SOHO overview

The mission

The Solar and Heliospheric Observatory (SOHO) is stationed 1.5 million kilometres away from the Earth. There, it constantly watches the Sun returning spectacular pictures and data of the storms that rage across its surface. SOHO's science ranges from the Sun's hot interior, through its visible surface and stormy atmosphere, and out to distant regions where the wind from the Sun battles with a breeze of atoms coming from among the stars. The SOHO mission is a joint ESA-NASA project.

What's special

The Sun that looks tranquil to the naked eye is really savage. Storms raging in its atmosphere are far bigger than anything known on the Earth. SOHO is on the sunward side of the Earth, where it enjoys an uninterrupted view of the Sun. Every day it sends thrilling images from which research scientists learn about the Sun's nature and behaviour. And experts around the world use SOHO images and data to help them predict bad space weather affecting our own planet.

The spacecraft is a big box crammed with instruments pointing continuously at the Sun. SOHO moves around the Sun in step with the Earth, by slowly orbiting around Lagrange Point No. 1, where the combined gravity of the Earth and Sun keep SOHO in an orbit locked to Earth-Sun line.

SOHO's science ranges from the Sun's hot interior, through its visible surface and stormy atmosphere, and out to distant regions where the wind from the Sun battles with a breeze of atoms coming from among the stars. Discoveries include complex currents of gas flowing beneath the visible surface, and rapid changes in the pattern of magnetic fields. In the atmosphere SOHO sees non-stop explosions and also remarkable shock waves and tornadoes.

After it went into space in 1995, SOHO was meant to operate until 1998, but it was so successful that ESA and NASA decided to prolong its life until 2007. This will allow SOHO to operate for a full 11-year solar cycle, from minimum (few dark sunspots) through maximum to minimum. A key objective of this 'SOHO Solar Cycle Mission' is to better understand the 11-year activity cycle of the Sun, from changes in its interior through its atmosphere and out into interplanetary space.

Three years into its mission, contact was lost with SOHO on June 25th 1998 after a sequence of incorrect commands during what should have been a routine manoeuvre. All attempts to re-establish contact with the spacecraft failed and no one knew where Soho was for certain, for four weeks. In August 1998, a powerful radar signal from Earth produced a faint echo from the spacecraft.

Soho was still in the right place and angled in such a way that sunlight would begin to fall on its solar cells again during the next two months, so enabling it to resume normal operations.

Other difficulties came with the loss of the gyroscopes used to control the spacecraft's orientation. Despite these problems, engineers have kept SOHO functioning with all its instruments performing well. Interestingly, SOHO was the first 3-axis stabilized spacecraft to be operated without any gyros.

The spacecraft

The total mass of the spacecraft at launch was 1 850 kg. Its overall length along the sun-pointing axis is 4.3 metres, and the span of the extended solar panels is 9.5 metres.

The instruments on board SOHO are:-

• CDS (Coronal Diagnostic Spectrometer) from Rutherford Appleton Laboratory, United Kingdom.
• CELIAS (Charge, Element, and Isotope Analysis System) from the University of Bern, Switzerland.
• COSTEP (Comprehensive Suprathermal and Energetic Particle Analyzer) from the University of Kiel, Germany.
• EIT (Extreme ultraviolet Imaging Telescope) from the Institut d'Astrophysique Spatiale, France.
• ERNE (Energetic and Relativistic Nuclei and Electron experiment) from the University of Turku, Finland.
• GOLF (Global Oscillations at Low Frequencies) from the Institut d'Astrophysique Spatiale, France.
• LASCO (Large Angle and Spectrometric Coronagraph) from the Naval Research Laboratory, USA
• MDI (Michelson Doppler Imager) from Stanford University, USA.
• SUMER (Solar Ultraviolet Measurements of Emitted Radiation) from the Max-Planck-Institut für Aeronomie, Germany
• SWAN (Solar Wind Anisotropies) from Service d'Aeronomie, France.
• UVCS (Ultraviolet Coronagraph Spectrometer) from Harvard-Smithsonian Center for Astrophysics, USA.
• VIRGO (Variability of Solar Irradiance and Gravity Oscillations) from PMO/WRC Davos, Switzerland.

The journey

SOHO had such a flawless launch that very little thruster fuel had to be expended on course corrections during the spacecraft's journey out to its operating position at a point in space called 'L1', 1.5 million kilometres from the Earth towards the Sun.

The history

SOHO was first proposed 13 years before its actual launch and indeed the roots of SOHO were laid in earlier studies, namely those of GRIST (Grazing Incidence Solar Telescope) and DISCO (Dual Spectral Irradiance and Solar Constant Orbiter). It is the combination of the objectives of these two missions that constitutes the core of the SOHO mission. Indeed, most ESA missions - especially those in 'new' fields - are the result of an evolution rather than a single proposal. Often such studies eventually involve the communities of several space-science disciplines. Aligning these communities behind one coherent proposal involves several steps and can thus take quite some time.

Already in June 1976, GRIST had been competing with a 'Solar Probe' as well as other studies involving other disciplines for further study. Solar Probe envisaged a set of instruments on a spacecraft that would approach the Sun once to within four solar radii. Although its Assessment Study cited four scientific disciplines interested in the mission: (i) solar wind and space plasma, (ii) solar atmosphere, (iii) solar and stellar interior, and (iv) experimental gravitation and relativity, Solar Probe was not followed up at the time.

The GRIST study, on the other hand, proceded to a feasibility stage. GRIST was preferred over Solar Probe because the wavelength range accessible through its optics was particularly useful for studying the hot outer solar atmosphere. GRIST was at that time designated for multiple flights on Spacelab, mounted on the Instrument Pointing System (IPS). Its smallest picture element in the ultraviolet was planned actually to exceed even that of SOHO.

Following the study of 1976-78, accommodation studies were made with the intention of flying GRIST alongside NASA's Solar Optical Telescope (SOT) on Spacelab. Like the Solar Probe, GRIST did not make it to project selection either. It was based on a collaboration with NASA, and in early 1981 it became a victim as part of ESA's response to NASA's unilateral cancellation of the US probe in the 'International Solar Polar Mission' (ISPM, the former 'Out-of-Ecliptic Mission', now called 'Ulysses'). GRIST was 'mothballed'. Fortunately, however, restricted studies (concentrating mainly on the spacecraft interface) of the main spectrometers of GRIST were further supported by ESA.

In the course of 1980, many leading solar physicists took part in two international conferences at which prospects for space observations of the Sun were discussed. They were rather specialised: one dealing mainly with aspects of 'Solar Irradiance' and the other on 'Solar Physics from Space'.

At nearly the same time, a group of French and American physicists observed the Sun continuously from Antarctica between 31 December 1979 and 5 January 1980 and succeeded in measuring aspects of Solar Physics in the best conditions available on Earth. These historic observations led to the decision to include the same sort of aspects onboard a newly proposed mission called DISCO. (These are now being performed on-board SOHO with the GOLF experiment).

It proposed to locate DISCO at the L1 Lagrangian point between the Sun and the Earth, which would be an ideal observing site. A miniaturised version of the South Pole experiment (then weighing several hundred kilos) could be sent as part of DISCO's payload, provided its weight could be considerably reduced.

DISCO's model payload was extended to contain a set of photometers and absolute radiometers. DISCO was also going to carry a far-ultraviolet spectrometer. It was conceived as a fairly small and cheap spin-stabilised spacecraft, weighing no more than 520 kg and, in the minds of its proponents, it was supposed to prove that ESA could also undertake small and inexpensive missions.

A first assessment was made and the results published in the Assessment Report in May 1981. At the completion of this study, DISCO had remained a relatively inexpensive spinning satellite, very similar in fact to a Cluster satellite. At its meeting in January 1983, the Solar System Working Group preferred DISCO to a competing Mars mission called 'Kepler'. However, DISCO eventually lost out to ISO (the Infrared Space Observatory) in the final evaluation by the Space Science Advisory Committee, and thus ISO was approved as a new project by the Science Programme Committee in March 1983.

SOHO itself developed as a mission through 1982 and 1983 combining many of the aspects of the missions described above. It became important even though its predecessors had not gone ahead because it developed momentum, together with the Cluster mission, as part of the International Solar- Terrestrial Physics (ISTP) Programme to be undertaken jointly by ESA, NASA and ISAS. In May 1984, the Survey Committee identified SOHO as a component of the Solar-Terrestrial Physics (STP) 'Cornerstone' of the ESA long-term programme 'Space Science: Horizon 2000'.

Partnerships

SOHO is part of the first Cornerstone project in ESA's science programme, in which the other part is the Cluster mission. Both are joint ESA-NASA projects in which ESA is the senior partner. SOHO and Cluster are also contributions to the International Solar-Terrestrial Physics Programme, to which ESA, NASA and the space agencies of Japan, Russia, Sweden and Denmark all contribute satellites monitoring the Sun and solar effects.

Of the spacecraft's 12 sets of instruments, nine come from multinational teams led by European scientists, and three from US-led teams. More than 1500 scientists from around the world have been involved with the SOHO programme, analysing and interpreting SOHO data for their research projects. .

SOHO was built for ESA by industrial companies in 14 European countries, led by Matra Marconi (now called ASTRIUM). The service module, with solar panels, thrusters, attitude control systems, communications and housekeeping functions, was prepared in Toulouse, France. The payload module carrying the scientific instruments was assembled in Portsmouth, United Kingdom, and mated with the service module in Toulouse, France. NASA launched SOHO and is responsible for tracking, telemetry reception and commanding.