Soho Credit - ESA

SOHO was a component of the Collaborative Solar-Terrestrial Research (COSTR) Program of the International Solar Terrestrial Physics (ISTP) Program. SOHO's specific objectives were to study and understand the solar corona, in particular its heating mechanism and its expansion into the solar wind; and to study the solar structure and interior dynamics from the Sun's core to the photosphere. SOHO had the first long duration unobstructed view of the sun from its halo orbit about the L1 Lagrange point 1.5 million kilometers ahead of the Earth. In this orbit, it avoided eclipses with the Earth that blocked the sun. The spacecraft was built by ESA, tracking and data acquisition were shared by NASA and ESA, and mission operations were conducted by NASA.

The spacecraft consisted of two modules and was three-axis stabilized to within 10 arcsec, with pointing stability of 1 arcsec per 15 minutes. The service module provided power, thermal and pointing control. The payload module contained twelve science instruments. Power was provided by dual solar panels, with a total output of 1150 W. The payload consumed 450 W on orbit. The velocity of the spacecraft relative to the sun was known to within 0.5 cm/sec.

Payloads:

• GOLF (Global Oscillations at Low Frequencies) - studied the internal structure of the sun by measuring the spectrum of global oscillations in the frequency range 10-7 to 10-2 Hz.
• VIRGO (Variability of Solar Irradiance and Gravity Oscillations) - provided continuous measurements of the solar total and spectral irradiance and spectral radiance variation, solar polar and equatorial diameters, and the frequencies, amplitudes and phases of oscillation modes in the frequency range of 1 uHz to 8 mHz
• SOI/MDI (Solar Oscillations Investigation/ Michelson Doppler Imager) - used a telescope with a 1024*1024 CCD camera to image the Sun. Data from the experiment was used to understand the static and dynamic properties of the Sun's convection zone and core, as well as contributing to knowledge of the solar magnetic field.
• SUMER (Solar Ultraviolet Measurements of Emitted Radiation) - measured profiles and intensities of extreme ultraviolet (EUV) lines emitted in the solar atmosphere. This data was used to understand plasma flow characteristics, turbulence and wave motions, plasma densities and temperatures, and structures and events associated with solar magnetic activity in the chromosphere, the transition zone and the corona.
• CDS (Coronal Diagnostic Spectrometer) - designed to obtain spectra-spectroheliograms in a number of lines in the EUV frequency region.
• EIT (Extreme Ultraviolet Imaging Telescope) - imaged the solar transition region and inner corona at 4 EUV frequencies (171, 195, 284, 304 angstroms). The instrument imaged active regions, filaments and prominences, coronal holes, coronal "bright points," polar plumes, and a variety of other solar features.
• UVCS (Ultraviolet Coronograph Spectrometer) - imaged the solar corona between 2 and 10 solar radii. Spectrographic images were used to calculate proton velocity distribution, proton outflow velocity, electron temperature, and specific ion outflow velocities and densities.
• LASCO (Large Angle Spectrometric Coronograph) - a set of three coronagraph telescopes that recorded white light and spectral images of the solar corona. Data from the experiment was used to understand how the corona was heated, how the solar wind was accelerated, what caused coronal transients, and how large-scale structures evolved.
• CELIAS (Charge, Element and Isotope Analysis System) - studied the composition of the solar wind and solar and interplanetary energetic particles. It also monitored the absolute EUV flux from the Sun.
• COSTEP (Comprehensive Suprathermal and Energetic Particle Analyzer)
• ERNE (Energetic and Relativistic Nuclei and Electron experiment)
• SWAN (Solar Wind Anisotrophies) - measured Lyman alpha UV light scattered by interplanetary hydrogen atoms.

Design Life: 2 years, 6 year goal. Typical orbit: 'Halo' about L1 Lagrange Point. Length: 3.65 m (11.97 ft). Maximum Diameter: 3.65 m (11.97 ft). Span: 9.50 m (31.10 ft). Mass: 1,850 kg (4,070 lb). Associated Launch Vehicle: Atlas IIAS.

• 1995 December 2 - SOHO - Launch Site: Cape Canaveral. Launch Vehicle: Atlas. Mass: 1,850 kg (4,070 lb). Perigee: 8,973 km (5,575 mi). Apogee: 671,400 km (417,100 mi). Inclination: 29.60 deg. Period: 29,196.30 min.

  Solar and Heliospheric Observatory; orbiting at L1 Lagrange point; solar physics. En route Earth-Sun L1 point Earth-Sun L1 libration point transfer trajectory. Inertial trajectory option.