-mail and the SOHO website 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 were presented in Solar Physics, volumes 170 and 175 (1997), edited by B. Fleck and Z. Švestka.

{{See also|List of Kreutz sungrazers}}

File:Sixteen Comets Touring the Inner Solar System.webm

As a consequence of its observing the Sun, SOHO (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/LASCO images online. SOHO had discovered over 2,700 comets by April 2014,{{cite web |date=15 September 2015 |title=3000th Comet Spotted by Solar and Heliospheric Observatory (SOHO) |url=http://www.nasa.gov/image-feature/3000th-comet-spotted-by-solar-and-heliospheric-observatory-soho |access-date=September 15, 2015 |publisher=NASA}} [https://web.archive.org/web/20140913102152/https://twitter.com/SungrazerComets/status/458317466998145027 (2,703 discoveries as of 21 April 2014)] {{PD-notice}}{{cite tweet|user=SungrazerComets|author=Karl Battams|number=458317466998145027|date=April 21, 2014 |title=As of April 21, 2014, the @ESA/@NASA SOHO satellite comet discovery count stands at 2,703! #Sungrazers}} with an average discovery rate of one every 2.59 days.{{cite tweet|user=SungrazerComets|author=Karl Battams|number=259370186770112512|date=October 19, 2012|title=Since the @ESA/@NASA SOHO mission launched in 1995, it has discovered a new comet every 2.59-days on average!}}

• SOHO-1000 (C/2005 P2)– 5 August 2005, Toni Scarmanto
• SOHO-2000 (C/2010 Y20) – 26 December 2010, Michał Kusiak
• SOHO-3000 (C/2015 ??) – 13 September 2015, Worachate Boonplod
• SOHO-4000 (C/2020 ??) – 15 September 2020, Trygve Prestgard
• SOHO-5000 (C/2024 ??) – 25 March 2024, Hanjie Tan

As of 13th December 2024, SOHO has found 5,124 comets.{{cite web | url=https://sungrazer.nrl.navy.mil/confs-Oct2024 | title=October 2024 Confirmations }}

The Max Planck Institute for Solar System Research contributed to SUMER, Large Angle and Spectrometric Coronagraph (LASCO), and CELIAS instruments. The Smithsonian Astrophysical Observatory (SAO) 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 Extreme ultraviolet Imaging Telescope (EIT), with a strong contribution to SUMER. A complete list of all the instruments, with links to their home institutions, is available at the [https://soho.nascom.nasa.gov/about/instruments.html SOHO Website].{{cite web|date=September 16, 2015|url=http://www.space.com/30562-soho-spacecraft-discovers-3000-comets.html|title= Whoa! Sun-Watching Spacecraft Finds 3,000th Comet|author=Mike Wall|website=Space.com |access-date=September 16, 2015}}

• Space Weather Follow On-Lagrange 1, scheduled for launch in December of 2025.
• Advanced Composition Explorer, launched 1997, still operational.
• Deep Space Climate Observatory (DSCOVR), launched 2015, orbiting in {{L1}}.
• Heliophysics
• High Resolution Coronal Imager (Hi-C), launched 2012, sub-orbital telescope.
• Parker Solar Probe, launched 2018, still operational.
• Phoebus group, international scientists aiming at detecting solar g modes
• SOHO 2333
• Solar Dynamics Observatory (SDO), launched 2010, still operational.
• Solar Orbiter, launched 2020, still operational.
• STEREO (Solar TErrestrial RElations Observatory), launched 2006, still operational.
• Transition Region and Coronal Explorer (TRACE), launched 1998, decommissioned 2010.
• Ulysses (spacecraft), launched 1990, decommissioned 2009.
• Wind (spacecraft), launched 1994, still operational.

{{Reflist|30em}}

• {{cite web | title=SOHO's Recovery – An Unprecedented Success Story|url=http://www.esa.int/esapub/bulletin/bullet97/vandenbu.pdf|access-date=March 11, 2006}}
• {{cite web | title=SOHO Mission Interruption Preliminary Status and Background Report – July 15, 1998|url=http://umbra.nascom.nasa.gov/soho/prelim_and_background_rept.html|access-date=March 11, 2006}}
• {{cite web | title=SOHO Mission Interruption Joint NASA/ESA Investigation Board Final Report – August 31, 1998|url=http://umbra.nascom.nasa.gov/soho/SOHO_final_report.html|access-date=March 11, 2006}}
• {{cite web | title=SOHO Recovery Team|url=http://sohowww.nascom.nasa.gov/operations/Recovery/recovery_team.html|access-date=March 11, 2006}} Image
• {{cite web|title=The SOHO Mission {{L1|nolink=yes}} Halo Orbit Recovery From the Attitude Control Anomalies of 1998|url=http://www.ieec.cat/hosted/web-libpoint/papers/roberts.pdf|access-date=July 25, 2007|archive-date=19 August 2014|archive-url=https://web.archive.org/web/20140819082614/http://www.ieec.cat/hosted/web-libpoint/papers/roberts.pdf|url-status=dead}}
• {{cite book|last1=Weiss|first1=K. A.|last2=Leveson|first2=N.|last3=Lundqvist|first3=K.|last4=Farid|first4=N.|last5=Stringfellow|first5=M.|title=20th DASC. 20th Digital Avionics Systems Conference (Cat. No.01CH37219) |chapter=An analysis of causation in aerospace accidents |volume=1|pages=4A3/1–4A3/12|date=2001|doi=10.1109/DASC.2001.963364|isbn=978-0-7803-7034-0|s2cid=111244003 }}
• {{cite journal|last=Leveson|first=N. G.|journal=Journal of Spacecraft and Rockets|title=The Role of Software in Spacecraft Accidents|volume=41|issue=4|pages=564–575|date=July 2004|bibcode=2004JSpRo..41..564L

|doi=10.2514/1.11950|citeseerx=10.1.1.202.8334}}

• {{cite journal|title=Risks to the Public in Computers and Related Systems|date=January 1999|pages=31–35|volume=24|issue=1|journal=Software Engineering Notes|first=Peter G.|last=Neumann |doi=10.1145/308769.308773|s2cid=36328273 }}

{{Commons category|SOHO (spacecraft)|Solar and Heliospheric Observatory}}

• [https://www.esa.int/Science_Exploration/Space_Science/SOHO ESA SOHO webpage: Stories, images, videos]
• [https://soho.nascom.nasa.gov/home.html NASA SOHO webpage: about, gallery, data, operations, etc.]
• {{cite web|title=A Description of the SOHO Mission|publisher=NASA's SOHO website|url=http://sohowww.nascom.nasa.gov/mission|access-date=October 24, 2005}}
• {{cite web|title=Latest SOHO Images|publisher=NASA's SOHO website|url=http://sohowww.nascom.nasa.gov/data/latestimages.html|access-date=October 24, 2005}}, free to use for educational and non-commercial purposes.
• [https://web.archive.org/web/20070802045133/http://solarsystem.nasa.gov/missions/profile.cfm?MCode=SOHO SOHO Mission Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
• {{cite web|title=Space Weather Now|publisher=National Weather Service – Space Environment Center|url=http://www.sec.noaa.gov/SWN/|access-date=October 24, 2005|archive-url=https://web.archive.org/web/20051023234951/http://www.sec.noaa.gov/SWN/|archive-date=October 23, 2005|url-status=dead}}
• {{cite web|title=The SOHO Mission L1 Halo Orbit Recovery From the Attitude Control Anomalies of 1998|url=http://europa.ieec.fcr.es/libpoint/papers/roberts.pdf|access-date=October 24, 2005|url-status=dead |archive-url=https://web.archive.org/web/20051021222238/http://europa.ieec.fcr.es/libpoint/papers/roberts.pdf|archive-date=October 21, 2005}}
• [http://www.suntrek.org Sun trek website] A useful resource about the Sun and its effect on the Earth
• {{cite journal|doi=10.1016/j.asr.2004.12.021|title=Coordinating with SOHO|journal=Advances in Space Research|volume=36|issue=8|pages=1557–1560|year=2005|last1=Haugan|first1=Stein V.H. |bibcode=2005AdSpR..36.1557H}}
• [http://www.nasa.gov/mission_pages/soho/comet-2000.html SOHO Spots 2000th Comet]
• [https://sungrazer.nrl.navy.mil/index.php?p=transits/transits_2013 Transits of Objects through the LASCO/C3 field of view (FOV) in 2013] (Giuseppe Pappa)
• [https://sungrazer.nrl.navy.mil/index.php?p=transits/noteables Notable objects in LASCO C3] and [https://lasco-www.nrl.navy.mil/index.php?p=sky/starmap LASCO Star Maps] (identify objects in the field of view for any day of the year)
• {{usurped|1=[https://web.archive.org/web/20120425080414/http://scienceforcitizens.net/blog/2011/10/you-can-discover-the-next-comet-from-your-couch/ You can discover the next comet... from your couch!]}} (science for citizens October 18, 2011)
• [https://twitter.com/Astroguyz/status/368722725310705665/photo/1 Ceres in LASCO C2] (17 August 2013)
• [http://fenyi.solarobs.unideb.hu/SDD/SDD.html Sunspot Database based on SOHO satellite observations from 1996 to 2011] (File:Flag of Hungary.svg)

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