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European Space Agency

The Resurrection of the Cluster Scientific Mission

J. Credland

Head of Scientific Projects Department, ESA Directorate for Scientific Programmes, ESTEC, Noordwijk, The Netherlands

R. Schmidt

Cluster Project Scientist, Space Science Department, ESA Directorate for Scientific Programmes, ESTEC, Noordwijk, The Netherlands

The Cluster mission was first proposed to the Agency in late 1982 and was subsequently selected, together with SOHO, as the Solar Terrestrial Science Programme (STSP), the first Cornerstone of ESA's Horizon 2000 Programme. This article gives an overview of the complex chain of events that have taken place between the loss of the original mission with the Ariane-5 launch failure and the recent approval of the recovery mission known as Cluster-II.


The launch of Cluster, one of the Cornerstone missions of the ESA Scientific Programme, was scheduled for the early morning of 4 June 1996. After a short delay during the countdown due to bad weather over the launch pad, Ariane-5 rose flawlessly to an altitude of 3.5 km, at which point a sudden swivelling of both solid-booster nozzles caused the vehicle to tilt sharply. The resulting intense aerodynamic structural loads caused the launcher to begin to break up, prompting the onboard safety systems to initiate self-destruction of all launcher elements. The original four-spacecraft Cluster mission was lost in the ensuing explosion.

Cluster-II is a new four-spacecraft mission for which the go-ahead was given on 3 April 1997, ten months after the loss of the original spacecraft. Cluster-II will be basically identical to the previous mission except for changes introduced as a result of the non-availability of components. The launch date for the new mission has been set for mid-2000. The four spacecraft will be launched by two Russian Soyuz rockets, from Baikonur in Kazachstan.

The original Cluster mission was planned to conduct an in-situ investigation of plasma processes in the Earth's magnetosphere using four identical spacecraft simultaneously. It would permit the accurate determination of three-dimensional and time-varying phenomena and make it possible to distinguish clearly between spatial and temporal variations for the first time. The four Cluster spacecraft were to be placed into nearly identical, highly eccentric polar orbits, with a nominal apogee of 19.6 Re, and a perigee of 4 Re. Such an orbit is essentially inertially fixed, so that in the course of the two-year mission a detailed examination could be made of all significant regions of the magnetosphere. If launched in summer, the plane of this orbit bisects the geomagnetic tail at apogee during the northern-hemisphere summer, and passes through the northern cusp region of the magnetosphere six months later.

The Cluster spacecraft also carried their own propulsion stages in order to perform all necessary manoeuvres for the transfer from the Geostationary Transfer Orbit (GTO) to the final polar orbit. In addition, it was required to change the in-orbit constellation of the satellites periodically by modifying the distances between them to match the scale lengths of the plasma phenomena being investigated. The relative separations would therefore vary from a few hundred kilometres to a few Earth radii. In the original programme baseline, all four spacecraft had to be injected into GTO on a single Ariane-5 launch vehicle and then transferred in pairs to their mission orbits via a complex series of orbital manoeuvres.

This article describes the sequence of events that eventually led to the approval of Cluster-II by ESA's Science Programme Committee (SPC). Numerous meetings took place in order to either prepare position papers or, more often, to re-act to new challenges raised by either ESA committees or national agencies. The following paragraphs highlight the most important milestones in the decision process, without addressing in detail the many technical and management meetings held with industry and the scientific institutions.

The ten-month period from the launch disaster to Cluster mission recovery was rich in excitement, disappointment and emotional discussion. It was also characterised by the excellent support from and collaboration with the scientific teams, the national agencies and industry, without whose help Cluster-II would never have materialised.

first Ariane-5 test flight
Figure 1. The first Ariane-5 test flight leaving the pad

In fact, Cluster-II will retain all of the critical mission parameters, including the original orbit. The launch scenario will, however, be totally different due to the use of the Soyuz vehicle. Two launchers, each carrying a pair of satellites (so-called payload 'stack'), will deliver them into a low circular orbit, about 250 km high and with a 51.8° inclination. A kick stage attached to each stack will raise the apogee to 18 000 km. Separation from the kick-stage will then take place and each satellite will be transferred to the final 4 by 19.6 Re orbit using its own propulsion capability.

The road to recovery

One day after the Ariane-5 launch failure, Roger Bonnet, the Director of ESA's Scientific Programme, met with the Cluster Principal Investigators to assure them of his full support regarding the recovery from the loss of Cluster. It was agreed that a Science Working Team (SWT) meeting would be held immediately after returning to Europe. This meeting took place on 17 and 18 June in Paris. Taking advantage of the fact that most ESA and Dornier engineers were still in Kourou, brainstorming meetings were held there to devise a cost-efficient recovery strategy based on maximum re-use of spare elements, including the payload.

Within a few days, the Cluster Project proposed a replacement mission to the scientific community based on the remaining structural model of the spacecraft, which could be made ready for flight in little more than a year. This spacecraft, initially called Cluster-5 but later renamed 'Phoenix', was considered to be the first of a new fleet of Cluster spacecraft. The rationale for recommending an early Phoenix launch was to keep the scientific and industrial teams together until approval for a full replacement mission could be secured.

This proposal did not find unanimous support among the science community because it was feared that the proposed launch of the single Phoenix would be seen as sufficient compensation for the loss of the four original spacecraft. With hindsight, the immediate start on the assembly of Phoenix turned out to be the correct approach, keeping both the scientific and industrial teams together as the battle for approval for three more spacecraft lasted far longer than was originally expected.

The scientific case for the quick launch of a single Phoenix spacecraft into a 'Cluster-type' orbit rested upon two central arguments: (a) primary investigation using modern high-resolution plasma and field instrumentation of key magnetospheric regions, principally the high-altitude dayside cusp, and (b) co-ordinated observations with a number of other key spacecraft from Japan, NASA and Russia making up the International Solar-Terrestrial Physics Programme (ISTP), and with the new ground-based infrastructure that was timed for Cluster. A further important collaborative aspect was the execution of coordinated measurements with ground-based observatories located in the northern hemisphere.

The Principal Investigators, meeting on 17 June for the first in a long series of emergency SWT meetings, stressed that an early launch of Phoenix could in no way achieve the scientific objectives of the four Cluster spacecraft. They re-confirmed the original Cluster scientific objectives and, after an extensive discussion of the scientific aspects of the recovery scenario, prepared a resolution listing two possible options that could meet those objectives:

  1. Re-fly the Cluster mission: this option would Use four re-built Cluster spacecraft and would totally recover the scientific objectives Of Cluster and a substantial portion of the ESA Solar Terrestrial Science Programme (STSP). This was the highest priority option and the one that would best exploit previous Investments.
  2. Re-build one Cluster spacecraft, namely Phoenix, and launch it together with three potentially smaller spacecraft provided through a special programme with national agencies.
Two important meetings took place on 21 June at ESA Headquarters in Paris. The recommendation by the SWT was presented to the Space Science Advisory Committee (SSAC), which met that morning. Having heard the recommendation of the Cluster Science Working Team, the SSAC advised the Science Programme Committee (SPC), which convened for an emergency meeting immediately afterwards, that, as an initial step, the Phoenix spacecraft should be made ready for flight without delay, with the aim of launching it in the summer of 1997. An early launch of Phoenix was considered essential for Europe to remain a valid partner in the International Solar Terrestrial Physics (ISTP) Programme.

The SPC did not reach a consensus on how to proceed and postponed the decision to its subsequent meeting in early July. It did, however, accept the SSAC's view that there was a strong case for Phoenix as part of ISTP, but several Delegations were reluctant to proceed with it without a simultaneous commitment to the remaining three spacecraft. The Chairman therefore asked the Delegates to consult their science communities and insisted that a decision on Phoenix needed to be made at the next meeting. He proposed that further iterations be undertaken during the summer regarding procurement of the remaining three or four spacecraft.

The Cluster community anxiously awaited the outcome of the 3 July meeting, at which the SPC was briefed on the costs of potential recovery missions for Cluster. The total cost of Phoenix, assuming a free launch in mid-1997, amounted to 30 MAU. The cost for rebuilding four Cluster satellites according to the original design, without changes to either the spacecraft or the payload, were predicted to be 351 MAU. If a completely new design of mission had to be implemented, then the estimated costs would rise to about 640 MAU. The discussions also focused on the potential impact of a recovery mission on the overall Scientific Programme, as the Directorate would not be given fresh money for the implementation of a recovery mission. It was clear that most future missions, except those already being implemented, would have to suffer delays and therefore solidarity and support from the other science communities that would be affected was extremely important.

The SPC finally recommended going ahead with the industrial contract for Phoenix and requested that studies be conducted to determine the most cost-efficient way of launching Phoenix and procuring the remaining three satellites to achieve a four-spacecraft mission by the year 2000.

four cluster in the swamps
Figure 2. Remains of one of the four Cluster spacecraft in the swamps of Kourou

This request triggered three main activities: immediate negotiations were started with Dornier regarding the procurement of Phoenix; in parallel, ESA also began negotiating cost reductions for a full implementation of Cluster, by procuring either three or four more spacecraft from Dornier, an activity termed Cluster-II Option 1; and studies were begun of realising the recovery mission by using nationally provided small satellites, an activity known as Cluster-II Option 2.

Following the SPC's recommendation, Prof. Gerhard Haerendel of the Max-Planck Institute in Garching (D) called in mid-July for a meeting of engineers and representatives of the national agencies to initiate a study of Option 2 with the aim of procuring small spacecraft able to fulfil all of the scientific objectives of the original Cluster mission. It appeared feasible to build spacecraft of about 1.4 m diameter which could carry the Cluster payload and be launched in pairs by Ukrainian Cyclone launch vehicles. It was decided to carry out a full study and provide a final report in time for the SPC's scheduled 28/29 November meeting.

The ESA Solar System Working Group (SSWG), which provides detailed scientific advice to the SSAC, met on 22 and 23 July for a special brainstorming session on the recovery of Cluster. The SSWG reiterated that the Cluster scientific objectives remained compelling and timely and it recommended the early launch of Phoenix and expressed its solidarity and willingness to assist in exploring solutions that would meet the most essential scientific needs of the European space-plasma community.

Phoenix the final stages integration
Figure 3. The Phoenix spacecraft during the final stages of its integration

In order to ensure that Cluster-II Option 1 would receive as much scientific attention as Option 2, Prof. André Balogh of Imperial College London (UK) was nominated as its co-ordinator, playing a similar role to that which Prof. Haerendel was undertaking for Option 2.

Prof. Haerendel presented the status of the Option 2 concept to the SWT on 5 September. He expressed his optimism concerning the accommodation of the entire payload and also informed them that he had asked small industries to provide quotations. The total weight of an Option 2 spacecraft was to be around 300 kg. The Principal Investigators offered their full support to his activity and also refined their strategy regarding the expected decisions to be taken at the November SPC. The conducting of a 'public-relations' campaign in the relevant Member States was agreed.

A few days later, the Working Group responsible for implementing the Cluster Science Data System was called together to discuss the impacts of the various options on the overall system. In view of the relatively good prospects of getting Phoenix as a minimum, and perhaps even Option 1 or Option 2 at a later stage, it was decided to maintain the computer systems in a ready-for-launch condition.

Along the way, a financially attractive option of launching one pair of spacecraft into a low earth polar orbit together with a SPOT spacecraft for a cost of 20 MAU had to be abandoned due to the inherent technical difficulties.

On 8 October, Arianespace submitted its formal quotes for the launch of Phoenix and Cluster-II. It offered a Phoenix-alone launch for 33.5 MAU. Cluster-II, based on two independent launches for a pair of spacecraft together with another passenger, was quoted at 90 MAU (both launches into a standard GTO orbit).

During the SWT of 17 October, after a long debate, the Principal Investigators agreed upon a reduced payload complement for the Cluster-II mission in order to alleviate the payload funding problems within certain national agencies. This complement later became known as the 'minimum payload'. Had the eventual overall funding situation for the payload been unsatisfactory, the minimum payload would have been flown and some Investigators would not have participated in the four-spacecraft recovery mission. They would, however, have had an instrument on at least one spacecraft. The repercussions of this approach were fully realised by the scientists and it was gratifying to observe the relatively unemotional and business-like manner in which these decisions were adopted.

Prof. Haerendel's Option 2 working group meanwhile had received several responses from industry. These proposals were evaluated and ranked during a weekend session on 12 - 14 October. The conclusions were documented and organised into a form in which they could be presented to the SPC.

The SSAC, meeting on 7 and 8 November, recommended that a cost cap of 210 MAU be imposed on the Cluster recovery programme in order to minimise its impact on the Agency's other already approved missions.

Meeting again on 13 November, the Principal Investigators expressed their satisfaction with the positive recommendations by both the SSWG and the SSAC. The Cluster recovery programme had now become a credible and realistic possibility. Still, there was some discussion about the relative scientific merits of Options 1 and 2, and the choice between them was creating some tension in the community, which had the potential to divide the otherwise united front of the Principal Investigators.

A major step forward occurred on 25 November when NASA's Associate Administrator for Space Science, Wesley Huntress, sent a letter to ESA confirming NASA's commitment to the Cluster recovery programme. He stated that NASA would fund its contributions to the minimum payload and expressed his hopes regarding a positive decision on the recovery programme.

The SPC was convening again on 27 and 28 November, and the hopes of the Cluster community were riding high. It appeared that all of the main problems had been resolved ahead of this meeting but, as so often in life, just hours before the meeting began it became apparent that one Member State was unable to fund its relatively large share of the payload, making a positive decision on Cluster-II very unlikely.

Nevertheless, the SPC approved the principle of recovering the Cluster mission; it authorised the ESA Executive to submit a procurement proposal to ESA's Industrial Policy Committee in January 1997 for the Option 1 programme. The ESA Executive was also asked to ascertain the availability of funds from the national agencies for the 'minimum payload' as identified by the Cluster SWT. The Cluster recovery programme, be it Option 1 or Option 2, would only be undertaken if the national agencies were able to commit the funds needed for rebuilding and operating the payloads.

The SPC endorsed the cost cap proposed by the SSAC and invited the Executive to continue to study Option 2 and report on the results at the SPC meeting scheduled for February 1997. The SPC also asked for details regarding a stand-alone launch of Phoenix, in order to enable the Committee to take a decision at its February meeting.

The SWT met again in January to review the overall funding situation in the Member States. It became very obvious that this issue would finally become the one that would make or break the recovery mission. The tide was against Cluster at this point and the optimism in the community therefore began to fade once again.

four original Cluster spacecraft
Figure 4. The four original Cluster spacecraft

The SPC, meeting on 18 and 19 February, confirmed worrying rumours as more Member States announced their inability to fund the payloads. In addition, launch opportunities on Ariane had receded for reasons completely outside the influence of ESA and the situation did not look good for Cluster's recovery. Arianespace had, however, suggested that ESA might look into the use of Russian Soyuz launchers, to be marketed in future by the STARSEM Consortium (consisting of Aérospatiale, Arianespace and two Russian companies). ESA's Director of Scientific Programmes, in an attempt to alleviate the payload funding problems, offered limited financial support to those Member States that contributed to the Cluster payload. Concluding a very intense debate, the SPC authorised ESA to continue the launcher-related study and to prepare a payload funding proposal for submission to a special SPC meeting in April. Should neither launcher nor payload funding be acceptable, then Phoenix should be launched as soon as possible and a detailed study of Option 2 initiated.

The SWT meetings of 4 and 26 February focussed on the payload funding issue and how to ensure that all contributing Member States would vote in favour of a recovery mission at the decisive meeting of the SPC in April.

The day of decision

After a long discussion at its 3 April meeting, the SPC approved Cluster-II Option 1 within a financial envelope of 214 MECU, using two Soyuz launch vehicles, and gave the go-ahead to start its implementation immediately. Ten months of struggle and hectic searching for solutions had been brought to a successful conclusion.

One of the key elements in bringing about this positive decision was the offer by the Director of Scientific Programmes to contribute to the funding of the payload elements, whereby ESA will support the Member States by covering about 40% of the total payload costs.

After many hectic SWTs, this body met again the next day in a more relaxed frame of mind for the first time in a year and discussed the implementation work for Cluster-II. The scientific community expressed its deep satisfaction with the positive decision and specifically acknowledged the support provided by Roger Bonnet, ESA's Director of Scientific Programmes.

Current status of the project

Cluster-II is now moving ahead at full speed. The industrial contract is being placed with Dornier (D). The Phoenix spacecraft has been completed and was scheduled to undergo environmental testing at IABG, Germany, when the positive decision about Cluster-II was taken. Following this decision, the test activities were interrupted and the spacecraft was put into storage. The Phoenix payload has been returned to its providers for refurbishment. As Phoenix has become an integral part of Cluster-II, some of its subsystems will also be modified to make it identical to the new set of spacecraft. The instrument teams have started their procurement and manufacturing activities. As some of the original electronic components are no longer available, a limited redesign will be necessary.

One of the major payload-related activities concerns the establishment of contracts between Dornier and some national agencies or scientific institutes. In view of legal regulations, ESA's financial contributions to the payload must be channelled through contracts between Dornier and the national institutions. The Agency has not supported instruments in this way in the past and therefore new approaches must be explored by Dornier and the Member States, requiring some additional effort.

In parallel with the industrial activities at Dornier, detailed discussions have started with STARSEM, the European-registered provider of the two Soyuz launchers. No major technical difficulties have been identified and the prospects for launching the four new Cluster spacecraft from Baikonur in the summer of 2000 look very promising.


The scientific community and all of the other teams involved in the original Cluster project are very excited by the possibility of re-flying this unique mission. It is the Member States, through the positive decision communicated by their representatives in the SPC, that have made this possible. We are confident that the ultimate scientific success of Cluster-II will demonstrate that their confidence and financial support have been well placed.

It must also not be forgotten that the implementation of Cluster-II is having an impact on other future scientific missions. The Cluster community is very grateful for the solidarity and support shown by those communities, who have had to accept delays in some of their own missions.

ESA Bulletin Nr. 91
Published August 1997.

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