Cluster Technical Challenges and Scientific Achievements

  • C. P. EscoubetEmail author
  • A. Masson
  • H. Laakso
  • M. G. G. T. Taylor
  • J. Volpp
  • D. Sieg
  • M. Hapgood
  • M. L. Goldstein
Living reference work entry


The Cluster mission has been operated successfully for 14 years. As the first science mission comprising four identical spacecraft, Cluster has faced many challenges during its lifetime. Initially, during the selection process where strong competition with SOHO was almost fatal to one of them, finally both missions were merged into the Solar Terrestrial Science Programme with strong support from NASA. The next challenge came during the manufacturing process where the task to produce four spacecraft in the time usually allocated to one demanded considerable flexibility in the production process. The first launch of Ariane V was not successful, and the rocket exploded 40s after takeoff. The great challenge for the Cluster scientists was to convince ESA, the National Agencies, and the science community that Cluster should be rebuilt identical to the original one. The fast rebuilding phase, in 3 years, and the 2nd launch on two Soyuz rockets, paved the way to numerous ESA launches afterward. Finally in the operational phase, the challenge was to operate four spacecraft with the funding for one, to solve serious anomalies, and to extend the spacecraft lifetime, now seven times its initial duration with some vital elements such as batteries not working at all. After the technical challenges, the key scientific achievements will be presented. The main goal of the Cluster mission is to study in three dimensions small-scale plasma structures in key plasma regions of the Earth’s geospace environment: solar wind and bow shock, magnetopause, polar cusps, magnetotail, plasmasphere, and auroral zone. Science highlights are presented such as ripples on the bow shock, 3D current measurements and Kelvin-Helmholtz waves at the magnetopause, bifurcated current sheet in the magnetotail, and the first measurement of the electron pressure tensor near a site of magnetic reconnection. In addition, Cluster results on understanding the impact of coronal mass ejections (CME) on the Earth’s environment will be shown. Finally, how the mission solved the challenge of distributing huge quantity of data through the Cluster Science Data System (CSDS) and the Cluster Archive will be presented. Those systems were implemented to provide, for the first time for a plasma physics mission, a permanent and public archive of all the high-resolution data from all instruments.


Sun-Earth connection Magnetosphere Multi-point measurements 


Cluster was selected, together with the Solar and Heliospheric Observatory (SOHO), as the Solar Terrestrial Science Programme, the first cornerstone of ESA’s Horizon 2000 program. The selection process took four years since both Cluster and SOHO were in competition and ESA funding was only available for one. Substantial descoping took place during Phase A where the payloads of both Cluster and SOHO were reduced. Furthermore, a large collaboration was opened with NASA, and a memorandum of understanding was agreed where NASA was providing the launch and conducting the operations of SOHO as well as providing support to Cluster and SOHO spacecraft and payload elements. The Science Programme Committee approved Cluster and SOHO in 1986, and, following an announcement of opportunity, the 11 Cluster instruments were selected in 1988.

Technical Challenges

Building, Integrating, and Testing Four Spacecraft

The main challenge of the Cluster mission was to produce four identical spacecraft in the time allocated and for the cost of one spacecraft (Credland et al. 1997). Since four spacecraft are not really a series, such as nowadays GPS or Galileo satellites, and at the same time are much more than one spacecraft, the production line has to be adapted in between a one-off, which is usually used in the Science program, and a series production line. The spacecraft were designed to be modular, where integration of instruments and subsystems could be rearranged to fit the delivery of the various units. All elements that constituted the spacecraft were so numerous that they had to be built in series. For instance, the four spacecraft were equipped with 16 deployable rigid booms for communications and magnetometers, 16 43 m long wire booms to measure electric field and waves, 320 m of pipes for the propulsion system connecting four main engines and 32 thrusters, 20 km of harness, 1,500 connectors, and 56,000 electrical connections.

An invitation to tender was issued by ESA in 1988 to build the four spacecraft, and the prime contractor, Dornier GmbH (Friedrichshafen, Germany), was selected in 1989. After a design phase of about 1.5 years, the building of the spacecraft took place between April 1991 and April 1995. Parallel integration of the four spacecraft was the norm as well as parallel system testing. The system level testing required over 2 years in IABG (Ottobrunn, Germany), making it one of the longest in an ESA program. When a problem was occurring with one subsystem, it was quickly exchanged with the second model, and the test could be continued without losing significant time waiting for repair. This flexibility in integration and testing was possible due to the full traceability of hardware and software. Since it was mandatory to achieve identical spacecraft and instruments, knowing that each unit may have been slightly different, performances and calibration curves of each unit had to be tracked carefully.

The Cluster spacecraft are large cylinders of 2.9 m diameter and 1.3 m height. Their mass at launch was 1,200 kg, including 650 kg of propellant to achieve the delta V of 2,300 m/s required by the mission. Most of the propellant (500 kg) was used to transfer the spacecraft from geo-transfer orbit to the operational orbit. The rest was used for attitude maneuvers and to change the separation between the spacecraft. For the orbit injection, a large 400 N engine was used a few times and once in the operational orbits, and after deploying the instrument and communication booms, only the 10 N thrusters were used. Figure 1 shows the four spacecraft after testing in the clean room at IABG, Ottobrunn (D).
Fig. 1

The four Cluster spacecraft displayed in the clean room at IABG (Ottobrunn, Germany). The background pair is in a stacked launch configuration, while the two front spacecraft show the instruments and booms on the main equipment platform

The original proposed payload was significantly different from the final one, mainly because the original proposal was based on a main spacecraft with a complete payload and three companions with a reduced payload. However, it turned out to be simpler and cheaper to build, integrate, test, and launch four identical spacecraft. The final payload flown on each spacecraft (Table 1) is therefore close to the original main spacecraft.
Table 1

The 11 instruments on each of the four Cluster spacecraft. History of PI ships is also given

Instrument/principal investigator (current and previous)

Mass (kg)

Power (W)

ASPOC (Spacecraft potential control)

K. Torkar (IRF, A)

W. Riedler (IRF, A) 1988–2001



CIS (Ion composition 0 < E < 40 keV)

I. Dandouras (IRAP/CNRS, F)

H. Reme (IRAP/CNRS, F) 1988–2007



EDI (Plasma drift velocity)

R. Torbert (UNH, USA)

G. Paschmann (MPE, D) 1988–2006



FGM (Magnetometer)

C. Carr (IC, UK)

E. Lucek (IC, UK) 2005–2012

A. Balogh (IC, UK) 1988–2005



PEACE (Electrons, 0 < E < 30 keV)

A. Fazakerley (MSSL, UK)

A. Johnstone (MSSL, UK) 1988–1997



RAPID (High energy electrons and ions)

P. Daly (Gottingen U., D)

B. Wilken (MPAe, D) 1988–1999



DWP a (Wave processor)

M. Balikhin (Sheffield, UK)

H. Alleyne (Sheffield, UK) 1996–2011

L. Woolliscroft (Sheffield, UK) 1988–1996



EFW a (Electric field and waves)

M. André (IRFU, S)

G. Gustafsson (IRFU, S) 1988–2000



STAFF a (Magnetic and electric fluctuations)

P. Canu (LPP, F)

N. Cornilleau-Werhlin (LPP, F) 1988–2010



WBD a (Electric field and wave forms)

J. Pickett (IOWA, USA)

D. Gurnett (IOWA, USA) 1988–2008



WHISPER a (Electron density and waves)

J.-L. Rauch (LPC2E, F)

J.G. Trotignon (LPC2E, F) 2007–2012

P. Decreau (LPC2E, F) 1988–2007






aMembers of the wave experiment consortium (WEC)

bIncluding power supply

Ariane V Launcher

Cluster and Ariane V were developed in parallel. It brought a number of challenges on the Cluster side since the specifications and especially shock requirements were not really known when Cluster was being built. The shock requirements were therefore very high, and the Cluster spacecraft and instruments had to be built extremely solidly in order for them to survive during launch. Furthermore, very late in the program when the Cluster spacecraft were built, Ariane V engineers realized that shocks could indeed be even higher during the separation of launcher elements. Cluster had to demonstrate that it could also survive these shocks.

The launch mass available for the four spacecraft was very constrained since for a long time Cluster had to be compatible with an Ariane 4 launch, which was the backup launcher. The spacecraft mass limit was 4,800 kg for the four spacecraft, and the total Cluster mass ended up at 4,707 kg. Continuous efforts were necessary on both the spacecraft and the payload side to keep the mass below the limit during the manufacturing process. Experiment box walls had to be made thinner, and some instruments even used magnesium for their electronic box (e.g., WBD). The monitoring of both project and instrument teams was essential to achieve the required result.

Payload: 55 Instruments to be Built, Calibrated, and Integrated

The payload of Cluster (Table 1) (Escoubet et al. 1997 and reference therein) posed a great challenge to the Europeans and American institutes involved. This was the first time that small teams of engineers were requested to build, test, and calibrate five identical suites of instruments (four flight units and one spare). Altogether, with 11 PI teams, the total number of instruments was 55. Something that could be done in industry by bringing more manpower could not be done easily in small institutes that had fixed numbers of engineers and scientists. The institutes had therefore to allocate more manpower to Cluster, but also the engineers had to work longer hours and sometimes weekends to deliver their instruments on time. The fact that one model had to be built, another one to be calibrated, another to be delivered, and the last one to be tested on the spacecraft at the same time posed great difficulties to the PI teams. It was almost unbearable to the point that when Cluster II had to be rebuilt, some engineers did not want to repeat the experience and it was necessary to train new people to do the work.

The major challenge, which was driven by the science goal to derive plasma parameters from the four Cluster measurements, was to make the instruments as identical as possible. Some instruments, which were simpler in design, were easier to produce; however, their requirements were also more difficult to achieve. For instance, the PEACE (electron instrument) PI (Johnstone et al. 1997) required that their two sensors on two different Cluster spacecraft to differ by less than 1 % in the same plasma. To achieve that goal, the design of the instrument required a positioning of the hemispheric deflectors to within 40 μm. The first model could only achieve 80 μm, and a complete redesign was performed to finally reach 37 μm. These efforts were essential to measure for the first time the divergence of the electron pressure tensor and to better understand the magnetic reconnection process (see below).

For the five wave instruments, their key requirement was to get the best time accuracy possible to compare measurements at high frequencies between the four spacecraft. The accuracy of the Cluster onboard clock was ±2 ms which was clearly not enough to compare wave data at 180 Hz (frequency of the wave form data collected by STAFF and EFW in burst mode). The Cluster spacecraft time accuracy could not be improved significantly at that time due to cost constraints. This is why the Digital Wave Processor (DWP) was charged to coordinate all five wave instruments and to improve significantly the time accuracy (Woolliscroft et al. 1997). This finally was achieved using DWP, the spacecraft time correlation performed by ESOC on regular basis and the NASA Wide Band (WBD) time measurements. The accuracy of the wave instrument time is now down to 20 μs (Yearby et al. 2013).

One of the prime objectives of the Cluster mission was to measure electric currents in space. To achieve it, a very precise measurement of the magnetic field was required since the magnetic field gradients between the spacecraft are used to deduce the current. With a very extensive electromagnetic cleanliness program (see below) and in-flight calibration, the goal of 0.2 nT accuracy was successfully achieved (Balogh et al. 2001; Carr et al. 2013).

Electromagnetic Cleanliness

The Cluster scientific objectives required measuring the electric and magnetic fields and electromagnetic waves in the plasma around the spacecraft with very high accuracy. Therefore the spacecraft and its subsystems as well as the instruments needed to be electromagnetically clean. The importance of this requirement was recognized very early in the Cluster development program, and a special Electromagnetic Cleanliness (EMC) review board was set up, made of scientists and engineers, to monitor the EMC program. First all components used to manufacture the instruments and subsystems were selected to be nonmagnetic, so far as was possible. Then magnetic moments of all units were measured before integration, and an overall spacecraft model was produced. The magnetometers, FGM (flux gate) and STAFF (search coil), were put at the tip of a 5 m solid boom to minimize any magnetic field produced by the spacecraft. However, verification was needed to see if the spacecraft would produce a strong magnetic field at the end of the boom. The 5 m boom had therefore to be deployed, which constituted a challenge for a rather heavy device in the Earth’s gravity (this boom was supposed to deploy only in space). A smart Dornier engineer found a great and simple system to overcome this problem: he hung the boom from a helium-filled balloon, which compensated for the weight of the boom. A small propeller was then attached to the boom to simulate deployment at the proper speed.

In the late 1980s, space batteries were usually made of nickel-cadmium (e.g., SOHO) and were strongly magnetic. On Cluster, which required a very low magnetic field produced by the spacecraft, such batteries could not be used. It was therefore decided to use silver-cadmium batteries, which were nonmagnetic. These batteries have been much less frequently used in space missions because they degrade quickly and can crack and expel electrolyte when overcharged. Consequences on operations will be detailed later.

Once the four spacecraft were assembled, they were brought to the IABG MSFA (Ottobrunn, Germany) Magnetic Test Facility where the Earth’s magnetic field is compensated and the real spacecraft magnetic field could then be measured. Three of the spacecraft were found outside of the specification of 0.25 nT at the end of the 5 m boom. They were in the range 0.6–1 nT. A long compensation program started where small magnets had to be glued on strong magnetic spot found on the spacecraft (e.g., the thruster valves) to decrease the spacecraft magnetic field. After this meticulous work, the spacecraft magnetic field at the end of the boom was found to be around 0.12 nT, better than the requirement of 0.25 nT.

Another important element in EMC is conductivity. In sunlight, a spacecraft produces photoelectrons on its sunlit side and therefore charges positively. On the other hand its shadowed side does not charge since there are no photons to extract electrons. If the spacecraft is not fully conductive, then there will be buildup of charges, and these will greatly affect the motion of low energy particles that must be measured by the plasma instruments. The Cluster spacecraft needed therefore to be fully conductive. All external layers of Cluster, solar panels, and the multilayer insulations (MLI) were coated with indium tin oxide to achieve that requirement. Then conductivity was checked over the surface of the spacecraft with many local measurements. During testing it was found that the MLI lost its conductivity when folded at spacecraft corners, and special conductive bridges were then added in those places to maintain conductivity.

Sensitive electromagnetic wave measurements also require a very clean spacecraft since some of the waves are coming from large distances away from the spacecraft and have a weak signal that can be swamped by spacecraft subsystems or instruments. Special radiated emission and radiated susceptibility tests were performed to verify that the spacecraft would not perturb the very sensitive wave instruments. After launch and after the first switch on of the STAFF instrument, the PI stated that Cluster was one of the cleanest spacecraft she had put an instrument on. This was a real achievement that all teams were very happy to hear.

However, some interference which was inherent to the experiment techniques employed could not be totally suppressed. For instance, the EDI instrument, measuring electric fields by sending an electron beam around the spacecraft, had an effect on the measurements of high-frequency electric waves and sometimes on the spacecraft potential. After analyzing the data, the PIs agreed to start time sharing: EDI would alternate every orbit their operation mode in low, normal, and high beam current, and the ASPOC instrument, controlling the spacecraft potential control, would be switched on during EDI high-current mode. This helped all PIs to get clean data for their investigation. Furthermore a special interference campaign was conducted at the end of the instrument commissioning where the active instruments run through various modes and instrument disturbances could be checked.

Cluster Rebuilt

After the failed launch on 4 June 1996 of the Ariane V carrying all four Cluster spacecraft, the team of engineers and scientists saw their hopes dashed in less that 40 s. However, shortly after the failure, the Cluster Science Working Team (SWT) and the project team investigated ways to recover the mission (Credland and Schmidt 1997; Schmidt et al. 1997a). To keep the teams in place and not to lose time if a decision to rebuild the four Cluster spacecraft would be taken, it was decided to refurbish the spare model of the spacecraft and instruments. This model was called Phoenix as a reminder to the four original Cluster spacecraft that had burned in the explosion of the first Ariane V rocket. However, the community, although agreeing to build Phoenix, was clearly stating that one spacecraft only would not recover the science objectives of Cluster. It was therefore decided to study two options to recover the original Cluster scientific objectives; the name given to this new mission was Cluster II.

The first option (Option 1) was to rebuild four original Cluster spacecraft, including Phoenix. The spacecraft and instruments would be identical to the original ones and could therefore be built in a fast track to be ready less than 3 years later. The launch could be a single one or multiple ones using spare capacity on already planned launches.

The second option (Option 2) was based on a small spacecraft platform to be launched in pairs on a Ukrainian Tsyklon rocket. A kickstage would raise apogee to 18.5 RE, and the fuel needed on the spacecraft would be limited to constellation and attitude maneuvers. Such option would have required changes to ESA’s management structure.

Option 1 was preferred by the Cluster SWT and by the majority of delegations in the Science Programme Committee since it carried less risk, was not substantially more expensive, had minimal cost for the payload, and could be launched one year earlier than Option 2. Furthermore ESA, after long negotiation, managed to keep the overall Cluster II option 1 cost to under 214 Meuros, including two Soyuz rockets for 60 Meuros and a contribution to the payload of 17.5 Meuros. In addition, NASA agreed to support rebuilding the US-provided instruments. This was accomplished, in part, by lifting several Quality Assurance requirements. At its meeting on 3 April 1997, the Science Programme Committee agreed to the recovery of the Cluster mission, called Cluster II, using identical spacecraft and payload as the original Cluster mission.

Phoenix and three new spacecraft were then built in three years and successfully launched on 16 July and 9 August 2000 with two Soyuz rockets from Baikonur (Escoubet et al. 2001). With Cluster I and Cluster II, the teams of engineers and scientists had built 8 science spacecraft and more than 100 instruments in a total of 11 years, which is an extraordinary achievement, rewarded by fundamental science and discoveries made over the past 14 years.

Orbit and Constellation Changes

The orbit of Cluster was designed to cross key regions of the Earth’s environment: bow shock, magnetopause, polar cusp, magnetotail, plasmasphere, and the auroral zone. The polar cusp, being above the pole of the Earth, required the orbit to have a high inclination, close to 90°. On the other hand, to study the magnetotail and bow shock required an apogee above 15 RE. The selected orbit was 4 × 19.6 RE with 90° inclination (Fig. 2, solid lines). Each spacecraft had a slightly different orbit to form two tetrahedra along the orbit in the key scientific regions: in the Northern polar cusp and Southern magnetopause in spring and at two places above and below the plasma sheet in the magnetotail in fall. Although requiring a bit more fuel to set up, these two tetrahedra had the great advantage to get an almost perfect tetrahedron in a large area of the magnetotail and the solar wind, which was essential for 3D measurements. With the Soyuz launch into a transfer orbit, a major change in the orbit was required to reach the nominal required operational orbit. This was done using the spacecraft themselves, which carried more than 600 kg of fuel representing more than half of each spacecraft mass (Table 2).
Fig. 2

Sketch of the Cluster orbits in 2001, when it was polar, and in 2010, when the inclination and perigee decreased. The yellow orbit is during fall when the apogee is in the tail, and the red orbit is during spring when the apogee is in the solar wind. Two tetrahedra were formed at two locations along the orbit to maintain a good 3D configuration throughout apogee

Table 2

Cluster II spacecraft mass











Dry mass (kg)





Propellant (kg)





Total launch mass





C2 and C4 are heavier since they carry the separation system for the other two spacecraft

Out of such large amount of fuel, about 80 % was used to increase the apogee and perigee altitudes and change the inclination. With the 63 kg of fuel left (not counting oxidizer) on each spacecraft, the separation distances between the spacecraft, as well as the points where the perfect tetrahedron was placed, were changed during the course of the mission (Fig. 3). At the beginning of the mission, constellation maneuvers were performed every 6 months; then after two years it was changed to once a year with two tetrahedra of different sizes for the tail and the polar cusp. A multi-scale formation is flown since 2006 with C1, C2, and C3 forming a large triangle of a few 1,000 km and the C4 distance with C3 being varied from a few km to a few 1,000 km. Since 2011, with the opening to Guest Investigator operations, maneuvers were performed every 3–4 months.
Fig. 3

Cluster constellation from the beginning of mission up to now. The distance between the spacecraft is given as a function of time: C1-C2-C3 separation distance in magenta and C3-C4 in green. The distance is given at one point along the orbit defined by the symbol and color in the legend

At the beginning of the mission, the smallest inter-spacecraft distance that was considered safe was 100 km. This was achieved at the beginning of 2002 and brought a wealth of new results and discoveries (Paschmann et al. 2005). However the science investigations required smaller distances, and the distance of 40 km was reached between two Cluster spacecraft in 2008–2009, 20 km in 2011, and finally 4 km in 2013. Careful preparation is required to achieve such small distance since it approaches the accuracy in knowing the position of the spacecraft. Furthermore, the Cluster spacecraft have very long wire booms of 88 m tip to tip, which makes their cross section fairly large. The requirement on the accuracy of the position, which is determined using ground station ranging with the spacecraft, was initially 10 km for an inter-spacecraft distance of less than 1,000 km. After launch, the European Space Operations Centre (ESOC) flight dynamics team could achieve an accuracy below 1 km. This experience could be useful for future multi-spacecraft missions, such as the soon-to-be-launched NASA MMS mission.

Operations: Four Spacecraft for the Cost of One

The major challenge in operating Cluster was the strong constraint to operate the four spacecraft for the cost of one. The operation teams, for the spacecraft at ESOC (Darmstadt, Germany) and for the science at the Joint Science Operation Centre (RAL, UK), were therefore built to operate, as much as possible, the four spacecraft in parallel. For instance, the payload commanding was prepared simultaneously on the four spacecraft. This is fine when everything goes according to plan but can create more stressful situations when anomalies occur, since the teams have to solve anomalies and at the same time still operate the other three spacecraft. Fortunately, the Cluster spacecraft have been very stable with their fast spin rate (15 rotations per minute) and solidly built to withstand harsh space conditions and have not had very many anomalies. However, due to the degradation of some key spacecraft elements such as batteries and solar panels, the flight control team have had to invent new procedures to return the science promised to the community. A few of these challenges are listed below.

As presented in section “Electromagnetic Cleanliness,” the Cluster spacecraft were required to be magnetically “clean” and therefore carried nonmagnetic silver-cadmium batteries. Due to the relatively rapid degradation of the cathodes in this type of battery, they were expected to last for only 3 years. Thanks to the careful management by the Cluster team, they kept providing power for 6 years. After that time, the ESOC flight operation team had to invent new procedures, which the satellite builder never thought of, to survive eclipses with very little or even without battery power.

The first step was to put a new thermal strategy in place: since active heating was not possible anymore during eclipse, it was decided to heat the fuel tanks before eclipse to keep the spacecraft reasonably warm during eclipse. The second step was to reduce drastically the power consumption. A new procedure, called “decoder only,” was introduced, requiring a minimum battery capacity to keep-alive lines, while all spacecraft subsystems, including the onboard computer, were switched off at eclipse entry. The spacecraft were then switched back on at eclipse exit. A few years later, when none of the batteries were usable, a more drastic procedure was introduced, which switched the spacecraft completely off before eclipse and switched it back on at eclipse exit. This procedure was then automated when eclipse seasons increased in length, up to 9 months in 2011 (Fig. 4). Four spacecraft had to be recovered, every 57 h, within a limited interval of ground station visibility. Speeding up the recovery allowed to keep the ESOC team small and gave more time to operate the scientific instruments, enabling measurements during 85 % of the orbit. At this time, 795 eclipses have been passed successfully, demonstrating the robustness of this procedure.
Fig. 4

Eclipses during 2010–2011. The long period of eclipses in 2011 lasted 9 months

Recovery of the Five Wave Instruments

On 5 March 2011, the digital wave processor (DWP), controlling the five wave instruments, was found switched off at acquisition of the signal of spacecraft 3. Subsequent attempts to switch it back on did not succeed since the current consumed by DWP was going over the spacecraft limit and the onboard current limiter was switching it off automatically. The cause was not clear: it could have been a short circuit or another problem with the instrument. After a few weeks of investigations by the instrument team and ESA, a new attempt was done to switch the instrument using the redundant power line, without success. The instrument technical manager looked then at all possibilities of failure and identified the most likely one: after the abnormal switch off of DWP, which controls the five wave instruments, all instrument relays were most probably stuck in a position on, preventing the subsequent switch on with their overall inrush current. A typical switch on of DWP switches on two or three instruments at once but not five. He went back to ground tests done in 1994 (almost 20 years back, available only on paper) and found the raise of current with three instruments, and he estimated the current raise to be up to 2 A within 11 ms for the five instruments. This was clearly well above the current limit of 1.29 A that the spacecraft could provide. Overconsumption of current is a difficult anomaly since, in case of short circuit, regular switch-on attempts increase the risk of losing the spacecraft if the current monitoring system fails.

The next objective was to get more information about the actual ramp-up of the instrument current at switch on. Sampling of current is done on Cluster every 5 s, and the ramp-up, before automatic switch off by the onboard monitoring, was most likely less than 0.1 s. The solution was found by using special application software that had been used to record pyro firing during the launch and early orbit phase. This was tested on spacecraft 4 first and then applied on spacecraft 3 in early April: the ramp-up of the failed instrument current was characterized with a time resolution of one sample every 0.5 ms and confirmed that the switch off occurred after 21 ms as expected if all five relays in the instrument were closed.

After intense discussion within the team, the idea emerged to try to switch on the redundant power line before the main one was switched off by the onboard current limiter, which would double the current available from the spacecraft. This would need to be done within 11 ms reaction time of the current limiter to have a chance of success. However, the fastest time allowed between two commands by the onboard computer was only 39 ms, too slow to recover the instrument. After further discussions, the team came up with a clever trick to “hack” the onboard computer and the power distribution unit. The trick was to execute the switch on of prime and redundant power lines with only one command, by changing the command register during the execution of the first command. Nobody knew, including industry experts who built the spacecraft, if that would work. However, everybody was convinced that it was the only solution. The first test was executed on another spacecraft with two spare current lines, and the trick worked: the two power lines could be switched on with one execute command in less than 2 ms. Everybody was then awaiting the “real” switch-on test with both current lines simultaneously, planned for the week after. When this was executed, the first test failed. Then the team tried it a second time, and the instrument switched on successfully on 1 June 2011. Fifty percent of the Cluster payload was finally recovered on Cluster 3.

Another example of delicate operation was following the science request, from the WHISPER (electric waves and sounder) PI, to tilt the axis of one of the Cluster spacecraft with respect to the others. The goal was to study propagation of an electromagnetic emission called nonthermal continuum and observe its polarization. Since this was not a configuration described in the spacecraft user manual, the instrument teams had to confirm that their instrument would cope with such a change, and ESOC, Astrium, and ESTEC experts had to verify that it was also possible on the spacecraft side. The main issues were the risk of not being able to slew back to the initial attitude in case of a hardware failure, the thermal behavior of the spacecraft and instruments, and the possibility to get sunlight directly into some instruments. After the feasibility and low risk were confirmed in 2006, it was found that May 2008 was the best period of time for these operations, when having the optimal spacecraft constellation and being outside of an eclipse season. Finally Cluster 3 was tilted by 45° for a month. This operation was successful and unique science could be performed. A science paper showing that only the tilted conditions give the accurate location of the source of nonthermal continuum has recently been published (Décréau et al. 2013).

Science Results

As part of a cornerstone, Cluster science was required to be a major step forward in fundamental plasma physics with many expected discoveries and a large community involved in data analysis. This is clearly being fulfilled with the total number of refereed papers currently above 1959 papers (Fig. 5) and the continuous growth of the community using the data. In the last five years, the publication rate has been above 175 papers/year, with a peak at 232 in 2011 demonstrating that Cluster data usage by the community continues to be very high even after 14 years in orbit. The new science targets and mission goals, obtained by the evolving orbit in combination with different separation strategies, have been a key driver of this vibrant scientific activity. Certainly the public access to all high-resolution data, about 1 year after acquisition, through the Cluster Active Archive has also been a determining factor for success. As of the end of 2013, 1,794 scientists from all over the world have been using the Cluster Active Archive.
Fig. 5

Cluster and Double star referred publications up to the end of 2013

The following sections will describe a few Cluster science highlights that demonstrate the unique capability of the four spacecraft constellation such as ripples on the bow shock, 3D current measurements and Kelvin-Helmholtz waves at the magnetopause, bifurcated current sheet in the magnetotail, and the first measurement of the electron pressure tensor near magnetic reconnection. In addition, Cluster results on acceleration of energetic electrons in the Earth’s magnetosphere when it was hit by a CME will be highlighted.

Ripples on the Bow Shock

The Earth’s bow shock is the first obstacle for the solar wind plasma when it enters the Earth’s environment. It is, however, a rather porous obstacle that mainly slows down and heats the plasma. The bow shock can also be the place where surface waves can form and propagate, similarly to waves on the ocean. Moullard et al. (2006) presented the first evidence of such waves at the bow shock when the Cluster spacecraft were approximately at 250 km from each other (Fig. 6). These waves were observed in the foot and the ramp of the bow shock on both magnetic field and plasma density data. Their period was 16 s, their wavelength 1,000–2,000 km, and they were propagating along the magnetic field. Two possible explanations have been proposed: these waves are produced by the shock itself, or since it was observed in the flank of the shock, the solar wind flow may have produced it through the shear instability. Such surface waves have been observed by Cluster almost everywhere on the boundaries of the magnetosphere, and they could only be fully characterized and distinguished from a boundary motion by having the four spacecraft in a constellation.
Fig. 6

Magnetic field magnitude (top panels) and electron number density (bottom panels) from C1 for two shock crossing events (From Moullard et al. 2006). The oscillations can be seen in the foot and ramp of the shock, when the density and magnetic field start to increase

Electric Current and Kelvin-Helmholtz Waves at the Magnetopause

The magnetopause is the boundary where the coupling between the solar wind and the Earth magnetic field takes place, and it is the place where a strong current (Chapman and Ferraro 1930) is flowing. Dunlop et al. (2001) first published four-point measurements of the magnetic field at the magnetopause during November 2000. They could show that the magnetopause speed along the normal was varying continuously from a minimum of 17 up to 124 km/s.

Haaland et al. (2004) presented a new method for estimating the orientation and motion of the magnetopause. They used Cluster when it was in a tetrahedron configuration of 100 km, in spring 2002, to obtain the minimum variance of the current density. The small separation distance was essential to make sure that all four spacecraft were inside the current layer of the magnetopause at the same time. The variance analysis was used to estimate the orientation of the magnetopause, and the integration of the current density gave the magnetopause velocity. The motion of the magnetopause on that event was found around 34 km/s, slightly larger than the plasma flow measured by the ion instrument of around 28 km/s, which would indicate an outflow of plasma from the magnetosphere into the solar wind (Fig. 7).
Fig. 7

Magnetopause crossing on 2 March 2002 (From Haaland et al. 2004). From top to bottom, the parameters shown are the electron density deduced from the spacecraft potential, the magnetic field components in GSE, the current density estimated from the curlometer technique, and div B which gives an indication of the validity of the current estimation

At the flanks of the magnetosphere, Hasegawa et al. (2004) observed for the first time Kelvin-Helmholtz (K-H) waves rolling up into vortices that would allow plasma transfer from the solar wind to the magnetosphere. This was demonstrated when the spacecraft located further inward observed higher density than the three others located outward. These observations were supported by a magnetohydrodynamic (MHD) simulation. Since there was no sign of plasma acceleration due to magnetic stress, they speculated that magnetic reconnection was not taking place locally in that event. A few years later however, Nykyri et al. (2006) found that reconnection was occurring inside a rolled-up vortex, and Hasegawa et al. (2009) found it on the trailing edge of the vortex. More recently, K-H vortices have been found even during southward IMF (Hwang et al. 2011) and at high latitudes (Hwang et al. 2012).

Bifurcated Current Sheet in the Plasmasheet

The four Cluster spacecraft have also significantly advanced our knowledge of the magnetotail and especially the plasmasheet, the big reservoir of plasma that regularly releases large quantity of energy toward the Earth and produces the northern lights. Once again, being able to distinguish between spatial and temporal variations using measurements at four points separated in space is a key aspect to understand the physics since it is a very dynamic region. These four-point measurements allow for the first time to distinguish the spatial characteristics of the plasmasheet from its quick motion. Runov et al. (2003) showed without ambiguity that the electric current is not maximum in the middle of the plasmasheet but a few 1,000 km away (Fig. 8); this is the so-called bifurcated current sheet. Bifurcations of the current sheet were proposed before (Sergeev et al. 1993; Hoshino et al. 1996), but Cluster observations helped to remove the ambiguities inherent in using single- or two-spacecraft measurements.
Fig. 8

Current sheet bifurcation on 29 August 2001 (From Runov et al. 2003). The top panel shows Bx as a function of time from the four spacecraft. The middle and bottom panels show two sketches of the bifurcated current sheet for a sausage or kink wave disturbance, respectively. Dark and light gray indicate the region of maximum current

Electron Pressure Tensor Near Magnetic Reconnection

Magnetic reconnection is a universal physical process, playing a major role in various phenomena such as star formation or solar explosions, but also preventing plasma confinement in fusion reactors on Earth. However, a lack of precise measurements at the heart of this physical process prevents a full understanding of this phenomenon.

At the heart of the reconnection process, magnetic field lines from different magnetic domains collide and tie together, changing the overall magnetic field topology, usually forming an X line. This topological change leads to the mixing of previously separated plasmas, like the entry of solar material into the magnetosphere. It also efficiently converts magnetic field energy to particle energy, generating plasma heating and reconnection jets as in the magnetotail. But the magnetic field is not the only physical parameter to consider. The electric field also plays a crucial role in the microphysics of reconnection since it accelerates particles.

After considerable efforts by the PEACE (electron) team to perfect the PEACE calibration, the multi-spacecraft nature of the Cluster mission was used to derive the first measurements of the divergence of the full electron pressure tensor (Henderson et al. 2008); this parameter is one of the contributions to the electric field. They were able to directly compare quantitatively, for the first time, this quantity to the Hall term (J × B). In agreement with simulations, they found in particular that both terms generate oppositely directed electric field contributions, clearly anti-correlated (Fig. 9, last panel). These observations used small inter-spacecraft distances of about 200 km. This fundamental property of the reconnection process can then be applied elsewhere in the solar system and beyond.
Fig. 9

The first four panels show the magnetic field from the four spacecraft (From Henderson et al. 2008). The 5th panel shows the electron pressure. The 6th panel shows the parallel electric field derived from the electron pressure tensor. The 7th to 9th panels show the perpendicular electric field deduced from the electron pressure tensor (black) and the Hall term (J × B). The Z component of the perpendicular electric field shows that the electric field from the electron pressure tensor and the one from the Hall term are anti-correlated

Acceleration of Electrons by Coronal Mass Ejections

Coronal mass ejections (CMEs) are huge clouds of plasma emitted by the Sun during solar storms. Their characteristics vary greatly, but CMEs can sometimes be as fast as 3,000 km/s and contain strong magnetic field and dense plasma. The effect on the Earth’s environment could be dramatic, since it induces fast and large changes of the Earth’s magnetosphere, which in turn energizes particles to very high energy. “Space Weather” was created to study the effect of the Sun on the Earth and on human systems. Cluster is not a space weather mission as such, but its measurements, especially the study of dynamic structures, are providing key input for the models that are being developed for space weather predictions. Cluster capabilities were enhanced when in 2002 the SPC agreed to add a second ground station to record the Cluster observations twenty-four hours a day, seven days a week (originally the mission was designed to cover about 50 % of the orbit focused on magnetospheric boundaries, bow shock, magnetopause, cusp, and plasmasheet). Without it the following observation would not have been possible.

CMEs are very often associated with interplanetary shocks that can then accelerate electrons to very high energy when they hit the Earth’s environment. Zong et al. (2009) observed a strong enhancement of energetic electrons in the magnetosphere associated with the passage of the CME shock (Fig. 10). After the shock passage the electric field oscillated in correlation with the flux of energetic electrons, suggesting that the magnetic field compression produced ultralow frequency (ULF) waves that then accelerated electrons. Energetic electrons can penetrate through components of spacecraft and produce deep dielectric charging. If vital components are affected, it could eventually kill a spacecraft. Measurements of energetic electron events and their associated plasma parameters are therefore fundamental to improve models and in the longer term predict their occurrence.
Fig. 10

Energetic electron flux measured by Cluster RAPID instruments around the time of the arrival of the interplanetary shock at 18:27 UT on 7 November 2004 (From Zong et al. 2009). The four top panels show the electron flux in energy-time spectrogram (C1) and in pitch-angle-time spectrogram (C2-C4). The black line on top of each spectrogram represents the azimuthal electric field. The bottom panel shows the magnetospheric magnetic field (Bz component) measured on the four spacecraft; its increase (compression) is clear after 18:27 UT

Cluster Guest Investigator Programme

As customary for space physics missions, the decisions on how to operate the spacecraft and instruments have been the role of the Science Working team made of the Principal Investigators and the Project Scientist. In 2010, however, as part of activities for that extension period, science operations were opened to scientists from the community, turning Cluster into an “observatory,” similarly to what is commonly done with astronomy missions. An Announcement of Opportunity was opened in July 2010, soliciting Guest Investigator (GI) proposals for special operations of the instruments or the spacecraft, including changing the separation between the spacecraft. Six GI proposals were selected (Table 3). The GI-proposed operations were executed from 2011 up to the end of December 2013. A new AO is in preparation and should be open in the first quarter of 2014.
Table 3

List of Cluster guest investigators selected in 2011

Guest investigator

GI proposal title


Implementation period

B. Walsh

High latitude magnetopause electrons

Boston University (USA)

Spring 2011

E. Yordanova

Small-scale turbulence

Institutet for Rymdfysik, Uppsala (Sweden)

February until April 2012

A. Retinò

Multi-scale observations of magnetic reconnection in the magnetosphere

LPP/UPMC/Ecole Polytechnique/CNRS (France)

May and August 2012

C. Foullon

Magnetopause boundary layer: evolution of plasma and turbulent characteristics along the flanks

Exeter University (UK)

November 2012

Z. Pu

Generation and 3D features of flux transfer events at the dayside magnetopause

Peking University (China)

January and February 2013

F. Pitout

Particle acceleration and field aligned currents in the cusp

IRAP/Paul Sabatier University/CNRS (France)

Autumn 2013

Cluster Open Access to all High-Resolution Data Sets

Since the beginning of the Cluster mission preparation, it was realized that for Cluster to be a success, data should be easily accessible to the community (Schmidt et al. 1990). The Cluster Science Data System (CSDS) was set up to achieve that objective (Schmidt et al. 1997b). CSDS started in the early 1990s, and, at that time, the network bandwidth was much lower than nowadays, only able to distribute e-mail and view simple web pages. It was therefore not suited to distribute the very large quantity of data collected by Cluster, in the range of 1–3 CDroms per day. The only solution was to get data burned on CDroms and to distribute these to PI and CoI institutes (around 70 at that time). It was a big challenge for ESOC to produce between 70 and 200 CDroms per day and to ship them to the scientists. A large operation of PCs and CDroms burners was put in place with substantial manual intervention. Since 2006 the raw data are automatically transferred via public Internet to the Cluster Active Archive (see below). Once at PI and CoI institutes, the data were calibrated, and key parameters at medium and low temporal resolution could be extracted. The CSDS Implementation Working Group had defined the parameters that each instrument would produce (Daly et al. 2005). This had the advantage of decreasing the amount of data from 700–2,100 MBytes down to 20 MBytes per day. The production and distribution of these products were performed by eight data centers, located close to PI institutes and spread worldwide (Fig. 11). Since each data center was processing only data for a few instruments, special software (Cluster data management system) was produced, enabling them to exchange data. This process ensured that all data centers had the full database from all instruments. For Cluster I, public Internet could not guarantee sufficiently fast exchange of data. Consequently, dedicated lines were provided by ESOC to interconnect the data centers. For Cluster II, starting in the late 1990s, public Internet was fast enough to be used.
Fig. 11

The Cluster Science Data System (CSDS) made of eight interconnected data centers (yellow), processing data for the 11 instruments indicated. Three additional operation centers at ESTEC, ESOC, and RAL are indicated in red

During the first two years (2001–2002) of the Cluster mission, it was realized that Cluster scientific output would be greatly enhanced if the science community would have access to all high-resolution data and not only to medium and low resolution. At that time the network capacity had grown by a few orders of magnitude, and it was not a problem anymore to send high quantity of data through public network. In early 2003, the ESA SPC agreed to the development of the Cluster Active Archive (CAA) that was designed to:
  • Maximize the scientific return from the mission by making all Cluster data available to the worldwide scientific community.

  • Ensure that the unique data set returned by the Cluster mission is preserved in a stable, long-term archive for scientific analysis beyond the end of the mission.

  • Provide this archive as a major contribution by ESA and the Cluster science community to the International Living With a Star program.

After a few years of development to define the metadata and the Cluster Exchange Format (ASCII based), to process the first few years of data, and to develop the user interface, the Cluster Active Archive was open to the public in February 2006 (Laakso et al. 2010). The science community using CAA data has been growing continuously since 2006 at a rate around 20 new users every month, and now more than 1,790 scientists are using the data. The download rate has also been continuously growing, and at the beginning of 2014, it was above 2 TB/month. Furthermore a large portion of Cluster-published papers (Fig. 3) are using data from CAA, and their number has clearly increased since 2006, the year of CAA opening.


The Cluster mission is one of the most successful space missions dedicated to the study of the Sun-Earth connection . This is primarily due to the determination of the Cluster scientists who never compromised on the number of spacecraft necessary to achieve the objectives: during the development of the original Cluster mission and its successor Cluster II, the total number of spacecraft was always challenged in order to decrease cost. The answer from scientists was however always the same: four spacecraft is the minimum. They are now continually rewarded by the results achieved by Cluster. Another key aspect that helped to maximize science return was the fast and easy access to data that was first achieved by CSDS and then with the CAA.

Cluster for the first time with four identical spacecraft has discovered many aspects of plasma physics by measuring for the first time the 3rd dimension. A few highlights have been presented in this paper such as ripples on the bow shock, 3D current measurements and Kelvin-Helmholtz waves at the magnetopause, bifurcated current sheet in the magnetotail, and the first measurement of the electron pressure tensor near magnetic reconnection. In addition, acceleration of energetic electrons in the Earth’s magnetosphere, which was hit by a CME, was demonstrated. Extreme solar storms and their associated CMEs could have dramatic effects on human life (see US NSF report “Severe space weather events-understanding societal and economic impacts, 2008” or the UK Royal Academy and engineering report “Extreme space weather: impacts on engineered systems and infrastructure, 2013”). The growing interest by governments, especially in the very rare extreme events, has made space weather a permanent agenda item of the United Nations Committee on the Peaceful Uses of Outer Space. Cluster is not a space weather mission as such, but its measurements, especially on the study of dynamic changes in the Earth’s environment, are providing key input for the models that are being developed for space weather predictions.




The authors thank the PI teams for keeping the instrument in very good shape after more than 14 years in space: K. Torkar (IWF, Austria), I. Dandouras (IRAP/CNRS, France), R. Torbert (UNH, USA), C. Carr (IC, UK), A. Fazakerley (MSSL, UK), P. Daly (Gottingen U., Germany), M. Balikhin (Sheffield, UK), M. André (IRFU, Sweden), P. Canu (LPP, France), J. Pickett (U. Iowa, USA), and J.-L. Rauch (LPC2E, France). We also thank the ESOC and JSOC teams for spacecraft and science operations as well as industry (Astrium, Germany) for their continuous spacecraft operation support. We also thank the archiving teams at ESTEC and ESAC and the CSDS teams at National data centres.


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Copyright information

© Springer International Publishing Switzerland 2014

Authors and Affiliations

  • C. P. Escoubet
    • 1
    Email author
  • A. Masson
    • 1
  • H. Laakso
    • 1
  • M. G. G. T. Taylor
    • 1
  • J. Volpp
    • 2
  • D. Sieg
    • 2
  • M. Hapgood
    • 3
  • M. L. Goldstein
    • 4
  1. 1.ESA/ESTECNoordwijkThe Netherlands
  2. 2.ESA/ESOCDarmstadtGermany
  3. 3.RAL Space/STFCHarwell, OxfordUK
  4. 4.NASA/GSFCGreenbeltUSA

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