Staring at the Sun
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.
Mission facts
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:
The second or 'in-situ particle' group tells us which particles make up the Sun's atmosphere, known as the corona:
The other six instruments all study and measure aspects of the Sun's atmosphere at different wavelengths:
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:
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:
