 Artist's impression of SOHO spacecraft | |||||||||||||||||||||||||||
| Names | SOHO | ||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Mission type | Solar observation | ||||||||||||||||||||||||||
| Operator | ESA / NASA | ||||||||||||||||||||||||||
| COSPAR ID | 1995-065A | ||||||||||||||||||||||||||
| SATCATno. | 23726 | ||||||||||||||||||||||||||
| Website | sohowww.nascom.nasa.gov | ||||||||||||||||||||||||||
| Mission duration | 2 years (planned) 29 years, 11 months and 21 days(in progress) | ||||||||||||||||||||||||||
| Spacecraft properties | |||||||||||||||||||||||||||
| Bus | SOHO | ||||||||||||||||||||||||||
| Manufacturer | Matra Marconi Space | ||||||||||||||||||||||||||
| Launch mass | 1,850 kg (4,080 lb)[1] | ||||||||||||||||||||||||||
| Payload mass | 610 kg (1,340 lb) | ||||||||||||||||||||||||||
| Dimensions | 4.3 × 2.7 × 3.7 m (14.1 × 8.9 × 12.1 ft) 9.5 m (31 ft) with solar arrays deployed | ||||||||||||||||||||||||||
| Power | 1500watts | ||||||||||||||||||||||||||
| Start of mission | |||||||||||||||||||||||||||
| Launch date | 2 December 1995, 08:08:01UTC | ||||||||||||||||||||||||||
| Rocket | Atlas IIAS (AC-121) | ||||||||||||||||||||||||||
| Launch site | Cape Canaveral,LC-36B | ||||||||||||||||||||||||||
| Contractor | Lockheed Martin | ||||||||||||||||||||||||||
| Entered service | May 1996 | ||||||||||||||||||||||||||
| Orbital parameters | |||||||||||||||||||||||||||
| Reference system | Sun–Earth L1 orbit | ||||||||||||||||||||||||||
| Regime | Halo orbit | ||||||||||||||||||||||||||
| Perigee altitude | 206,448 km (128,281 mi) | ||||||||||||||||||||||||||
| Apogee altitude | 668,672 km (415,494 mi) | ||||||||||||||||||||||||||
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 SOHO mission patch Huygens → | |||||||||||||||||||||||||||
TheSolar and Heliospheric Observatory (SOHO) is aEuropean Space Agency (ESA) spacecraft built by a European industrial consortium led byMatra Marconi Space (nowAirbus Defence and Space) that was launched on aLockheed MartinAtlas IIASlaunch vehicle on 2 December 1995, to study theSun. It has also discovered more than 5,000comets.[2] It began normal operations in May 1996. It is a joint project between theEuropean Space Agency (ESA) andNASA. SOHO was part of the International Solar Terrestrial Physics Program (ISTP). Originally planned as a two-year mission, SOHO continues to operate after 29 years inspace; the mission has been extended until the end of 2025, subject to review and confirmation by ESA's Science Programme Committee.[3]
In addition to its scientific mission, it is a main source of near-real-time solar data forspace weather prediction. Along withAditya-L1,Wind,Advanced Composition Explorer (ACE),Deep Space Climate Observatory (DSCOVR) and other satellites, SOHO is one of five spacecraft in the vicinity of theEarth–SunL1 point, a point of gravitational balance located approximately 0.99astronomical 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 itsreaction wheels as a kind of virtualgyroscope; the technique was adopted after an on-board emergency in 1998 that nearly resulted in the loss of the spacecraft.
The three main scientific objectives of SOHO are:[citation needed]
The SOHO spacecraft is in ahalo orbit around theSun–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 thecentripetal force needed for an object to have the sameorbital period in its orbit around the Sun as the Earth, with the result that the object will stay in that relative position.[citation needed]
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 astable 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 ellipticalhalo 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.[citation needed]
In normal operation, the spacecraft transmits a continuous 200kbit/s data stream of photographs and other measurements via the NASADeep Space Network of ground stations. SOHO's data about solar activity are used to predictcoronal mass ejection (CME) arrival times at Earth, soelectrical grids and satellites can be protected from their damaging effects. CMEs directed toward the earth may producegeomagnetic storms, which in turn producegeomagnetically induced currents, in the most extreme cases creating black-outs, etc.
In 2003, ESA reported the failure of the antennaY-axisstepper motor, necessary for pointing thehigh-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'slow-gain antennas together with the larger 34 m (112 ft) and 70 m (230 ft)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.[5]
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:[6][7]
TheMax 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. TheLockheed Martin Solar and Astrophysics Laboratory (LMSAL) built the MDI instrument in collaboration with the solar group atStanford University. TheInstitut d'astrophysique spatiale is theprincipal investigator of GOLF andExtreme 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 theSOHO Website.[9][2]
Observations from some of the instruments can be formatted as images, most of which are readily available on theinternet for either public or research use (seethe official website). Others, such asspectra and measurements of particles in thesolar wind, do not lend themselves so readily to this. These images range inwavelength orfrequency fromoptical (Hα) toExtreme ultraviolet (EUV). Images taken partly or exclusively with non-visible wavelengths are shown on the SOHO page and elsewhere infalse 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 inSolar Physics, volumes 170 and 175 (1997), edited by B. Fleck and Z. Švestka.[citation needed]
| Comet discoveries[10][11] | |||||||
| Year | # | ||||||
|---|---|---|---|---|---|---|---|
| 2013 | 213 | ||||||
| 2012 | 222 | ||||||
| 2011 | 216 | ||||||
| 2010 | 209 | ||||||
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,[12][13] with an average discovery rate of one every 2.59 days.[14]
Source:[15]
As of 1 April 2025, SOHO has found 5,189 comets.[16]
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 lostlock on the Sun and entered an emergencyattitude control mode called Emergency Sun Reacquisition (ESR). The SOHO Team attempted to recover the observatory, but SOHO entered theemergency 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.[17]
ExpertESA personnel were immediately dispatched fromEurope to theUnited States to direct operations.[18] Days passed without contact from SOHO. On 23 July 1998, theArecibo Observatory andGoldstone Solar System Radar combined to locate SOHO withradar and to determine its location andattitude. SOHO was close to its predicted position, oriented with its side versus the usual frontOptical Surface Reflector panel pointing toward the Sun, and was rotating at onerevolution every 53 seconds. Once SOHO was located, plans for contacting SOHO were formed. On 3 August, acarrier was detected from SOHO, the first signal since 25 June 1998. After days of charging thebattery, a successful attempt was made tomodulate the carrier and downlinktelemetry 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.[19]
The Recovery Team began by allocating the limited electrical power. After this, SOHO's anomalous orientation in space was determined. Thawing the frozenhydrazine fuel tank using SOHO's thermal control heaters began on 12 August 1998. Thawing pipes and thethrusters 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.[18]
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 7 kg (15 lb) of fuel weekly, while the ESA developed a new gyroless operations mode that was successfully implemented on 1 February 1999.[18]
This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain.
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