Figure 1. The SOHO spacecraft in launch configuration prior to acoustic testing at Intespace in Toulouse (F)
The Solar and Heliospheric Observatory (SOHO) forms part of the Solar-Terrestrial Science Programme (STSP), a collaborative effort between ESA and NASA. The STSP is the first 'Cornerstone' of ESA's long-term programme 'Space Science: Horizon 2000'.
The principal scientific objectives of the SOHO mission are: (a) to achieve a better under-standing of the structure and dynamics of the solar interior using helioseismology techniques, and (b) to gain greater insight into the physical processes that form and heat the Sun's corona, maintain it and give rise to acceleration of its particles into the solar wind.
To achieve these goals, SOHO carries a payload consisting of twelve complementary instruments. It is an 1850 kg, three-axis-stabilised spacecraft, powered by solar panels delivering 1150 W. The payload itself weighs about 650 kg and will have a power consumption of 500 W once in orbit. It will produce a continuous science data stream of 40 kbit/s, which will increase by 160 kbit/s whenever the Solar Oscillations Imaging (SOI) instrument is operated in its high-rate mode.
SOHO will be launched by an Atlas-IIAS vehicle and will subsequently be placed into a halo orbit around the Sun Earth L1 Lagrangian point, where it will be pointed continuously at the Sun's centre with an accuracy of 10 arcsec. The pointing stability will be better than 1 arcsec over 15 min intervals. Planning, coordination and operation of both the spacecraft and the scientific payload will be conducted from the Experiment Operations Facility (EOF) at NASA's Goddard Space Flight Center (GSFC), and the telemetry data will be received by NASA's Deep-Space Network (DSN).
SOHO's set of instruments had to be developed, and their design performances verified by analysis and test, taking into account strict cost and schedule constraints, without, however, compromising the mission objectives. This article describes the corresponding programme leading up to flight-qualification of the experiments and their performance validation within the overall spacecraft system environment.
Table 1. The SOHO mission
Mission objective Investigation of the Sun from its interior out to and
including solar wing
Mission shares ESA: spacecraft plus nine experiments supplied by Member States
ESA: Launcher, ground segment and three experiments.
Launch November 1995
Mission lifetime =>2 years (onboard consumables for up to 6 years)
The Solar and Heliospheric Observatory (SOHO) mission was first proposed in November 1982 as a comprehensive, high-resolution spectroscopic investigation of the upper solar atmosphere, in response to one of the ESA Scientific Programme's regular Calls for Mission Proposals.
A Phase-A feasibility study followed, from July 1984 to October 1985. The Science Study Team responsible for the study was composed of both European and US scientists, supported by ESA and NASA. Meeting on 6 - 7 February 1986, ESA's Science Programme Committee (SPC) approved the Solar-Terrestrial Science Programme (STSP) as the first 'Cornerstone' mission of the Agency's long-term 'Space Science: Horizon 2000' programme and a mission to be implemented in collaboration with NASA. The STSP is composed of two missions SOHO and Cluster, the latter consisting of four identical spacecraft to study turbulence and small-scale plasma structures in the magnetosphere in three dimensions (described elsewhere in this issue).
SOHO, together with Cluster and the Geotail, Wind and Polar spacecraft, constitute the International Solar-Terrestrial Physics (ISTP) Programme, a cooperative scientific satellite project involving ESA, ISAS (Japan) and NASA. The ISTP is aimed at gaining improved understanding of the physics of solar-terrestrial relations by coordinated, simultaneous investigations of the Sun Earth space environment over an extended period of time.
A joint ESA/NASA Announcement of Opportunity for the STSP missions was issued on 1 March 1987, calling for 'Proposals for Investigations'. The proposals received were evaluated on the grounds of their scientific and technical merits, and the payloads were selected following the recommendations of the joint ESA/NASA advisory bodies. ESA and NASA subsequently announced the composition of the SOHO and Cluster payloads in March 1988. The SOHO Science Working Team (SWT), composed of all of the Principal Investigators (PIs), met for the first time from 27 to 30 June 1988.
A consortium of European industries, led by Matra of France as the Prime Contractor, started the industrial definition phase (Phase-B) on 1 December 1989. The main industrial development phase (Phase-C/D) started 14 months later, in early 1991. The Structural Model (SM) programme was completed in 1993, and the Engineering Model (EM) programme in early 1994. The first Flight Model (FM) instruments were delivered in December 1993. The Assembly, Integration, and Validation (AIV) activities for the flight-model spacecraft have taken place in 1994 and the first half of 1995. The SOHO spacecraft was shipped from Toulouse (F) to Kennedy Space Center at Cape Canaveral, ready for the launch campaign, on 1 August 1995.
Figure 1 shows the SOHO satellite in launch configuration prior to acoustic testing at Intespace in Toulouse. Figure 2 shows the SOHO Payload Module (PLM) during integration at Matra Marconi in Toulouse. Figure 3 shows the positions of the twelve payload instruments schematically.
Figure 1. The SOHO spacecraft in launch configuration prior to acoustic testing at Intespace in Toulouse (F)
Figure 2. The SOHO payload module, without thermal blankets, after integration and testing at Matra Marconi Space in Toulouse (F)
Figure 3. Schematic of the SOHO spacecraft, showing the locations of the twelve experiments
The SOHO mission's three principal scientific objectives are:
The spacecraft's scientific payload consists of a set of state-of-the-art instruments, developed and furnished by twelve international Principal Investigator (PI) consortia, involving 39 institutes from fifteen countries: Belgium, Denmark, Finland, France, Germany, Ireland, Italy, Japan, Netherlands, Norway, Russia, Spain, Switzerland, the United Kingdom, and the United States. Nine of the consortia have been led by European PIs and three by US PIs.
The experiments aboard SOHO can be divided into three main groups, according to their respective areas of research: helioseismology instruments, solar-corona instruments, and solar-wind in-situ instruments (Table 2).
Table 2. The SOHO scientific instruments
There are three helioseismology experiments, designed to provide high-precision measurements of solar oscillations, which are difficult, or even impossible, to obtain from the ground.
GOLF (Global Oscillations at Low Frequencies) will use a very stable sodium-vapour resonance scattering spectrometer to observe low-degree oscillation modes (I less than or equal to 3) of the global solar velocity field with a sensitivity of better than 1 mm/s over the complete frequency range from 0.1 micron Hz to 6 MHz (periods from 3 min to 100 days). It will also measure the long-term variations in the global average of the line-of-sight magnetic field with a precision of 1 mG.
VIRGO (Variability of solar IRradiance and Gravity Oscillations) will perform high-sensitivity observations of p- and if detectable g- mode solar-intensity oscillations with a three-channel Sun-photometer measuring the spectral irradiance at 402, 500 and 862 nm, and with the 12-resolution-element Luminosity Oscillations Imager (LOI) (I less than or equal to 7). The relative accuracy of these data will be better than 1 ppm (for a 10's integration time). VIRGO will also measure the solar constant with an absolute accuracy of better than 0.15% using two different types of absolute radiometers (PMO6 and CROM).
Both GOLF and VIRGO lay particular emphasis on the very-low-frequency domain of low-order p- and g-modes, which penetrate deep into the solar core. They are difficult to observe from the Earth because of noise effects introduced by the Earth's diurnal rotation and the transparency and seeing fluctuations caused by the Earth's atmosphere.
The Solar Oscillations Imager (SOI) focusses on the intermediate to very high degree p-modes. By sampling the Ni 676.8 nm line with a wide-field tunable Michelson interferometer, SOI will provide high-precision solar images (1024 1024 pixels) of the line-of-sight velocity, line intensity, continuum intensity, longitudinal magnetic-field components, and limb position. It can be operated in a full-disk mode (2 inches -equivalent pixel size) to resolve modes in the range where I is between 3 and 1500, or in a high-resolution mode (0.65 inches pixel size) to resolve modes as high as I=4500. The high-resolution field-of-view is roughly 650 square and will be centred about 160 north of the Sun's equator on the central meridian.
SOI will run four different observing programs. The 'structure program' will provide a continuous 5 kbit/s data stream of various spatial and temporal averages of the full-disk velocity and intensity images. It will be running at all times. The dynamics program will operate for 60 consecutive days each year with continuous high-rate telemetry (+160 kbit/s). A full-disk velocity image and either a full-disk intensity image or a high-resolution velocity image will be transmitted every minute. The 'campaign program' will be conducted for 8 consecutive hours each day when the high-rate telemetry is available. This is a very flexible operating mode for performing a variety of more narrowly focussed scientific investigations (e.g. studies of meso- and super-granulation, active-region seismology, etc.). Finally, the 'magnetic-field programme' will provide several real-time magnetograms per day for planning purposes and correlative studies.
The three helioseismology instruments complement each other in several respects. While MDI will measure oscillations over the full range of degrees up to I = 4500, GOLF and VIRGO are expected to provide greater stability for the measurement of low-degree oscillations. GOLF and MDI will measure oscillations of the line-of-sight velocity, while VIRGO will measure intensity oscillations (both radiance and irradiance). GOLF and VIRGO complement each other because, given the difficulty that one can expect in identifying gravity modes, it may well prove essential to have two different approaches (velocity oscillations from GOLF and intensity oscillations from VIRGO) to achieve a convincing result.
The remote sensing of the solar atmosphere will be carried out with a set of telescopes and spectrometers that will produce the data necessary to study the dynamic phenomena that take place in and above the chromosphere. The plasma will be studied by making spectroscopic measurements and high-resolution images at different levels of the solar atmosphere. Plasma diagnostics obtained with these instruments will provide temperature, density and velocity measurements of the material in the outer solar atmosphere.
In the past, the coronal observations with the best spatial and spectral resolution have covered only limited spectral ranges and were obtained from rockets, i.e. on a snapshot basis. SOHO will provide what is now desperately needed: an extended and concerted investigation of the physical structures, the dynamics and evolution of the transition region and corona on a synoptic basis. In addition, given its ability to make both (remote-sensing) coronal and (in-situ) interplanetary measurements, SOHO will help to establish the nature of the relationship between conditions in the regions where the solar wind originates and the observed properties at 1 AU distance, in particular the elusive acceleration.
SUMER (Solar Ultraviolet Measurements of Emitted Radiation) is an ultraviolet (UV) telescope equipped with a normal-incidence spectrometer. It will study plasma flows, temperatures, densities, and wave motions in the upper chromosphere, transition region and lower corona with high spatial (1.5 inches) and temporal (typically 10 s) resolution, by measuring line profiles and intensities of UV lines from 500 to 1600 Angstrom The spectral coverage varies between 20 and 44 Angstrom with a spectral resolving power of lambda/delta lambda = 18 800-40 000. It should be possible with SUMER to measure velocity fields in the transition region and corona down to 1 km/s.
At shorter wavelengths of 150 to 800 Angstrom, CDS (Coronal Diagnostic Spectrometer), a Walter II grazing-incidence telescope equipped with both a normal-incidence and a grazing-incidence spectrometer, will measure the absolute and relative intensities of selected EUV lines to determine the temperatures and densities of various coronal structures. The telescope's spatial resolution is about 3", and its spectral resolution varies between 2000 and 10000.
EIT (Extreme-ultraviolet Imaging Telescope) will obtain full-Sun, high-resolution EUV images in four emission lines (Fe IX 171 Angstrom , Fe XII 195 Angstrom , Fe XV 284 Angstrom, and He II 304 Angstrom ) corresponding to four different temperature regimes. It will thus provide the morphological context for the spectral observations to be made by SUMER and CDS. The wavelength separation is achieved by multilayer reflecting coatings deposited on the four quadrants of the telescope mirrors, and a rotatable mask to select the quadrant illuminated by the Sun. A 1024 1024 CCD camera with an effective pixel size of 2.6" is used as the detector.
UVCS (Ultra-Violet Coronagraph Spectro-meter) is an occulted telescope equipped with high-resolution spectrometers to perform spectroscopic observations of the solar corona out to 10 solar radii, in order to locate and characterise the coronal source regions of the solar wind, to identify and understand the dominant physical processes that accelerate it, and to understand how the coronal plasma is heated. One of the gratings is optimised for line profile measurements of Ly-alfa, another for line intensity measurements in the range 944-1070 Angstrom.
LASCO (Large Angle and Spectrometric COronagraph) is a triple coronagraph with nested, concentric annular fields of view with progressively larger included angles. The fields of view of the three coronagraphs C1, C2 and C3 are 1.1-3, 1.5-6 and 3-30 solar radii, respectively. All three coronagraphs will use 1024x1024 CCD cameras as detectors. C1 will not only be the first spaceborne mirror coronagraph , but also the first spaceborne coronagraph with its own spectroscopic capabilities. It is equipped with a Fabry-Perot interferometer to perform spectroscopic measurements with a spectral resolution of about 700 mAngstrom in the lines Fe XIV 5303 Angstrom , Fe X 6374 Angstrom, Ca XV 5964 Angstrom , NaD2 , and Halfa.
SWAN (Solar Wind ANisotropies) will measure the latitude distribution of the solar-wind mass flux from solar equator to solar pole by mapping the emissivity of the interplanetary Ly-alfa light.
These will measure the composition of the solar wind and energetic particles 'in-situ' to determine the elemental and isotopic abundances, the ionic charge states and velocity distributions of ions originating in the solar atmosphere. The energy ranges covered will allow the ion acceleration and fractionation processes to be studied under the various conditions that cause their acceleration from the slow solar wind through solar flares.
CELIAS (Charge, ELement and Isotope Analysis System) consists of three mass- and charge- discriminating sensors based on the time-of-flight technique, making use of electrostatic deflection, post-acceleration and residual-energy measurements. It will measure the mass, ionic charge and energy of the low-and high-speed solar wind, of suprathermal ions, and of low-energy flare particles. It also carries the SEM (Solar Extreme-ultra-violet Monitor), a very stable photodiode spectrometer which will continuously measure the full-disk solar flux in the Hell 304 Angstrom line as well as the absolute integral flux between 170 and 700 Angstrom.
In order to study the energy-release and particle-acceleration processes in the solar atmosphere, as well as particle propagation in the interplanetary medium, COSTEP (COmprehensive Supra-Thermal and Ener-getic Particle analyser) will measure the energy spectra of electrons (up to 5 MeV), protons and He nuclei (up to 53 MeV/nuc).
ERNE (Energetic and Relativistic Nuclei and Electron experiment), having the same scientific objectives as COSTEP, will measure the energy spectra of elements in the range Z=1-30 (up to 540 MeV/nuc), abundance ratios of isotopes, as well as the anisotropy of the particle flux.
In summary, the coronal remote-sensing and the in-situ experiments on board SOHO will provide a comprehensive data set with which to study the solar wind from its source at the Sun and out through the heliosphere. The solar imagers and spectrographs will allow study of the morphology, magnetic structure, and heating and particle-acceleration processes occurring at the Sun. At the same time, it will be possible with the particle experiments to make direct measurements of the particle composition and energy spectrum in the solar wind.
The assembly, integration and verification (AIV) programme consisted of validating the spacecraft system performances by means of a set of tests and analytical methods (Fig. 4). This SOHO baseline AIV programme served as the foundation for the full qualification of the spacecraft system design, using two development models: the Structural and Engineering Model (SM + EM) and one Flight Model (FM).
Figure 4. The baseline Assembly, Integration and Verification (AIV) programme for SOHO
To minimise risk, the development models were of flight-build standard, but in order to constrain costs were not equipped with hi-rel parts.
These models were employed in the qualification and acceptance programme process as follows:
The analyses performed at system level were defined to support the validation of the design performance and the test/environmental level predictions in the following areas:
In support of these system tasks, the Principal Investigators produced a set of detailed structural and thermal models according to well-defined specifications provided by the Project Team.
The system test programme leading to the full qualification of the spacecraft and payload designs is summarised in Table 3.
Table 3. The SOHO model and test-parameter philosophy
In order to fulfil the test objectives defined by the system programme, the experiments had to be developed and produced according to a model-compliant philosophy and build standard summarised in Table 4.
Table 4. The SOHO experiment development options
Due to both the diversity and complexity of the SOHO instruments and the firm budget limitations, two different development- path options were proposed to the Principal Investigators:
The resulting test matrix covered the following disciplines:
Based on the excellent results obtained during the system test campaign, the test and analytical programme described above can be deemed to have been adequate and very successful.
The experiment design verification and qualification process was generally achieved within the cost and schedule envelopes defined and without compromising the system performances or disclosing design weak-nesses at a late stage which could have endangered programme continuity.
Adequate design margins were also confirmed, further enhancing confidence in the ability of the SOHO payload to meet the scientific objectives laid down for this demanding mission to the Sun.
ESA Bulletin Nr. 84.