- Solar and Heliospheric Observatory
Infobox Space telescope
name = Solar and Heliospheric Observatory (SOHO)
location = L1
height = 1.5×106 km (heliocentric)
period = 1 Earth year
2 December 1995
launch_vehicle = Atlas IIAS
opticalthrough UV, also magneticinformation
mass = 1,850 kg (610 kg payload)
instrument_1_name = GOLF
solar core oscillations nowrap|(Doppler-sensitive photometer)
instrument_2_name = VIRGO
instrument_2_characteristics = core oscillations (photometric imager)
instrument_3_name = MDI
instrument_3_characteristics = oscillations and magnetic fields (Doppler imager)
instrument_4_name = SUMER
coronal physics (UV spectrograph)
instrument_5_name = CDS
instrument_5_characteristics = nowrap|corona/
chromospherephysics (UV spectrograph)
instrument_6_name = EIT
instrument_6_characteristics = low corona and
instrument_7_name = UVCS
solar windacceleration (UV spectrograph)
instrument_8_name = LASCO
instrument_8_characteristics = low to outer corona (two
visible lightcameras, one imaging Fabry-Pérot interferometer)
instrument_9_name = SWAN
instrument_9_characteristics = nowrap|solar wind density (UV camera)
instrument_10_name = CELIAS COSTEP ERNE
instrument_10_characteristics = nowrap|solar wind
ions (material samplers)
website = [http://sohowww.nascom.nasa.gov/ sohowww.nascom.nasa.gov/] The Solar and Heliospheric Observatory (SOHO) is a
spacecraftthat was launched on a Lockheed MartinAtlas IIAS launch vehicle on December 2 1995to 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 timesolar data for space weatherprediction. Along with the GGS Wind and Advanced Composition Explorer("ACE"), "SOHO" is one of three spacecraft currently in the vicinity of the Earth- SunL1 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 1998that nearly resulted in the loss of the spacecraft.
The 610 kg "SOHO" spacecraft is in a
halo orbitaround the Sun- EarthL1 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 centripetalforce needed for an object to have the same orbital periodin 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/s²) more than at the Earth (5.9 mm/s²), while the reduction of required centripetal force is half of this (59 µm/s²). The sum of both effects is balanced by the gravity of the Earth, which is here also 177 µm/s².
Although sometimes described as being at L1, the "SOHO"
satelliteis 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 Networkof 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 currentcreating black-outs, etc.).
In 2003 ESA reported the failure of the antenna Y-axis
stepper motor, necessary for pointing the high gain antennaand 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 | work=ESA news | title=Antenna anomaly may affect SOHO scientific data transmission | url=http://spdext.estec.esa.nl/content/news/index.cfm?aid=1&cid=1&oid=32426 |accessmonthday=14 March | accessyear=2005] 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. [cite web | work=ESA news | title="SOHO" 's antenna anomaly: things are much better than expected | url=http://www.esa.int/export/esaCP/SEM0VZWO4HD_index_0.html | accessmonthday=14 March | accessyear=2005 ]
Near Loss of SOHO
The SOHO [http://umbra.nascom.nasa.gov/soho/SOHO_final_report.html Mission Interruption] sequence of [http://sohowww.nascom.nasa.gov/operations/Recovery/recovery_diary.html events] began on
24 June 1998, while the SOHO Team was conducting a series of spacecraft gyroscopecalibrations and maneuvers. Operations proceeded until 23:16 UTC when SOHO lost lock on the Sun, and entered an emergency attitude controlmode called Emergency Sun Reacquisition (ESR). The SOHO Team attempted to recover the observatory, but SOHO entered the emergency mode again on June 2502:35 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 [http://umbra.nascom.nasa.gov/soho/prelim_and_background_rept.html mission interruption] had begun. SOHO was spinning, losing electrical power, and no longer pointing at the Sun.
ESApersonnel were immediately dispatched from Europeto the United Statesto direct operations. Days passed without contact from SOHO. On July 23, the Arecibo Observatoryand 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 Reflectorpanel pointing toward the Sun, and was rotating at one RPM. Once SOHO was located, plans for contacting SOHO were formed. On 3 Augusta 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 telemetryon August 8. After instrument temperatures were downlinked on August 9, data analysiswas performed, and planning for the SOHO recovery began in earnest.
The [http://sohowww.nascom.nasa.gov/operations/Recovery/recovery_team.html SOHO Recovery Team] began by allocating the limited electrical power. After this, SOHO's anomalous orientation in space was determined. Thawing the frozen
hydrazinefuel tank using SOHO's thermal controlheaters 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 25at 19:52 UTC. Recovery of the instruments began on October 5with SUMER, and ended on October 24 1998with CELIAS.
Only one gyro remained operational after this recovery, and on
December 21that gyro failed. Attitude control was accomplished with manual thruster firings that consumed 7kg of fuel weekly, while ESAdeveloped a new gyroless operationsmode that was successfully implemented on February 1 1999.
*cite web | title=SOHO's Recovery - An Unprecedented Success Story | url=http://www.esa.int/esapub/bulletin/bullet97/vandenbu.pdf | accessdate = 2006-03-11 -PDF
*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 | accessdate = 2006-03-11
*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 | accessdate = 2006-03-11
*cite web | title=SOHO Recovery Team | url=http://sohowww.nascom.nasa.gov/operations/Recovery/recovery_team.html | accessdate = 2006-03-11 Image
*cite web | title=The SOHO Mission L1 Halo Orbit Recovery From the Attitude Control Anomalies of 1998 | url=http://www.ieec.fcr.es/libpoint/papers/roberts.pdf | accessdate = 2007-07-25
last = Weiss
first = K. A.
coauthors = Leveson, N.; Lundqvist, K.; Farid, N.; Stringfellow, M.
title = An analysis of causation in aerospace accidents
booktitle = Digital Avionics Systems, 2001. DASC. The 20th Conference Vol. 1
year = 2001
month = 10
*cite journal | last = Leveson | first = N. G. | journal = AIAA Journal of Spacecraft and Rockets | title = The Role of Software in Spacecraft Accidents | volume = 41 | issue = 4 | month = July | year = 2004
*cite journal | title = Risks to the Public in Computers and Related Systems | month = January | year = 1999 | pages = 31–35 | volume = 24 | issue = 1 | journal = Software Engineering Notes | first = Peter G. | last = Neumann | doi = 10.1145/308769.308773
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 windand associated phenomena in the vicinity of L1. CELIAS and CEPAC are used for " in situ" solar wind observations.
* Probing the interior structure of the Sun. GOLF, MDI, and VIRGO are used for
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:
*Coronal Diagnostic Spectrometer ( [http://solar.bnsc.rl.ac.uk/ CDS] ) 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] ) which studies the ion and electron composition of the solar wind. (COSTEP and ENRE are sometimes referred to together as the COSTEP-ERNE Particle Analyzer Collaboration ( [http://www.srl.utu.fi/projects/cepac/main_english.html CEPAC] ).
*Extreme ultraviolet Imaging Telescope ( [http://umbra.nascom.nasa.gov/eit/ EIT] ) which studies the low coronial structure and activity.
*Energetic and Relativistic Nuclei and Electron experiment ( [http://www.srl.utu.fi/projects/erne/index_english.html ERNE] ) which studies the ion and electron composition of the solar wind. (See note above in COSTEP entry.)
*Global Oscillations at Low Frequencies ( [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 experiment ( [http://lasco-www.nrl.navy.mil/ 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
photosphereto learn about the convection zonewhich forms the outer layer of the interior of the sun and about the magnetic fields which control the structure of the corona. The MDI is the biggest producer of data by far on "SOHO". In fact, two of SOHO's virtual channels are named after MDI, VC2 (MDI-M) carries MDI magnetogramdata, and VC3 (MDI-H) carries MDI Helioseismologydata.
*Solar Ultraviolet Measurement of Emitted Radiation ( [http://www.mps.mpg.de/en/projekte/soho/sumer/ SUMER] ) which measures plasma flows, temperature and density in the corona.
*Solar Wind ANisotropies ( [http://www.fmi.fi/research_space/space_7.html 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 also readily available on the
internetfor 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 winddo not lend themselves so readily to this. These images range in wavelengthor frequencyfrom 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.
As a consequence of its observing the Sun, "SOHO" (specifically the LASCO instrument) has inadvertently discovered comets by blocking out the Sun's glare. Approximately one-half of all known comets have been discovered by "SOHO". Recently, it discovered its 1500th
Max Planck Institute for Solar System Researchcontributed to SUMER, LASCO and CELIAS instruments. The Smithsonian Astrophysical Observatorybuilt the UVCS instrument. The Lockheed Martin Solar and Astrophysics Laboratory (LMSAL)built the MDI instrument in collaboration with the solar group at Stanford University.
Solar Maximum Mission
* [http://soho.esac.esa.int ESA "SOHO" webpage]
*, free to use for educational and non-commercial purposes.
* [http://solarsystem.nasa.gov/missions/profile.cfm?MCode=SOHO SOHO Mission Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* - PDF
* [http://www.suntrek.org Sun trek website] A useful resource about the Sun and its effect on the Earth
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