SOHO (Solar Heliospheric Observatory)

Staring at the Sun

• In operation
• Launched on 2 December 1995
• Currently extended until December 2009

The Solar and Heliospheric Observatory (SOHO) recently celebrated its twelfth anniversary in space. The joint European Space Agency (ESA)/NASA project is one of the most successful and longest lasting space science missions of all time. The mission was originally designed to last only two years!

SOHO is a space-based observatory. The spacecraft has been viewing and investigating the Sun from its deep core through to its outer atmosphere - the corona. It has also been investigating the origin of the solar wind - the stream of charged particles that blow outward through the Solar System.

Every day SOHO sends back stunning images that tell scientists more about the Sun's nature and its impact on our own planet. The data it sends back also helps us to predict space weather.

SOHO is operated from a permanent vantage point 1.5 million km on the sunward side of the Earth. It orbits in a halo around the first Lagrangian point – a place in space where the Sun and Earth’s gravitational forces balance each other.

Mission highlights include revealing the first ever images of a star’s turbulent outer shell - known as its convection zone - and the structure of sunspots below the surface. During its time in orbit, SOHO has photographed currents of gas beneath the Sun's outer surface and shown that there are non-stop explosions in the Sun's atmosphere.

SOHO has also become the most prolific finder of comets in the history of astronomy. This was never an objective of the mission but by 2005 SOHO celebrated its tenth birthday by photographing its 1,000th comet.

The Sun, and its influence on the Earth, is being studied in unprecedented detail. SOHO, Hinode and STEREO are observing the Sun directly while Cluster is examining the interaction between the solar wind and the Earth’s magnetic bubble – the magnetosphere.

The latest on the mission (including images) can be found on NASA's SOHO website.

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Mission facts

• The Sun is a giant nuclear reactor: every second millions of tons of hydrogen fuse together to produce helium. This process releases energy, providing us with the heat and light that makes life possible on Earth.
  
  
• Every second a million tonnes of hot plasma and charged particles (electrons and ions) escape the Sun’s gravity. This material is known as the ‘solar wind’. Fortunately, the Earth has a magnetic field which reaches out into space called the magnetosphere. This acts like a giant shield.
  
  
• However, these millions of charged particles can still have an effect. This is known as ‘space weather’.
  
  
• The most visible sign of space weather is the aurora. Space weather can be extremely variable and depends on solar activity.
  
  
• Every so often the Sun belches out billions of tonnes of particles in events called Coronal Mass Ejections. If one of these heads towards the Earth it can trigger a disturbance of the Earth's magnetic field called a geomagnetic storm. Large geomagnetic storms can knock-out orbiting satellites.
  
  
• SOHO has proved remarkably robust. In 1998, control was lost following the failure of all the onboard gyroscopes. However, new software was uploaded to the spacecraft which allowed it to operate without gyroscopes. This recovery was a first for any spacecraft.

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Technology

SOHO was built for ESA by industrial companies in 14 European countries, led by Matra Marconi (now part of Astrium Limited). The payload module carrying the scientific instruments was assembled in Portsmouth.

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

SOHO has 12 instruments on board. These are split into three groups, each with its own set of scientific objectives.

The ‘helioseismology group’ tells us more about the nature of the Sun's interior, and is made up of the following instruments:

• Global Oscillations at Low Frequencies (GOLF)
• Variability of Solar Irradiance (VIRGO)
• Michelson Doppler Imager (MDI)

The second or ‘in-situ particle’ group tells us which particles make up the Sun's atmosphere, known as the corona:

• Charge Element Isotope Analysis (CELIAS)
• Suprathermal & Energetic Particle Analyser (COSTEP)
• Energetic Particle Analyser (ERNE)

The other six instruments all study and measure aspects of the Sun's atmosphere at different wavelengths:

• Solar Ultraviolet Measurement of Emitted Radiation (SUMER)
• Coronal Diagnostic Spectrometer (CDS)
• Extreme Ultraviolet Imaging Telescope (EIT)
• Ultraviolet Corona and Spectrometer (UVCS)
• Large Angle Spectrometer Coronagraph (LASCO)
• Solar Wind Anisotropies (SWAN)

Together the three groups help scientists build up a complete picture of the Sun - from its blistering core, right out to the stream of particles that it shoots out into the Solar System - the solar wind.

Full details of the instruments are available on the ESA website.

UK involvement

The UK has always been a major player in the SOHO project, from the design and construction to the operation and science.

The UK has been heavily involved in the development and investigation of the Coronal Diagnostic Spectrometer (CDS) experiment. This has produced a huge amount of data. It has taken some ten million images and generated some 34,000 science studies! CDS addresses two of the three main goals of SOHO, namely:

• why does the corona exist; and
• how are the solar wind-streams accelerated?

STFC Rutherford Appleton Laboratory (RAL) is leading the team for the instrument, and assembled the group that designed and built the instrument. The UK members included:

• Armagh Observatory
• University of Birmingham
• Imperial College, University of London
• Mullard Space Science Laboratory
• Oxford University
• Queen's University, Belfast
• University of Central Lancashire
• University of Glasgow