10 Years of SOHO
02 December 2005
On 2 December 1995 the joint ESA-NASA Solar Heliospheric Observatory (SOHO) was launched by an Atlas II-AS rocket from Cape Canaveral in Florida, US.
SOHO was designed to answer the following three fundamental scientific questions about the Sun:
Clues on the solar interior come from studying seismic waves that are produced in the turbulent outer shell of the Sun and which appear as ripples on its surface.
To obtain a 24 hours a day, 365 days a year view of the Sun SOHO is placed at a permanent vantage point 1.5 million kilometers sunward of the Earth in a halo orbit around the L1 Lagrangian point. SOHO was initially designed to observe the Sun continuously for at least two years. All previous solar observatories have orbited the Earth, from where their observations were periodically interrupted as our planet eclipsed the Sun. The advantage of SOHO has been its continual monitoring of the Sun throughout the current solar cycle.
SOHO is made up of two modules. The Service Module forms the lower portion of the spacecraft and provides power, thermal control, pointing and telecommunications for the whole spacecraft and support for the solar panels. The Payload Module sits above it and houses all the scientific instruments.
Control of the spacecraft was lost in June 1998, and only restored three months later through efforts of the SOHO recovery team. All 12 instruments were still us-able, most with no ill effects. Two of the three on-board gyroscopes failed immediately and a third in December 1998. After that, new on-board software that no longer relies on gyroscopes was installed in February 1999. It allowed the spacecraft to return to full scientific operations, while providing an even greater margin of safety for spacecraft operations. This made SOHO the first three-axis stabilised spacecraft operated without gyroscopes, breaking new ground for future spacecraft designs.
The scientific payload of SOHO comprises 12 complementary instruments, developed and furnished by 12 international consortia involving 29 institutes from 15 countries. Nine consortia are led by European scientists, the remaining three by US scientists. More than 1500 scientists in countries all around the world are either directly involved in SOHO's instruments or have used SOHO data in their research programmes.
The solar interior
GOLF and VIRGO both perform long and uninterrupted series of oscillations measurements of the full solar disk, respectively in velocity and in the irradiance domain. In this way, information is obtained about the solar nucleus. SOI/MDI measure oscillations on the surface of the Sun with high angular resolution. This permits us to obtain precise information about the Sun's convection zone - the outer layer of the solar interior.
The solar atmosphere
SUMER, CDS, EIT, UVCS, and LASCO constitute a combination of telescopes, spectrometers and coronagraphs that observe the hot atmosphere of the Sun, the corona, extending far above the visible surface. SUMER, CDS and EIT observe the inner corona. UVCS and LASCO observe both inner and outer corona. They obtain measurements of the temperature, density, composition and velocity in the corona, and follow the evolution of the structures with high resolution.
The solar wind
CELIAS, COSTEP and ERNE analyze in situ the charge state and isotopic composition of ions in the solar wind, and the charge and isotopic composition of energetic particles generated by the Sun. SWAN make maps of the hydrogen density in the heliosphere from ten solar diameters. It uses telescopes sensitive to a particular wavelength of hydrogen, allowing the large-scale structure of the solar wind streams to be measured.
SOHO has provided an unprecedented breadth and depth of information about the Sun, from its interior, through the hot and dynamic atmosphere, to the solar wind and its interaction with the interstellar medium. These findings have been documented in an impressive, still growing body of scientific and popular literature.
Some of the key results include: