Space exploration archive
The Solar and Heliospheric Observatory (SOHO)
The Solar and Heliospheric Observatory (SOHO) is a spacecraft that is currently investigating the entire Sun, from the core to the outer corona and the solar wind. The project is a joint venture between the European Space Agency (ESA) and the US National Aeronautics and Space Administration (NASA).
This image shows a large eruptive prominence seen in ultraviolet light at a wavelength of 304Å with an image of the Earth added for size comparison. This prominence from 24 July 1999 extended over 35 Earth diameters out from the Sun. If material from erupting prominences reaches the Earth it can affect communications, navigation systems and even power grids while also producing auroras visible in the night skies.
On 2 December 1995 an Atlas II-AS rocket launched SOHO Cape Canaveral in Florida. It was inserted into a halo orbit around the L1 Lagrangian point between the Earth and Sun in February 1996. Lagrangian points are positions in the orbital plane of two massive objects – such as a planet and a star – where a third body of negligible mass can remain in equilibrium (i.e. experience no net gravitational force from either body.) As a result of this, SOHO maintains an orbit locked to the Earth-Sun line.
SOHO is positioned 1.5 million kilometres on the sunward side of the Earth and thus can view the Sun continuously. Previous solar observatories have all orbited the Earth and their observations were interrupted when they moved into the Earth’s shadow. SOHO was built for ESA by Europe's aerospace industry. The data are shared between ESA and NASA, while the day-to-day operations of the spacecraft are carried out by the American agency.
The spacecraft consists of two modules. The service module, which was constructed in Toulouse (France), houses the systems that are vital to the running and maintenance of the spacecraft. These include thrusters, communication systems, various control systems and solar panels. The second or payload module, which was constructed in Portsmouth (UK), is home to twelve different scientific instruments that are constantly directed towards the Sun. The total mass of the spacecraft is about 1850 kg. It is 4.3 metres long, and when the solar panels are extended, it is 9.5 metres in diameter.
SOHO's twelve instruments send back vast amounts of data and images which are used by scientists to learn more about the Sun. This information also has everyday practical uses in helping scientists to predict increased solar activity which has effects on the terrestrial environment, for example disrupting satellites and thus interfering with telecommunications and television broadcasts.
There are three different sets of instruments aboard SOHO, each of which focuses on a different area of the Sun. Five instruments are concerned with collecting data from the solar atmosphere or solar corona and the underlying photosphere. These instruments comprise telescopes, spectrometers and graphs that record the temperature, density, composition and movements of the corona. SOHO has obtained very exciting images and movies of structures in the corona such as prominences and solar flares (coronal mass ejections). Comets have also been seen plunging into the Sun.
Three instruments examine the solar interior. Scientists can learn about the interior of the sun by examining wave oscillations or sunquakes – this is helioseismology. Much information has also been collected about the Sun’s convection zone – the outermost layer of the interior of the Sun. The third set of instruments is devoted to examining the solar wind and to some extent the outer solar corona which extends to many times the diameter of the sun. As the solar wind flows pass SOHO, these instruments 'sample’ the various particles and ions that are present. This helps scientists to understand how the solar wind is accelerated.
SOHO completed its original two-year mission in April 1998 but it was so successful that ESA and NASA have extended the mission several times. The latest plan sees the spacecraft operating until December 2009. This means that scientists studying the data sent back can compare the behaviour of the Sun from 1996 when it had few sunspots; to 2001 when there was a peak of sunspot activity, through the next minimum in 2006 and then towards the next maximum predicted for 2011.
For all those involved in the project there were a few nervous months in 1998. In June that year, during a routine maintenance operation, all contact with SOHO was suddenly lost. After several weeks and an extensive search using NASA's Deep Space Network the spacecraft was eventually located by radar and communications were re-established. Amazingly, NASA and ESA scientists managed to restore power to the spacecraft and bring all the instruments back on-line after four months.
SOHO has and continues to be a very successful mission and all 12 instruments still send back vast amounts of data every day.
