Solar and Heliospheric Observatory

Solar and Heliospheric Observatory (SOHO)

COSPAR ID	1995-065A
SATCAT no.	23726
Website	sohowww.nascom.nasa.gov/
Start of mission
Launch date	December 2, 1995

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. 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 time solar data for space weather prediction. 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 0.99 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.

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. 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. 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. 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.).

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. 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.

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, 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. 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. 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 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.

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. 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.

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 7kg of fuel weekly, while ESA developed a new gyroless operations mode that was successfully implemented on February 1, 1999.

Additional References

• "SOHO's Recovery - An Unprecedented Success Story" (PDF). Retrieved 2006-03-11. -PDF
• "SOHO Mission Interruption Preliminary Status and Background Report - July 15, 1998". Retrieved 2006-03-11.
• "SOHO Mission Interruption Joint NASA/ESA Investigation Board Final Report - August 31, 1998". Retrieved 2006-03-11.
• "SOHO Recovery Team". Retrieved 2006-03-11. Image
• "The SOHO Mission L1 Halo Orbit Recovery From the Attitude Control Anomalies of 1998" (PDF). Retrieved 2007-07-25.

• Weiss, K. A. (2006-01-09). "An analysis of causation in aerospace accidents". Digital Avionics Systems, 2001. DASC. The 20th Conference Vol. 1. {{cite conference}}: Unknown parameter |booktitle= ignored (|book-title= suggested) (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)CS1 maint: date and year (link)
• Leveson, N. G. (2004). "The Role of Software in Spacecraft Accidents". AIAA Journal of Spacecraft and Rockets. 41 (4). {{cite journal}}: Unknown parameter |month= ignored (help)
• Neumann, Peter G. (1999). "Risks to the Public in Computers and Related Systems". Software Engineering Notes. 24 (1): 31–35. doi:10.1145/308769.308773. {{cite journal}}: Unknown parameter |month= ignored (help)

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 CEPAC are used for "in situ" solar wind observations.
• Probing the interior structure of the Sun. GOLF, MDI, and VIRGO are used for helioseismology.

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 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.
• 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.

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.

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 comet.

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.

See also

• Solar Maximum Mission

External links

• ESA SOHO webpage
• "A Description of the SOHO Mission". NASA's SOHO website. {{cite web}}: Italic or bold markup not allowed in: |work= (help); Unknown parameter |accessdaymonth= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
• "Latest SOHO Images". NASA's SOHO website. {{cite web}}: Unknown parameter |accessdaymonth= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help), 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. {{cite web}}: Unknown parameter |accessdaymonth= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
• "The SOHO Mission L1 Halo Orbit Recovery From the Attitude Control Anomalies of 1998" (PDF). {{cite web}}: Unknown parameter |accessdaymonth= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help) - PDF
• Sun trek website A useful resource about the Sun and its effect on the Earth