The Solar and Heliospheric Observatory (SOHO) is a spacecraft that was launched on a Lockheed Martin Atlas IIAS launch vehicle on December 2, 1995 to study the Sun, and began normal operations in May 1996. A heliocentric orbit is an Orbit around the Sun. In our Solar System, all Planets Comets and Asteroids are in such orbits In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays In Physics, magnetism is one of the Phenomena by which Materials exert attractive or repulsive Forces on other Materials. The core of the Sun is considered to extend from the center to about 0 Oscillation is the repetitive variation typically in Time, of some measure about a central value (often a point of Equilibrium) or between two or more different states A corona is a type of plasma " atmosphere " of the Sun or other celestial body extending millions of Kilometres into space most easily A spectrometer is an Optical instrument used to measure properties of Light over a specific portion of the Electromagnetic spectrum, typically used The chromosphere (literally "colour sphere" is a thin layer of the Sun 's atmosphere just above the Photosphere, roughly 10000 kilometres deep The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge A spacecraft is a Vehicle or machine designed for Spaceflight. Lockheed Martin ( is a large multinational Aerospace manufacturer and advanced technology Company formed in 1995 by the merger of Atlas II was a member of the Atlas family of Launch vehicles which evolved from the successful Atlas missile program of the 1950s Events 1409 - The University of Leipzig opens 1755 - The second Eddystone Lighthouse is destroyed by fire Year 1995 ( MCMXCV) was a Common year starting on Sunday. Events of 1995 The Sun (Sol is the Star at the center of the Solar System. Year 1996 ( MCMXCVI) was a Leap year starting on Monday (link will display full 1996 Gregorian calendar) It is a joint project of international cooperation between the European Space Agency (ESA) and NASA. The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program Originally planned as a two-year mission, SOHO currently continues to operate after over ten years in space. Space is the extent within which Matter is physically extended and objects and Events have positions relative to one another In addition to its scientific mission, it is currently the main source of near-real time solar data for space weather prediction. Space weather is the concept of changing environmental conditions in Outer space. 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. Wind is the flow of Air or other Gases that compose an Atmosphere (including but not limited to the Earth's) Advanced Composition Explorer (ACE is a Space exploration mission being conducted as part of the Explorer program to study Matter In situ EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 The Sun (Sol is the Star at the center of the Solar System. 99 astronomical unit (AU)s from the Sun and 0. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the 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. A reaction wheel is a type of Flywheel used primarily by Spacecraft to change their Angular momentum without using fuel for Rockets or other reaction A gyroscope is a device for measuring or maintaining orientation, based on the principles of Angular momentum. Year 1998 ( MCMXCVIII) was a Common year starting on Thursday (link will display full 1998 Gregorian calendar)

Orbit

The 610 kg 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. A halo orbit is a periodic three-dimensional Orbit near the L1 L2 or L3 Lagrange points in the three-body problem of The Sun (Sol is the Star at the center of the Solar System. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 The Sun (Sol is the Star at the center of the Solar System. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 The centripetal force is the external force required to make a body follow a curved path The orbital period is the time taken for a given object to make one complete Orbit about another object 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 satellite 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. This article is about artificial satellites For natural satellites also known as moons see Natural satellite. 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 Network of ground stations. The Deep Space Network, or DSN, is an international network of communication facilities that supports Interplanetary Spacecraft missions and 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. A solar flare is a violent explosion in a star's (like the Sun 's atmosphere releasing as much Energy as 6 × 1025 Joules Solar flares Electric power transmission, a process in the delivery of Electricity to consumers is the bulk transfer of electrical power This article is about artificial satellites For natural satellites also known as moons see Natural satellite. A geomagnetic storm or solar storm is a temporary disturbance of the Earth 's Magnetosphere caused by a disturbance in Space weather. Geomagnetically induced currents (GIC affecting the normal operation of long technological conductor systems are a manifestation at ground level of Space weather ).

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. In Mathematics, the Cartesian coordinate system (also called rectangular coordinate system) is used to determine each point uniquely in a plane A stepper motor (or step motor) is a brushless, synchronous Electric The high-gain antenna (HGA is an antenna with a focused narrow radiowave beam width At the time, it was thought that the antenna anomaly might cause two to three week data-blackouts every three months. ^[1] 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. The low-gain antenna (LGA is an antenna with a broad radiowave beam width ^[2]

Near Loss of SOHO

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. Events 972 - Battle of Cedynia, the first documented victory of Polish forces takes place Year 1998 ( MCMXCVIII) was a Common year starting on Thursday (link will display full 1998 Gregorian calendar) A gyroscope is a device for measuring or maintaining orientation, based on the principles of Angular momentum. 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 attitude of a body is its orientation as perceived in a certain Frame of reference; providing a vector along which a spacecraft is pointing is a description of its attitude The SOHO Team attempted to recover the observatory, but SOHO entered the emergency mode again on June 25 02:35 UTC. Events 524 - Battle of Vézeronce, the Franks defeat the Burgundians 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. The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member The United States of America —commonly referred to as the 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. Events 1632 - Three hundred colonists bound for New France depart from Dieppe France. The Arecibo Observatory is a very sensitive Radio telescope located approximately south-southwest from the town of Arecibo in Puerto Rico. The Deep Space Network, or DSN, is an international network of communication facilities that supports Interplanetary Spacecraft missions and Radar is a system that uses electromagnetic waves to identify the range altitude direction or speed of both moving and fixed objects such as Aircraft, ships 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 3 August a carrier was detected from SOHO, the first signal since June 25. Events 8 - Roman Empire General Tiberius defeats Dalmatians on the river Bathinus. In Telecommunications, a carrier wave, or carrier is a Waveform (usually Sinusoidal) that is modulated (modified with an input signal Events 524 - Battle of Vézeronce, the Franks defeat the Burgundians After days of charging the battery, a successful attempt was made to modulate the carrier and downlink telemetry on August 8. In electronics a battery is a combination of two or more Electrochemical cells which store chemical Energy which can be converted into electrical energy In Telecommunications, modulation is the process of varying a periodic Waveform, i Telemetry (synonymous with Telematics) is a Technology that allows the remote measurement and reporting of Information of interest to the system designer Events 1220 - Sweden is defeated by Estonian tribes in the Battle of Lihula. After instrument temperatures were downlinked on August 9, data analysis was performed, and planning for the SOHO recovery began in earnest. Events 48 BC - Caesar's civil war: Battle of Pharsalus - Julius Caesar decisively defeats Pompey at Pharsalus Data analysis is the process of looking at and summarizing Data with the intent to extract useful Information and develop conclusions

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. Hydrazine is a Chemical compound with the formula N2H4 It has an Ammonia -like odor and is derived from the same industrial chemistry Events 1099 - First Crusade: Battle of Ascalon - Crusaders under the command of Godfrey of Bouillon defeat Fatimid Thawing pipes and the thrusters was next, and SOHO was re-oriented towards the Sun on September 16. Events 1400 - Owain Glyndŵr is declared Prince of Wales by his followers 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. Events 303 - On a voyage preaching the Gospel, Saint Fermin of Pamplona is beheaded in Amiens, France Recovery of the instruments began on October 5 with SUMER, and ended on October 24, 1998 with CELIAS. Events 869 - The Fourth Council of Constantinople is convened to decide about what to do about Patriarch Photius of Constantinople Events 69 - Second Battle of Bedriacum, forces under Antonius Primus the commander of the Danube armies loyal to Vespasian, defeat Year 1998 ( MCMXCVIII) was a Common year starting on Thursday (link will display full 1998 Gregorian calendar)

Only one gyro remained operational after this recovery, and on December 21 that gyro failed. A gyroscope is a device for measuring or maintaining orientation, based on the principles of Angular momentum. Events 69 - The end of the Year of the four emperors: Following Galba, Otho and Vitellius, Vespasian Attitude control was accomplished with manual thruster firings that consumed 7kg of fuel weekly, while ESA developed a new gyroless operations mode that was successfully implemented on February 1, 1999. The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member Events 1327 - Teenaged Edward III is crowned King of England, but the country is ruled by his mother Queen Year 1999 ( MCMXCIX) was a Common year starting on Friday (link will display full 1999 Gregorian calendar)

Additional References

• SOHO's Recovery - An Unprecedented Success Story. Retrieved on 2006-03-11. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1425 BC - Thutmose III, Pharaoh of Egypt, dies (according to the Low Chronology of the 18th Dynasty -PDF
• SOHO Mission Interruption Preliminary Status and Background Report - July 15, 1998. Events 1099 - First Crusade: Christian soldiers take the Church of the Holy Sepulchre in Jerusalem after the final Year 1998 ( MCMXCVIII) was a Common year starting on Thursday (link will display full 1998 Gregorian calendar) Retrieved on 2006-03-11. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1425 BC - Thutmose III, Pharaoh of Egypt, dies (according to the Low Chronology of the 18th Dynasty
• SOHO Mission Interruption Joint NASA/ESA Investigation Board Final Report - August 31, 1998. Events 1056 - Byzantine Empress Theodora becomes ill dying suddenly a few days later without children to succeed the Throne Year 1998 ( MCMXCVIII) was a Common year starting on Thursday (link will display full 1998 Gregorian calendar) Retrieved on 2006-03-11. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1425 BC - Thutmose III, Pharaoh of Egypt, dies (according to the Low Chronology of the 18th Dynasty
• SOHO Recovery Team. Retrieved on 2006-03-11. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1425 BC - Thutmose III, Pharaoh of Egypt, dies (according to the Low Chronology of the 18th Dynasty Image
• The SOHO Mission L1 Halo Orbit Recovery From the Attitude Control Anomalies of 1998. Retrieved on 2007-07-25. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 285 - Diocletian appoints Maximian as Caesar, co-ruler

• Weiss, K. A. ; Leveson, N. ; Lundqvist, K. ; Farid, N. ; Stringfellow, M. (2006-01-09). Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 475 - Byzantine Emperor Zeno is forced to flee his capital at Constantinople. "An analysis of causation in aerospace accidents". Digital Avionics Systems, 2001. DASC. The 20th Conference Vol. 1.  
• Leveson, N. G. (July 2004). "The Role of Software in Spacecraft Accidents". AIAA Journal of Spacecraft and Rockets 41 (4).  
• Neumann, Peter G. (January 1999). "Risks to the Public in Computers and Related Systems". Software Engineering Notes 24 (1): 31-35.  

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. The chromosphere (literally "colour sphere" is a thin layer of the Sun 's atmosphere just above the Photosphere, roughly 10000 kilometres deep The solar transition region is a region of the Sun 's atmosphere between the Chromosphere and Corona. A corona is a type of plasma " atmosphere " of the Sun or other celestial body extending millions of Kilometres into space most easily CDS, EIT, LASCO, SUMER, SWAN, and UVCS are used for this solar atmosphere remote sensing. The Sun (Sol is the Star at the center of the Solar System.
• Making observations of solar wind and associated phenomena in the vicinity of L1. The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun. CELIAS and CEPAC are used for "in situ" solar wind observations. In situ (ɪn siːˈtuː is a Latin phrase meaning in the place.
• Probing the interior structure of the Sun. GOLF, MDI, and VIRGO are used for helioseismology. Helioseismology is the study of the propagation of Pressure waves in the Sun.

Instruments

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 (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 ENRE are sometimes referred to together as the COSTEP-ERNE Particle Analyzer Collaboration (CEPAC).
• Extreme ultraviolet Imaging Telescope (EIT) which studies the low coronial 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 experiment (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 convection zone of a Star is the range of radii in which energy is transported primarily by Convection. The Sun (Sol is the Star at the center of the Solar System. A corona is a type of plasma " atmosphere " of the Sun or other celestial body extending millions of Kilometres into space most easily 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 magnetogram data, and VC3 (MDI-H) carries MDI Helioseismology data. In Telecommunications, a logical channel number (LCN also known as virtual channel in North America, is a channel designation which differs A magnetogram is a pictorial representation of the variations in strength of a magnetic field Helioseismology is the study of the propagation of Pressure waves in the Sun.
• 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.

The SOHO instruments are seen at the top of this illustration.

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). The Internet is a global system of interconnected Computer networks Others such as spectra and measurements of particles in the solar wind do not lend themselves so readily to this. An energy spectrum is a distribution of Energy among a large assemblage of particles The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun. These images range in wavelength or frequency from optical (Hα) to extreme ultraviolet (UV). In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. Frequency is a measure of the number of occurrences of a repeating event per unit Time. In Physics and Astronomy, H-alpha, also written Hα, is a specific Emission line created by Hydrogen at 6562 Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays Images taken partly or exclusively with non-visible wavelengths are shown on the SOHO page and elsewhere in false color. A false-color image is an image that depicts a subject in Colors that differ from those a faithful full-color photograph would show 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 1000th comet. A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — ^[3]

Instrument contributors

The Max Planck Institute for Solar System Research contributed to SUMER, LASCO and CELIAS instruments. The Max Planck Institute for Solar System Research (abbreviation MPS; German: Max-Planck-Institut für Sonnensystemforschung is a Research institute The Smithsonian Astrophysical Observatory built the UVCS instrument. The Smithsonian Astrophysical Observatory (SAO is a "research institute" of the Smithsonian Institution headquartered in Cambridge Massachusetts, The Lockheed Martin Solar and Astrophysics Laboratory (LMSAL) built the MDI instrument in collaboration with the solar group at Stanford University. The Lockheed Martin Solar and Astrophysics Laboratory ( LMSAL) is part of the Lockheed-Martin Advanced Technology Center (ATC that is known primarily for its scientific Leland Stanford Junior University, commonly known as Stanford University or simply Stanford, is a private Research university located in

See also

• Solar Maximum Mission

References

1. ^ Antenna anomaly may affect SOHO scientific data transmission. The Solar Maximum Mission Satellite (or SolarMax was designed to investigate solar phenomenon particularly Solar flares. ESA news. Retrieved on 14 March, 2005.
2. ^ SOHO's antenna anomaly: things are much better than expected. ESA news. Retrieved on 14 March, 2005.
3. ^ History's Greatest Comet Hunter Discovers 1,000th Comet. NASA news. Retrieved on 17 August, 2005.

External links

• ESA SOHO webpage
• A Description of the SOHO Mission. NASA's SOHO website. Retrieved on 24 October, 2005.
• Latest SOHO Images. NASA's SOHO website. Retrieved on 24 October, 2005. , free to use for educational and non-commercial purposes.
• SOHO Mission Profile by NASA's Solar System Exploration
• Space Weather Now. National Weather Service - Space Environment Center. Retrieved on 24 October, 2005.
• The SOHO Mission L1 Halo Orbit Recovery From the Attitude Control Anomalies of 1998. Retrieved on 24 October, 2005. - PDF
• Sun trek website A useful resource about the Sun and its effect on the Earth