The roots of the SOHO mission and the story leading to the comprehensive observatory that the spacecraft is today are summarised here. Now fully operational in its halo orbit around the first Lagrangian point (L1) between the Earth and the Sun, SOHO is providing the international scientific community with the unique opportunity, and also challenge, of understanding the Sun and heliosphere as one complex, global system. It is a superb tool with which to investigate our daylight star and its circumstellar environment, from the Sun's centre, through its visible surface and tenuous corona, and out into the heliosphere to distances corresponding to more than ten times the orbital distance of the Earth.
The Solar and Heliospheric Observatory, SOHO, is the most comprehensive space mission ever devoted to the study of the Sun and its nearby cosmic environment known as the heliosphere. From the vantage point of a halo orbit around the first Lagrangian point, L1 (cf. Fig. 1), SOHO's twelve scientific instruments observe and measure structures and processes occurring inside as well as outside the Sun, and which reach well beyond the Earth's orbit into the heliosphere. The SOHO spacecraft is immersed in the solar-wind streams and provides an extremely stable platform - stabilised to a fraction of one arc- second (an angle equivalent to the breadth of a hair viewed from 20 m away!) - for these instruments, which are small observatories in their own right.
Figure 1. SOHO orbits around the first Lagrangian point L1, which is located on the Earth-Sun line, 1.5 x 10(exp 6) km (i.e. 1.5 Gm)
from the Earth. This distance corresponds to 5 light-seconds, or 0.01 Astronomical Units (AU), i.e. 1% of the way from the Earth to the Sun.
The observing programme for the ensemble of instruments, the most modern in their categories, is established and coordinated through a sequence of monthly, weekly and daily reviews. In this way, orderly long-term planning is achieved, yet the SOHO investigators can also respond to 'targets of opportunity' offered by the ever-changing conditions in the solar atmosphere, and exploit new knowledge gained (or lessons learned) from earlier SOHO observations: critical observations can be confirmed or improved upon without undue delay.
The SOHO mission is an international collaboration between ESA, European national authorities and NASA. ESA took the lead in the collaboration between the two large agencies by procuring the spacecraft (including integration of the twelve instruments and environmental testing of the satellite) through European Industry. The instruments were built under the leadership of Principal Investigators - nine of them funded by European national authorities, and three by NASA. Further support was given by Co-investigators and Associated Scientists with European and US national funding. NASA provided the SOHO launch aboard an Atlas-2AS vehicle and it also takes care of mission operations as well as communications with the satellite via the Deep Space Network. Overall responsibility for the mission remains with ESA.
Solar physicists and engineers from universities and scientific institutions on both sides of the Atlantic who were involved in the design, construction and calibration of the experiments and in the preparations for the data analysis, are also participating in the scientific operations.
The circa 250 original Principal Investigators, Co-Investigators and Associated Scientists are now, as the data are coming in, being joined by an even larger number of scientists who are participating in the observations and making use of SOHO data in their work.
On 14 February, SOHO reached its location at Lagrangian point L1, 1.5 Gm from Earth. As new fundamental knowledge about the Sun and heliosphere now becomes available, this is an opportune moment to recall the steps that led to the approval of this first mission of the first Cornerstone of ESA's Long-Term Plan Horizon 2000 fully devoted to studying the Sun.
Accordingly, we will address here the questions:
The foreword to the report on the SOHO Phase-A Study prepared for the presentation of candidate missions for project selection in January 1986 summarises very well the developments that took place during the most decisive approval phases. The first three paragraphs read:
Upon the recommendation of the SSAC (Space Science Advisory Committee), the SPC (Science Programme Committee) approved in November 1983 the Executive's proposal to proceed with a Phase-A Study for SOHO.
At the same time, it was recommended to explore the possibility of including SOHO and Cluster in an International Solar- Terrestrial Physics (ISTP) Programme to be undertaken jointly by ESA, NASA and ISAS. In May 1984, the Survey Committee identified SOHO as a component of the Solar-Terrestrial Physics (STP) 'Cornerstone' of the ESA long-term programme 'Space Science: Horizon 2000'.'
These paragraphs reveal that SOHO was proposed 13 years before its actual launch, but that within less than three years it had become part of a Horizon 2000 Cornerstone. They do not, however, reflect the fact that the roots of SOHO were laid in earlier studies, namely those of GRIST (Grazing Incidence Solar Telescope) and DISCO (Dual Spectral Irradiance and Solar Constant Orbiter). It is the combination of the objectives of these two missions that constitutes the core of the SOHO mission.
Indeed, most ESA missions - especially those in 'new' fields - are the result of an evolution rather than a single proposal. Often such studies eventually involve the communities of several space-science disciplines. Aligning these communities behind one coherent proposal involves several steps and can thus take quite some time.
In fact, already in June 1976, GRIST had been competing with a 'Solar Probe' as well as other studies involving other disciplines) for further study, under Phase-A. Solar Probe envisaged a set of instruments on a spacecraft that would approach the Sun once to within four solar radii. Although its Assessment Study cited four scientific disciplines interested in the mission: (i) solar wind and space plasma, (ii) solar atmosphere, (iii) solar and stellar interior, and (iv) experimental gravitation and relativity, Solar Probe was not followed up at the time.
The GRIST study, on the other hand, went into Phase-A. It foresaw a razing-incidence telescope (feeding several focal-plane instruments) that was to be mounted on the Instrument Pointing Systems (IPS) and flown as part of Spacelab payload. Once of the reasons for GRIST's preference over Solar Probe was that the wavelength range accessible through grazing-incidence optics is particularly powerful for spectroscopic diagnostics of the hot outer solar atmosphere. Spectroscopy in this domain had long been neglected on major solar satellites (Skylab 1973, Solar Maximum Mission 1980, Yohkoh 1991), partly because of experimental difficulties. ((In fact, an instrument covering the extreme ultraviolet, which had been under development in the USA, had to be abandoned due to severe cost overruns. The pressure to develop a similar instrument was therefore very strong).
For the Phase-A Study of GRIST, a team of scientific consultants (A. Gabriel, U. Grossmann-Doerth, M. Huber, M. Malinovsky, G. Tondello and F. van Beek) was selected through an open solicitation in the scientific community. This team worked together with ESA staff (G. Haskell and G. Whitcomb) in writing specifications for the industrial study, which was carried out by the British Aircraft Corporation (later BAe) together with the UK National Physical Laboratory and the University of Leicester. GRIST was at that time designated for multiple flights on Spacelab, mounted on the Instrument Pointing System (IPS). Its smallest picture element in the ultraviolet was planned to be 1 x 1 arcsec² - a performance somewhat exceeding even that of SOHO.
Following the Phase-A Study (1976-78), accommodation studies were made with the intention of flying GRIST alongside NASA's Solar Optical Telescope (SOT) on Spacelab. V. Domingo joined the above group as Study Scientist during that time.
Like the Solar Probe, GRIST did not make it to project selection either: being based on a collaboration with NASA, in early 1981 it became a victim as part of ESA's response to NASA's unilateral cancellation of the US probe in the 'International Solar Polar Mission' (ISPM, the former 'Out-of-Ecliptic Mission', now called 'Ulysses'); GRIST was 'moth-balled'. Fortunately, however, restricted studies (concentrating mainly on the spacecraft interface) of the main spectrometers of GRIST were further supported by ESA.
In the course of 1980, many leading solar physicists took part in two international conferences at which prospects for space observations of the Sun were discussed. The first, an ESLAB Symposium dealing mainly with aspects of the solar irradiance, was held in Scheveningen (NL) in September 1980; the second, on 'Solar Physics from Space' and addressing mainly coronal observations, was organised in Zurich (CH) in November 1980.
In July that year, in response to an ESA Call for Mission Proposals, a group of French and Belgian scientists (R.M. Bonnet, D. Crommelynck, J.P. Delaboudinière and G. Thuillier) proposed a mission dedicated to the study of spectral irradiance and the solar constant. This was considered an important objective in view of the possible climatic effect of a long-term variation in solar irradiance.
Simultaneously, the heliospheric community was becoming aware of the fact that, although the solar wind in the ecliptic plane had been almost continuously monitored for the past fifteen years, there was every chance that this surveillance might cease before ISPM. This anxiety was expressed in a resolution unanimously adopted at the ISPM Science Working Team meeting in October 1980. They urged that priority be given to complementary baseline measurements by a spacecraft in the ecliptic at about 1 AU distance during the heliographic high-latitude passes of the ISPM spacecraft, at that time envisaged for 1989/1990 (the actual passes occurred five years later).
At nearly the same time, in the austral summer of 1979-1980, a group of French and American physicists observed the Sun continuously from Antarctica between 31 December 1979 and 5 January 1980. They thereby succeeded in measuring the global velocity oscillations of the Sun with an unprecedented signal-to- noise ratio. These historic observations led to the decision to include helioseismology velocity observations on-board DISCO, as are now being performed on-board SOHO with the GOLF experiment.
At a meeting between the late P. Delache from the Nice Group and R.M. Bonnet at the Institut d'Astrophysique in Paris, it was proposed to locate DISCO at the L1 Lagrangian point between the Sun and the Earth, which would be an ideal observing site for these velocity observations because of the spacecraft's low radial velocity relative to the Sun. A miniaturised version of the South Pole experiment (then weighing several hundred kilos) could be embarked as part of DISCO's payload, provided its weight could be considerably reduced.
In addition, the potential for helioseismology of solar brightness oscillations, as evidenced by the high quality of the solar-constant data obtained by the ACRIM instrument on SMM, offered a unique asset to the mission which could, for the first time, attempt to detect the Sun's global oscillation modes and shed new light on the intriguing solar neutrino deficit issue. An instrument measuring brightness oscillations would therefore add a substantial helioseismology element to the radiance and irradiance instruments. Accordingly, DISCO's model payload was extended to contain a set of photometers and absolute radiometers to perform measurements of the total and spectral irradiance in selected bands and to detect solar oscillations in visible light, as are now being performed by the VIRGO experiment onboard SOHO.
DISCO was also going to carry a far-ultraviolet spectrometer to study coronal holes. It was conceived as a fairly small and cheap spin-stabilised spacecraft, weighing no more than 520 kg (dry weight) and, in the minds of its proponents, it was supposed to prove that ESA could also undertake small and inexpensive missions (already in 1980!).
A first assessment was made and the results published in the Assessment Report in May 1981. At its meeting in June 1981, the Science Advisory Committee (SAC) recommended a re-assessment of the mission, addressing the following topics in order of priority: (i) solar seismology (brightness and velocity), (ii) baseline in-ecliptic measurements in support of the ISPM mission, and (iii) measurements of the solar irradiance. The SAC also insisted that the overall cost of the revised DISCO mission must not exceed that of the original proposal. The resulting model payload had been selected to be as consistent as possible with these objectives (Fig. 2).
Figure 2. The concept DISCO spacecraft at the time of the
reassessment study
The science team for the study was made up of A. Balogh, R.M. Bonnet, P. Delache, C. Fröhlich and C. Harvey. D. Wyn- Roberts was the study engineer from ESTEC and V. Domingo the study scientist. The SPC decided in February 1982 to proceed with a Phase-A Study, which was conducted between April and November 1982 by British Aerospace.
Upon completion of this study, DISCO had remained a relatively inexpensive spinning satellite, very similar in fact to a Cluster satellite. At its meeting in January 1983, the Solar System Working Group preferred DISCO to a competing Mars mission called 'Kepler'. However, DISCO eventually lost out to ISO (the Infrared Space Observatory) in the final evaluation by the Space Science Advisory Committee, and thus ISO was approved as a new project by the Science Programme Committee in March 1983.
ISO, eventually launched in November 1995, just two weeks before SOHO, was an exceptional case. It was proposed by the 'new' infrared community, and went straight from mission proposal via Assessment and Phase-A Studies to approval as a new project. Although selected before Horizon 2000 and thus carried as a pre- selected 'medium-size mission' in the Horizon-2000 Plan, its overall cost was close to that of a Cornerstone today.
SOHO aims to answer the following three fundamental questions about the Sun:
It also addresses the influence of the Sun on its environment, the heliosphere, as well as the ecliptic plane in which the planets and their moons orbit the Sun.
The orbit of SOHO around the first Lagrangian point, L1, at a distance of 0.01 Astronomical Units from Earth (cf. Fig. 1), is central to the mission design: it provides a perfect vantage point for the investigations required to answer the above three questions. The satellite is located outside the absorbing, blurring and scattering terrestrial atmosphere, and outside the magnetic shield of the Earth's magnetosphere. Consequently, SOHO has access to the entire electromagnetic and particle spectrum of the Sun. In addition, observations are continuous, as there are no occultations of the spacecraft's line of sight. Furthermore, the relative velocity between satellite and Sun is small and varies slowly, a key element as discussed in the DISCO studies for helioseismology velocity measurements.
For all of these reasons SOHO, designed as a three-axis- stabilised spacecraft continuously pointing to the Sun with a stability that has turned out to be even better than the design value of 1 arcsec, is an ideal platform. It permits uninterrupted investigation of the ultraviolet and soft X radiation and the particle streams that are formed in the hot outer layers of the Sun's atmosphere and optimum conditions for probing the solar interior by the method of helioseismology.
In loose analogy to the above three basic questions to be answered by the mission, SOHO carries three payload segments:
As noted earlier, the relationship between the investigations of the three questions listed initially and the three payload segments is not a strict one, because studies of the acceleration of the solar wind and of the structure of the corona require both remote- and in-situ sensing of the solar-wind streams.
Following a schedule dictated by budget availability within the ESA Science Programme, on 6 July 1982 ESA's Director of Science, E. Trendelenburg, released a Call for Mission Proposals in relation to the new planning cycle. However, the early submittal date of November 1982, i.e. at a time when DISCO was in its last months of candidacy for project selection, created an undesirable conflict for solar physicists. Since they had to submit the proposal for SOHO before a decision on DISCO had been taken, they would automatically lessen the chances of DISCO's selection, or might even be told that no new solar mission was needed until a decision on DISCO was available. It was therefore proposed to ESA, by R. Bonnet, M. Huber and A. Gabriel among others, that the call for new mission ideas should be postponed until after the selection of the next project (which, as mentioned above, took place in March 1983). However, the budget-availability deadline could not be missed and ESA declined to modify its schedule.
As a consequence, the solar-physics community started discussing a new mission which would combine some of the objectives of GRIST and of DISCO. Initially, the model payload of the new mission on a spacecraft in low Earth orbit consisted primarily of high- resolution ultraviolet spectroscopic equipment and, accordingly, the mission was dubbed the Solar High-Resolution Observatory, or SOHO (but with 'high-resolution' not yet replaced by 'heliospheric'). This mission proposal was motivated by the persistent lack of solar investigations in the extreme- ultraviolet (or more precisely, in the grazing-incidence) domain, as well as by then recent, enigmatic measurements of Doppler- shifted coronal emission (implying a solar-wind outflow starting already in the inner corona). In addition, innovative measurements of transverse coronal outflows by the so-called 'Doppler-dimming method' had just been demonstrated. Moreover, the advantages of placing a coronal payload on a free-flyer like SOHO (with a mission duration of at least a few years) rather than on even a series of ten-day Spacelab flights had been dreamt of during the GRIST study.
Although the scientists involved in the SOHO proposal (who were primarily interested in solar physics) did not wish to be seen to be jeopardising the chances of DISCO's potential selection, they agreed with their colleagues to envisage including the helioseismology objectives in those of the new mission at a later time. In that case SOHO would be placed at a Lagrangian point outside the magneto-sphere and could then be renamed the Solar and Heliospheric Observatory if DISCO was not selected.
In December 1982 it was recommended that SOHO (in its high- resolution version) be pursued as an Assessment Study. In order to create a larger base for an eventual project, it was recommended by the Solar System Working Group that a particle payload segment be included in the model payload, as in DISCO's case. V. Domingo and D. Wyn-Roberts became study scientist and engineer, respectively, the latter being succeeded by J. Ellwood during the Phase-A Study.
During the first months of the SOHO study, in February 1983, it became clear that: (i) helioseismology should definitely be added to the set of spectroscopic solar telescopes forming the original payload; (ii) SOHO should, like DISCO, be placed in a halo orbit around L1 in order to be compatible with the helioseismological objectives, and (iii) in the new orbit, the 'particles-and- fields' instruments should be devoted to solar-wind composition measurements, to the study of solar energetic particles, as well as to the investigation of waves in the interplanetary medium. The science team, composed of H.F. van Beek, P. Delache, M. Huber, M. Malinovsky-Arduini, B. Patchett, H.B. van der Raay and R. Schwenn, already reflected this multi-disciplinary approach.
The studies of DISCO and SOHO coincided with the cancellation by NASA of their ISPM satellite and its aftermath. This was a time of extreme tension between Europe and the USA, and there was little interest in ESA in starting a new cooperative venture with NASA, which explains why DISCO and SOHO (in its Assessment Phase) were studied as purely European missions.
This is not to say that the scientific communities on opposite sides of the Atlantic did not discuss future potential cooperation on missions whose objectives were very similar. For example, NASA had a mission to study the interior of the Sun, the Solar Internal Dynamics Mission, whose objectives were indeed in the spirit of DISCO, with probably more emphasis on the study of the solar dynamics. After a meeting organised by NASA in early 1982 in Boulder, at which most of the active US scientists in the field were present, together with the DISCO scientists, it was decided that a special study group would be convened to define more precisely the aims of this mission.
NASA also conducted a definition study of a Solar Interplanetary Satellite (SIS) carrying the payload of their cancelled ISPM spacecraft and operating in a drift orbit at 1 AU some 90° behind the Earth. SIS was under consideration for a new start in NASA's 1984 Fiscal Year, but was never approved.
In the course of the discussions that took place between European and US scientists, the latter insisted that DISCO should include a high-resolution imaging instrument (the equivalent of what is now the MDI on SOHO) in order to study the high-order modes of the velocity oscillations. They strongly favoured transforming DISCO into a three-axis-stabilised satellite, which unfortunately would have put the project into the class of expensive missions, an option that was of course strongly opposed by the Europeans. In addition, a controversy developed on the NASA side, which was not fully convinced of the need to go into space to perform helioseismology observations when excellent results could be obtained from the ground, as demonstrated by the South-Pole observations. This controversy lasted for quite some time until NASA (D. Bohlin) recognised in the summer of 1983 at a meeting in Snowmass (Colorado) that space-based measurement would offer much better observing conditions and a strongly enhanced signal- to-noise ratio, as SOHO is proving today.
As the ISPM crisis slowly settled down, it was proposed that SOHO should be pursued with two potential launchers in mind (Ariane and the Space Shuttle). As a result, both the European and the US solar space communities could identify with the mission. Thus, the Phase-A Study was initially made with the participation of US scientists, with the support of NASA. This was the so-called 'ISTP-phase' (named after the International Solar-Terrestrial Physics Programme). When it became clear, some time before the end of the study, that NASA was not able to obtain a new start for the SOHO mission, a rider study for a purely European mission was performed. The results of this study for a European SOHO were those presented at the time of project selection in early 1986.
Before this date, there were several activities of great significance for the fate of SOHO, namely the International Solar-Terrestrial Working Group (that led to a Joint ESA/ ISAS/NASA Planning Group for the ISTP Programme being set up) as well as the preparations for the long-term scientific programme Space Science: Horizon 2000, to which we will now turn.
At one of the regular consultation meetings between ESA and NASA in June 1983, it was agreed that an integrated look should be taken at the large number of missions under study in the USA, Europe and Japan in the area of solar-terrestrial physics. NASA and ESA therefore organised a preparatory meeting in September 1983, to which the Japanese Institute for Space and Astronautical Science (ISAS) was invited.
After extensive discussion and a rather painful rationalisation process, the 'International Solar-Terrestrial Physics (ISTP) Programme' (Fig. 3) was formulated. It embodied a reduced version of NASA's previous 'Open' programme, now consisting of four spacecraft - 'Wind' (measuring the solar wind and space plasma properties near the Lagrangian point L1; cf. Fig. 1), 'Equator' and 'Polar' (in near-Earth orbits) and 'Geotail'. New 'add-ons' were Cluster and SOHO. The late Stanley Shawhan, who chaired this trilateral meeting and masterfully guided the rationalisation process, must be considered spiritus rector of the ISTP programme.
Figure 3. The original schedule for the International Solar-Terrestrial
Science Programme, as elaborated at the trilateral ESA/NASA/ISAS
Preparatory Meeting on Solar-Terrestrial Science,
held in Washington DC on 26 and 27 September 1983. The two last
missions, designated (ESA) 'Solar' and 'Multipoint', are the
equivalent of SOHO and Cluster. Note that all of the missions
shown on this figure from 1983 are now either flying, ready for
launch (Cluster), or under development (Equator-S)
It was argued in that preparatory meeting, by G. Haerendel, that SOHO and Cluster ought to be flown together: both were addressing the same physical structures and processes by remotely sensing the coronal plasma, by in-situ measurements of the solar wind, and by in-situ investigations in three dimensions of the magnetospheric plasma. It was pointed out, however, that the two missions were under Phase-A study only and that, given the usual ESA selection process, it would be almost impossible for both SOHO and Cluster to be selected as projects. (As we shall see, this impediment could be overcome thanks to the introduction of Cornerstone missions within Horizon 2000.)
Some of the scientists who had helped to formulate the ISTP programme at the preparatory meeting of September 1983 were asked to participate in a Joint Planning Group for ISTP, which then slowly built up the programme. As implied in the last but one paragraph, the origins of the ESA/NASA collaboration on SOHO and Cluster (the two missions that later became the first Cornerstone of Horizon 2000) can also be traced back to the Joint Planning Group.
The long term-programme that became known as 'Space Science: Horizon 2000' was a large community effort, guided and finally formulated by a Survey Committee composed of senior European space scientists, including all of the members of the ESA Space Science Advisory Committee. The procedure was started with a call to the wide scientific community for mission concepts, which were to form the basis for the Survey Committee's deliberations. After examining these mission concepts, Topical Teams drafted long-term plans for their respective disciplines.
The Topical Team for Solar and Heliospheric Physics concluded that 'a vigorous solar and heliospheric research programme commensurate with the capability, vitality and needs of the corresponding European community should be given strong support', and that 'the SOHO mission ... would provide major support for the International Solar-Terrestrial Physics Programme.'
The Space-Plasma Topical Team stated that 'of the missions under study, SOHO and Cluster (were) the ones of most interest to the plasma physicist'. They added: 'although each of them can stand on its own, much is to be gained if they are carried out as a European contribution to the International Solar-Terrestrial Physics Programme ...'.
At the final meeting of the Survey Committee in May 1984 in Venice, there were originally only three Cornerstones foreseen. These were what today are the XMM and the FIRST missions (covering X-ray and far-infrared astronomy), as well as a large, but not yet defined planetary mission. Given the European leadership in cometary research and the vigorous community who were working on meteorites and lunar samples, it was decided to define this third Cornerstone to be a 'Mission to Primitive Bodies including the Return of Pristine Materials'.
It was a surprise when a fourth Cornerstone, consisting of the SOHO and Cluster missions, and originally called the 'Solar- Terrestrial Physics (STP) Cornerstone', was introduced by M. Huber - then chairman of the Solar System Working Group - following a comment by CERN's L. van Hove regarding the bias toward astronomy (to the detriment of the solar-system sciences) that was inherent in the original plan with just three Cornerstones. It may well be that it was not immediately clear to all meeting participants what the STP Cornerstone was actually to be, in particular that it took up G. Haerendel's idea to combine SOHO and Cluster in one programme. In any case, the Executive returned next morning still with their original plan of three Cornerstones. This immediately attracted criticism from K. Fredga, B. Hultqvist (who had already given strong support to the idea of the STP Cornerstone the day before) and others. The ensuing discussions produced an admission of the feasibility of the STP proposal from the cost and schedule viewpoints and it was therefore reinstated. It quite naturally became the first Cornerstone, being renamed the 'Solar-Terrestrial Science Programme' (STSP) to make it a distinct element of the much larger ISTP Programme. The inclusion of this Cornerstone balanced the Horizon 2000 Programme between the disciplines represented by active researchers at the time.
Prior to the 1984 Survey Committee recommendation that SOHO and Cluster be combined to form the STSP Cornerstone, the two projects had been studied separately at both Assessment and Phase-A levels. In parallel, but within the framework of the Assessment Studies, discussions had taken place with NASA concerning a potential joint ESA/NASA/ISAS International Solar-Terrestrial Programme (ISTP) (see earlier). The Phase-A studies were conducted bearing in mind a potential ISTP collaboration between ESA and NASA.
Following the combination of the two projects as a Horizon 2000 Cornerstone, and when NASA's involvement became uncertain (for lack of a new start), further studies were conducted to investigate potential solutions for a purely European Solar- Terrestrial Physics Programme (ESTP). The conclusions were presented to the ESA Science Programme Committee (SPC) in February 1986. The SPC confirmed the choice of SOHO and Cluster as the first Cornerstone of Horizon 2000, but unfortunately the studies had resulted in an estimated cost of 555 Million Accounting Units (1984 economic conditions) for the European version and 562 MAU for the international option, not including the launch vehicle. In the worst-case scenario, there-fore, where launchers would have to be purchased at full cost, the additional funds required could amount to approximately 200 MAU.
The main differences between the European and international options reflected a more comprehensive science payload and a potential STS (Shuttle) plus Upper Stage launch for the latter option. This placed additional technical/cost demands on the SOHO spacecraft and flight operations in particular.
In addition to these Phase-A results, an independent cost estimate, which included more recent developments in internal costs, was made and this amounted to some 590 MAU for the International STSP with-out the launch vehicles. It was this estimate that was taken as the technical and cost baseline.
The SPC wished, however, to impose the Cornerstone financial limit of 400 MAU (again at 1984 economic conditions) for the total ESA Science Programme contribution corresponding to either recommendation of the Survey Committee. Consequently, major cost surgery was needed and the SPC requested the Executive to vigorously pursue the financial goals set, without serious erosion of the science objectives. From now on, the cost of SOHO and Cluster was treated as a package.
The problem was to be approached from two directions. The first task was to involve the scientific community in a scientific and technical descoping exercise, with the intention of reducing the Cluster and SOHO spacecraft development and operations costs. The second task was to explore with NASA (the Phase-A partner) the possibility of increasing the international share in the missions and thereby reduce the overall cost to ESA.
In 1986 a scientific committee (Science Advisory Group, SAG) was set-up under the Chairmanship of D. Southwood to review and rationalise the scientific and technical requirements. In parallel, discussions were initiated with the NASA ISTP Project in pursuance of increasing the NASA share. Within both initiatives, a number of options were listed for technical and cost study.
Whilst the SAG was primarily focussed on scientific rationalisation of the mission, there was a degree of overlap with the activity concerning increased international participation. NASA had experienced financial problems in obtaining continued approval for their Equator mission, and wished to find a solution to this problem within the context of an arrangement for increased participation in STSP. Specifically, NASA requested that ESA explore the possibility of one of the four Cluster spacecraft performing an 'Equator mission' prior to joining the three remaining Cluster spacecraft and then completing the planned mission of STSP. This request was maintained under review by the SAG. Clearly, it had obvious technical/cost consequences originating from a combination of the two payloads and introducing a capability for Cluster to operate in two very different orbits over a longer period than the original Cluster requirement. As negotiations progressed, the desire to accommodate Equator requirements into the Cluster mission constituted the greatest risk in being able to reach an agreement on a package of increased NASA participation: the cost and technical risk to the Cluster mission became the primary ESA concern in seeking to accommodate the Equator requirements. Following extensive studies of the various options in both domains during 1987, by the end of the year promising proposals for reducing costs were emerging.
Major factors of scientific and technical rationalisation recommended by the SAG included:
For Cluster:
For SOHO:
Figure 4. The SOHO spacecraft in launch configuration
Concerning the dialogue with NASA, progress was made along several avenues, including:
Unfortunately at the time, the request concerning Equator was becoming a major block to reaching agreement. It became evident, however, through the multiple interactions, studies and reviews, that the risk to Cluster was not in the interests of either party, and consequently NASA reluctantly agreed to drop the request.
Clearly, even though major savings to ESA were becoming viable, the full launch cost for Cluster was still a problem. The mission needed a full Ariane-4, resulting in some 100 MAU of additional cost which simply could not be met. Following extensive representation with the Ariane Programme, agreement was finally reached that Cluster could be launched as an APEX passenger on one of the Ariane-5 qualification flights, at a cost of about 13 MAU to cover APEX charges.
By November 1987, the complete package of science descoping/rationalisation, expanded international cooperation, together with the APEX opportunity for Cluster, offered a real opportunity for cost reductions within STSP, resulting in an estimated cost to ESA of 500 MAU, including launch vehicles: a saving of some 250 MAU! However, the problem still remained of how to meet the SPC limit of 400 MAU.
In dialogue with the SPC, it had been concluded that for the particular case of STSP, a special Cornerstone target revision from the canonical 400 MAU to 460 MAU (1984 economic conditions) could be permitted, but further actions had to be undertaken to identify further reductions below the 500 MAU estimate. In the meantime, the Payload Announcement of Opportunity could be released.
It was decided that the best approach for studying further cost reductions would be a joint effort by the ESA project and the scientific community, represented by a small committee under the chairmanship of H. Balsiger.
A very detailed and careful joint review of all project costs and cost-estimating history was conducted, and several economies were introduced into the project team (manpower, other ESA facility and manpower costs, together with estimates for the industrial elements). This resulted by end-1987/ early-1988 in a total STSP estimate of 484 MAU (1984 economic conditions), compared with the SPC's revised limit of 460 MAU.
Whilst this estimate did not fully match the desired limit, it was felt that sufficient confidence now existed in both the technical and cost project baselines that STSP should go ahead and that the estimates should be further revised at the end of Phase-B, when the real industrial costs would be known.
During the process of pre-Phase B and Phase-B itself, some of the agreements concerning both scientific rationalisation and international cooperation were modified, but in introducing these into the Phase-B, working in close co-operation with Industry, the final cost estimate presented to the SPC amounted to 474 MAU, a figure that it found acceptable. Both the Cluster and SOHO projects effectively completed their Phase-B in the spring of 1991 with viable solutions from the standpoints of science, full mission technical integrity and cost. The continuous pressure on cost initiated in 1986 had resulted in a mission saving to ESA of approximately 275 MAU and had provided a practical implementation opportunity for the first Cornerstone of Horizon 2000.
There were three remarkable aspects to SOHO's payload selection. The first concerned the decision of the Director of Science to remove the plasma payload, including (to the great dismay of the USA's N. Ness and Germany's H. Rosenbauer) the magnetometer. This saved considerable cost by avoiding a magnetic-cleanliness programme quite a dramatic issue given the numerous mechanisms in the large optical instruments.
The second major event in the selection process was the re- institution by the Solar System Working Group of the proposed Ultraviolet Coronograph Spectrometer (UVCS). The peer review committee recommended flying a visible-light coronograph (LASCO) only, since both UVCS and LASCO had been proposed by American PIs and flying both of these instruments was claimed to be placing too much of a strain on NASA's resources. However, the novelty of the UVCS measurements and the substantial Italian participation in and contribution to the UVCS instrument eventually led to its reinstatement into the payload.
The third issue concerned the Extreme-ultraviolet Imaging Telescope (EIT), which was preferred over the X-ray Telescope (XT). While the latter design featured a telescope imaging the entire solar disc and lower corona using the more conventional grazing-incidence optics, EIT was to be a rather novel design of normal-incidence optics with selectively reflecting thin-layer coatings peaked around four prominent solar lines in the EUV, a design never before flown on a satellite. Eventually, the better image definition at the solar limb afforded by the EIT was the reason for preferring the more novel design, although it obviously carried greater risk.
A rather dramatic decision had to be taken when it turned out that the industrial contractor building the imaging detectors for two of the instruments - SUMER and UVCS - was not capable of meeting its technical commitments within schedule (already during the GRIST study it was proposed that ESA's Technology Research Programme should pursue the development of European UV imaging detectors suitable for the count rates associated with solar observations, but no such effort materialised). Barely one and a half years before launch, it was decided to replace the original design with so-called 'XDL-detectors'. They were built in a University of California (Berkeley) laboratory, without formal product assurance in order to meet the schedule and to stay within the budgetary constraints.
The very involved and eventful history of SOHO has led to a payload and overall mission design are state of the art in terms of solar space observations and the latest developments in solar physics, especially as regards the new discipline of helioseismology. The data now coming in from the ensemble of observing instruments on-board SOHO are redeeming the careful planning and fully justifying the considerable effort and finance invested in the realisation of this important mission.
The enthusiastic international atmosphere prevailing at the Experiment Operations Facility (EOF), where the investigator teams plan the joint observing programme and receive and evaluate the quick-look data, are a magnificent testimony to the power of science to transcend national and discipline boundaries and establish its own international and multi-disciplinary culture!
An article devoted to the early scientific results from SOHO will appear in the next issue of ESA Bulletin.
ESA Bulletin Nr. 86.