The Solar Heliospheric Observatory (SOHO) was launched into space on December 2, 1995. This was a project of international cooperation between the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). The project included the study of the Sun´s interior, the outer corona and the source regions of the solar wind. To make these scientific studies, SOHO was launched with 12 complementary instruments on board.
SOHO´s design is based on a modular concept, including two principal elements: the service module which provides essential elements, such as thrusters, power and communications, and the payload module, housing the 12 instrument packages. SOHO is a three-axis stabilized spacecraft that perpetually faces the Sun. SOHO´s dimensions are 4.3×2.7×3.7 meters, although, it can reach 9.5 meters with solar arrays deployed. Its weight at launch was 1850 kilograms (4078 pounds). The cost of the SOHO project totaled approximately one thousand million Euros (1.2 billion dollars).
SOHO was originally designed to give answers to three basic scientific questions regarding the Sun´s activity, including the structure and dynamics of the solar interior; how the solar corona reaches extreme high temperatures of about 1,000,000 C°; and the causes of the solar wind acceleration. During the past 15 years, SOHO has not only answered these questions, but it has also revolutionized man´s understanding of the Sun-Earth interactions, has provided useful information of Sun´s activity in advance to help forecast space weather, and has discovered more than 2000 comets.
SOHO was strategically placed into a halo orbit at the L1 Lagrangian point 1.5 million km (930,000 miles) from Earth, where it was able to observe the Sun´s activity without interruption 24 hours of the day. SOHO ´s original lifetime span included two years; however, due to first successful results, the mission has been extended five times. In June 1998, control of SOHO was lost; however, with the effort of a recovery team, all 12 instruments on board were recovered with no plausible damage. This incident made SOHO the first three-axis stabilized spacecraft to use its reaction wheels as gyroscopes, opening new ground for future spacecraft designs.
The scientific load of SOHO includes 12 complementary instruments, including the Coronal Diagnostic Spectrometer (CDS) which detects emission lines from ions and atoms in the transition and corona regions. The Charge Element and Isotope Analysis System (CELIAS) which constantly examines the density and composition of particles present in the solar wind, and warn of incoming solar storms. The Comprehensive Suprathermal and Energetic Particle Analyzer (COSTEP) which identifies and classifies energetic particle assortments of solar, interplanetary and galactic origin.
The Extreme Ultraviolet Imaging Telescope (EIT) provides images of the Sun in the ultraviolet spectrum of four selected colors, and maps the plasma in the transition and low corona regions at extreme temperatures of 80,000-2.500,000 C°. The Energetic and Relativistic Nuclei and Electron Experiment (ERNE) which measures high energy particles coming from the Sun and Milky Way. It´s a complementary instrument to COSTEP. The Global Oscillations at Low Frequencies (GOLF) which studies the inner structure of the Sun by measuring oscillations of the entire solar disc.
The Large Angle and Spectrometric Coronograph (LASCO) which observes the corona from near the solar surface to a distance of about 21 million kilometers (13 million miles). This instrument blocks direct sunlight, creating a sort of eclipse which permits the observation of comets approaching the Sun. the Michelson Doppler Imager/Solar Oscillations Investigation (MDI/SOI) which records the vertical tides of the Sun´s surface at a million distinct points per minute. MDI measures the longitudinal section of the Sun´s magnetic field, as well.
The Solar Ultraviolet Measurements of Emitted Radiation (SUMER) which performs detailed spectroscopic diagnostics, including density, temperature, flows and dynamics of the Sun´s atmosphere. The Solar Wind Anisotropies (SWAN) points away from the sun, studying the interaction between the solar wind and hydrogen. The Ultraviolet Coronograph Spectrometer (UVCS) makes measurements of the solar corona in ultraviolet light, providing information of highly ionized coronal plasma. The Variability of Solar Irradiance and Gravity Oscillations (VIRGO) measures total irradiance of the Sun.
SOHO is operated on Earth from NASA´s Goddard Space Flight Center (GSFC). In its operation, there are a great number of scientists and engineers from NASA, research laboratories, universities and partner industries, all of which work under the supervision and responsibility of ESA. Overall control on Earth is provided by NASA´s Deep Space Network Antennae, situated in Goldstone, California, Madrid Spain and Canberra Australia.
