Cluster (Four Spacecraft Constellation in Concert with SOHO)

Cluster is a collaborative ESA/NASA multi-spacecraft mission within ESA's `Solar Terrestrial Science Program' (STSP), part of ISTP (International Solar Terrestrial Physics Programme). The objective is the observation of key interaction processes between two cosmic plasmas \[study of small-scale structures (from a few to a few tens of ion Larmor radii) in the Earth's plasma environment\]. The goal is to study the physical processes involved in the interaction between the solar wind and the magnetosphere by visiting key regions like the polar cusps and the magnetotail (mapping in three dimensions the plasma structures contained in these regions). Other regions of measurement are: a) solar wind and bow shock, b) magnetopause, and c) auroral zone. The simultaneous four-point measurements (with four S/C) also allow the derivation of differential plasma quantities for the first time. ^{1) 2)}

More detailed objectives call for the mapping of the small-scale plasma structures and current densities at:

• The solar wind bow shock: Study of the propagation of electric waves through the bow shock and magnetosheath

• The magnetopause, characterizing the motion and local geometry of the magnetopause, and identify the mechanism whereby plasma infiltrates the magnetopause

• The polar cusps, studying the behavior of postulated plasma vortices

• The magnetotail: observation of ion beams, and calculation of the magnitude of field aligned currents, in the plasma sheet boundary layer. Studies of the disruption of cross-tail currents during substorms, and the consequences for the plasma sheet

• The auroral zones. Determination of the sources of magnetospheric plasma, such as the polar wind, the cleft ion fountain, and nightside auroral zone.

Figure 1: Artist's view of the Cluster spacecraft constellation (image credit: ESA)

Background:

Cluster-I: The launch failure of Ariane-5 on June 4, 1996 with the Cluster satellites onboard and the ESA decision of a recovery program - are the reasons for a division into Cluster-I and Cluster-II programs.

The Cluster-I launch on June 4, 1996 with the first Ariane-5 flight from Kourou turned out to be a failure. 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 rocket vehicle to break up, prompting the onboard safety systems to initiate self-destruction. The entire four-spacecraft mission was lost.

Cluster-II program: In April 1997 ESA decided to go ahead with a recovery program, a new four-spacecraft mission, called Cluster-II, retaining all of the critical mission parameters. The Cluster-II mission comprises the Phoenix spacecraft (built with spares from the four original Cluster satellites) and three identical new satellites (construction and test of four S/C in 18 months, the industrial consortium was led by Astrium GmbH of Friedrichshafen, Germany). The overall objective is to investigate the physical interaction between the sun and the Earth, with four spacecraft flying in tetrahedral formation. ^{3) 4) 5) 6) 7) 8)}

Note: The new S/C was named after the mythical bird “Phoenix.” In ancient Egypt and in classical antiquity, phoenix is a fabulous long-lived bird associated with the worship of the sun. As its end approached, the phoenix fashioned a nest of aromatic boughs and spices, set it on fire, and was consumed in the flames. From the pyre miraculously sprang a new phoenix, which, after embalming its father's ashes in an egg of myrrh, flew with the ashes to Heliopolis (”City of the Sun”) in Egypt, where it deposited them on the altar in the temple of the Egyptian god of the sun, Re.

Spacecraft:

The four identical Cluster S/C are spin-stabilized (15 rpm, stringent requirements on electromagnetic cleanliness). Conductive surfaces and an extremely low S/C-generated electromagnetic background noise are mandatory for accurate electric field and cold plasma measurements.

Figure 2: Illustration of the Cluster spacecraft

Like their predecessor spacecraft of Cluster-I, each Cluster-II S/C is cylindrical in shape with a diameter of 2.9 m and a height of 1.3 m (launch mass = 1200 kg, payload = 71 kg, S/C (dry) = 550 kg, propellant = 650 kg), solar array power = 224 W (payload power 47 W), battery type: 5 identical silver-cadmium batteries, each of them is built of 14 cells in series with 16 Ah capacity (total battery capacity of about 80 Ah). S/C design life of 5 years (nominal operational lifetime of 27 months).

The S/C cylinder structure consists of aluminum honeycomb covered with a skin of carbon-fiber reinforced plastic (CFRP). The equipment panel inside this cylinder supports the main engine, two high-pressure fuel tanks and other parts of the propulsion system. Six spherical fuel tanks made from titanium are attached to the outside of this central cylinder. The fuel they carry accounts for more than half the launch weight of each spacecraft. Each spacecraft has a single main thruster (400 N) and eight smaller (10 N) thrusters for smaller changes of orbit. All thrusters use mono-ethyl hydrazine and mixed oxides of nitrogen for propellant. The large propellant mass is needed for an extensive series of maneuvers to reach the operational orbit and, during the course of the mission, to change the relative spacing of the S/C (constellation maintenance). The four spacecraft are spin-stabilized at 15 rpm. The in-orbit configuration is characterized by two 5 m experiment radial booms, four 50 m experiment wire booms and two axial telecommunications antenna booms. The S/C attitudes ensure a solar aspect angle of about 90º.

Figure 3: Cutaway of Cluster S/C main equipment platform showing FGM (1), EDI (2), and ASPOC (3) instruments (image credit: ESA)

The platform accommodates on one side the instruments. Each satellite carries two high capacity redundant tape recorders (data return of 50% per orbit). Two rigid booms, each 5 m, carry the magnetometers. Two pairs of wire booms, each with a tip-to-tip length of 100 m, permit electric field measurements (background magnetic field of×about 0.25 nT is aim).

Table 1: Overview of spacecraft parameters

Figure 4: Cutaway of a Cluster S/C showing its main structural features (image credit: ESA)

Launch: The launch of the Cluster-II mission used two Soyuz launch vehicles (with the Fregat upper stage and two S/C at a time) from Baikonur, Kazakhstan (launch provider: Starsem). ⁹⁾

• The launch of the first mission took place on July 16, 2000.

• The second pair of S/C was launched on Aug. 9, 2000.

The launch sequence called first for the injection of the S/C into an intermediate transfer orbit in pairs of two. Then, a series of propulsive maneuvers brought the four S/C from their initial transfer orbits into their mission orbits.

On the second launch day (Aug. 9, 2000), the four Cluster spacecraft were named by ESA as: Rumba (S/C 1), Salsa (S/C 2), Samba (S/C 3), and Tango (S/C 4). The names of the dances were suggested by Raymond Cotton of Bristol, UK (the winner of a naming competition); they are supposed to reflect the way in which the four satellites are dancing in formation around the heavens during their mission. The two S/C of the first launch carry the names Salsa (S/C 2) and Samba (S/C 3); the spacecraft of second launch have the names Rumba (S/C 1) and Tango (S/C 4).

Figure 5: Artist's rendition of the Fregat upper stage launching two Cluster spacecraft (image credit: ESA)

Orbit: HEO (Highly Elliptical Orbit), nominal apogee = 18.7 R_E (119,000 km), perigee = 3 R_E (19,000 km), inclination=90º, period of 57 hours (3420 min). The orbit for each S/C is selected so that each is located at a vertex of a predetermined tetrahedron when crossing the regions of interest within the magnetosphere. The size of this tetrahedron is varied from 200 km up to about 19,000 km during the course of the mission. The S/C, after release, use their own on-board propulsion systems to reach the final operational orbit. Since the Cluster-II orbit is fixed in the inertial system, the rotation of the Earth around the sun causes the S/C to cross the various near-Earth plasma regions, such as the Earth magnetotail soon after launch and the polar cusp and solar wind six months later.

Figure 6: Orbit plot and spacecraft configuration in neutral sheet as of June 2007 (image credit: ESA)

Figure 7: Artist's view of the Sun-Earth interaction studied by the four Cluster spacecraft (image credit: ESA)

Ground Segment:

Operations of four S/C are provided by two centers: ESA/ESOC in Darmstadt, Germany for mission operations, and JSOC (Joint Science Operations Center), located at the Rutherford Appleton Laboratory, Didcot, UK. The function of JSOC is the coordination of science operations. Its main task is to merge the input from the PI teams into a command schedule. In addition, JSOC monitors the health of the payload instruments and disseminates information of the mission.

RF communications: Science data are downlinked at a rate of 16.9 kbit/s real-time (105 kbit/s burst mode) or stored on one 2.25 Gbit solid-state recorder for replay (Note: the original two solid-state recorders were replaced by a single recorder of a new design). The S-band system operates at 2025-2110/2200-2290 MHz (up/down) at 10 W. The downlink data rate is variable from 2 to 262 kbit/s. Data from the four S/C is synchronized via a highly stable on-board clock. - The Villafranca ground station (Spain) of ESA is the prime station for all TT&C functions. Further support is provided by NASA's Deep Space Network (as well as other ground stations) during critical initial mission phases. About 1 GByte of science data per day is downlinked.

In the ground segment, the Cluster science data are available to the scientific community through CSDS (Cluster Science Data System). The CSDS consists of eight nationally funded and operated data centers. These are:

• Austrian Cluster Data Center, IWF, Graz (ASPOC)

• Chinese Cluster Data Center, Beijing (ASPOC)

• French Cluster Center, CNES, Toulouse (CIS, STAFF, WHISPER)

• German Cluster Data Center, MPE, Garching (EDI, RAPID)

• Scandinavian Data Center, KTH (Royal Institute of Technology), Stockholm (EFW)

• Hungarian Data Center, KFKI, Budapest

• UK Cluster Data Center, RAL, Didcot and QMW, London (DWP, FGM, PEACE)

• US Cluster Data Center, GSFC, Greenbelt, MD (WBD)

Status of mission:

The four Cluster spacecraft and their instruments are operational in 2010 (in their 10th year in orbit). The Cluster mission is extended to Dec. 31, 2012. ¹⁰⁾

• Cluster safely passed through its long eclipse season in October and November 2009, including a number of complete power-downs and restarts of the spacecraft. In the autumn, the apogee was lowered by 5000 km to improve data download volume. During winter, Cluster made the first plasma measurements in the auroral acceleration regions with four satellites and passed into the dayside magnetosphere to examine the magnetopause, magnetosheath and bow-shock. ¹¹⁾

• The spacecraft and instruments are operational in 2009. The Cluster mission has been extended twice in the past, up to June 2009. In early Feb. 2009, ESA provided another mission extension until Dec. 31, 2009. The new extension will make it possible to study the auroral regions above Earth’s poles and widen the investigations of the magnetosphere — its inner region in particular. ¹²⁾

Thanks to Cluster, scientists have reached an unprecedented understanding of the way solar activity affects the near-Earth environment. Cluster pioneered measurements of electric currents in space, revealed the nature of black aurorae, and discovered that plasma — a gas of charged particles surrounding Earth — makes ‘waves’.

• In the spring of 2009, ESA's Cluster mission revealed, for the first time, how turbulence develops in space just outside the Earth's magnetic environment. This result improves the understanding of turbulence, a key physical process by which energy throughout the Universe is transported from large scales at which it is input, to small scales where it is dissipated. ¹³⁾

The solar wind, when it reaches a magnetized planet, is first decelerated from supersonic to subsonic speed by a shock wave (called the bow shock), located in front of the magnetopause. The region between the bow shock and the magnetopause is called the magnetosheath. This region is one of the most turbulent environments of near-Earth space, making it an excellent laboratory in which to study turbulence. Characterizing the properties of the magnetic turbulence in this region is of prime importance to understand its role in fundamental processes such as energy dissipation or the acceleration of particles to high-energies.

The Cluster mission showed that extreme solar activity drastically compresses the magnetosphere and modifies the composition of ions in the near-Earth environment. A new model is needed to take these flare changes into consideration and to deduct how how these changes affect orbiting satellites, including the GPS system. - Under normal solar conditions, satellites orbit within the magnetosphere — the protective magnetic bubble carved out by Earth’s magnetic field. But when solar activity increases, the picture changes significantly: the magnetosphere gets compressed and particles get energized, exposing satellites to higher doses of radiation that can perturb signal reception. This is why monitoring and forecasting its impact on near-Earth space is becoming increasingly critical to safeguard daily life on Earth. ¹⁴⁾

• Out of 19 successive eclipses only 12 could be run nominally. ¹⁵⁾

• The 4 Cluster spacecraft are providing a detailed three-dimensional map of the magnetosphere, with surprising results (observation of magnetic reconnection events).

• December 29, 2006 marked the 1000th orbit around Earth of the Cluster constellation. ¹⁶⁾

• On Feb. 10, 2005, the ESA Science Program Committee approved the extension of the Cluster mission, pushing back the end date from December 2005 to December 2009. This extension will allow the first measurements of space plasmas at both small and large scales simultaneously and the sampling of geospace regions never crossed before by four spacecraft flying in close formation. ¹⁷⁾

• In the fall of 2006, the S/C went through a long ecliptic season. During this period, S/C 1 (Rumba) had very weak batteries. The counteract this, ESOC defined a new mode of operation called “decoder only” where the computer and all other subsystems are switched off. This warm-up period lasted from Sept. 15-23, 2006. The other three S/C recorded data as usual between eclipses. ¹⁸⁾

• December 29, 2006, marked the 1000th orbit around the Earth of the four ESA satellites composing the Cluster mission. Launched in the summer 2000, these spacecraft are delivering a unique harvest of in-situ scientific data of the Earth environment, from 25 000 km to 125 000 km in altitude. ¹⁹⁾

• The batteries of the satellites are well beyond their design lives and are starting to fail - the power situation first became critical during the long eclipses in September 2006. The battery aboard one could not power the heaters or computer, so new options had to be developed to avoid dangerous low temperatures and to regain control after each eclipse.

During eclipses, each satellite is powered by five silver-cadmium batteries. In the early 1990's, when Cluster was designed, these were the only non-magnetic batteries available (as Cluster's instruments were intended to measure magnetic fields, the internal fields had to be minimized). Their short lifetime of typically 2.5 years is limited by the amount of cadmium, which is gradually dissolved by the aggressive electrolyte. Hence, a new strategy for heating the satellites was developed and validated. ²⁰⁾

Sensor complement: (FGM, STAFF, EFW, WHISPER, WBD, DWP, EDI, CIS, PEACE, RAPID, ASPOC)

The instrument complement remained identical to that of the original version of Cluster-I. Each of the four Cluster-II spacecraft carries an identical set of 11 scientific instruments.

A Wave Experiment Consortium (WEC) was formed to get maximum scientific return from the available spacecraft instruments. WEC comprises five coordinated experiments designed for measuring electric and magnetic fluctuations, and small-scale structures within critical layers in the Earth's magnetosphere. These WEC experiments are: STAFF, EFW, WHISPER, DWP, and WBD. ^{21) 22)}

FGM (Fluxgate Magnetometer):

PI: A. Balogh, Imperial College, London, UK. Study of small-scale structures and processes in the Earth's environment. Objective: Provision of intercalibrated measurements of the magnetic field vector B at the four Cluster S/C. Identical instrumentation on all S/C (two triaxial fluxgate sensors and a data processing unit). Measurement of B wave from DC to 10 Hz, resolution => 6 pT. The magnetometers have eight possible operating ranges; of these, five are used on the Cluster magnetometers. The ranges were selected to provide good resolution in the solar wind (with expected field magnitudes between 3 and 30 nT), and up to the highest field values expected in the magnetosphere along the Cluster orbit (up to about 1,000 nT).

Table 2: Operating ranges of the FGM instrument

Figure 8: View of the FGM instrument (image credit: ESA)

STAFF (Spatio-Temporal Analysis of Field Fluctuations):

PI: N. Cornilleau-Wehrlin, CRPE/CNET, France). Objectives: Study of wave-particle interaction in the region where the solar wind meets the Earth's magnetosphere. STAFF looks at waves (rapid variations in the magnetic fields), particularly in regions where the charged particles of the solar wind interact with the magnetosphere. Measurement of B wave from up to 10 Hz, compressed data up to 4 kHz, cross-correlator for <E, B>. The STAFF instrument, located at the end of the 5 m boom, consists of a three-axis search coil magnetometer (measurement up to 4 kHz), and a spectrum analyzer to perform the auto and cross correlations between electric and magnetic components on board each satellite. STAFF is one of the five complementary experiments which form the Wave Experiment Consortium (WEC). - The tetrahedral orbit configuration of the Cluster spacecraft permits the study of the frontiers between the solar wind and Earth's magnetosphere. ^{23) 24)}

Figure 9: Illustration of the STAFF package (image credit: ESA)

EFW (Electric Fields and Waves):

PI: M. André, IRF-U (Swedish Institute of Space Physics), Uppsala, Sweden. Objectives: measurement of electric field and density fluctuations, determination of electron density and temperature, study of nonlinear processes that result in acceleration of the plasma, study of large-scale phenomena of data from all four S/C. The instrument consists of double probes, mounted on two pairs of wire booms, each 100 m tip-to-tip. Measurement of E wave from 10 Hz, compressed data up to 100 kHz, sensitivity < 50 nV/m (Hz)^1/2. EFW measurement modes: ^{25) 26)}

- Instantaneous spin plane components of the electric field vector, over a dynamic range of 0.1 - 700 mV/m, and with variable time resolution down to 0.1 ms.

- Low energy plasma density, over a dynamic range at least 1 to 100 cm^-3

- Electric field and density fluctuations in double layers of small amplitude, over dynamic ranges of 0.1 to 50 mV/m for the fields, and 1 - 50% for the relative density fluctuations, time resolution of 0.1 ms on some occasions

- Waves, ranging from electrostatic ion cyclotron emissions having amplitudes as large as 60 mV/m at frequencies as low as 50 mHz, to lower hybrid emissions at several hundred Hz and with amplitudes as small as a few µV/m.

- Time delays between signals from up to four different antenna elements on the same S/C, with a time resolution of 25 µs on some occasions.

- S/C potential

Figure 10: Illustration of the EFW experiment, 4 units and electronic boxes (image credit: ESA)

WHISPER (Waves of High Frequency and Sounder for Probing of Density by Relaxation):

PI: P. M. E. Décréau, LPCE (Laboratoire de Physique et Chimie de l'Environnement), Orléans, France. Objectives: accurate measurement of total plasma density within the range from 0.2 - 80 cm^-3 (prime objective), continuous survey of one electric component of plasma waves in the frequency range from 2 - 80 kHz with an accuracy of about 160 Hz (secondary objective).

WHISPER employs the method of relaxation sounding (using parts of the EFW wire booms). The analysis of density variations is performed via active sounding of plasma resonances (a radar technique). Brief radio pulses, sent out from two 50 m long wire antennas, trigger oscillations or 'echoes', which are detected after a short delay. Their frequencies reveal the particle concentration in the medium. In addition, Whisper monitors natural wave activity in the frequency range (2 to 80 kHz) covered by the sounder.

Expected measurement results:

- identification of regions in space and mass transport

- spatial extension and drift speed

- density fluctuations

- wave mode identification

- coldest component of the electron density

Passive measurements of natural plasma waves up to 400 kHz. WHISPER is one of the five complementary experiments which form the Cluster WEC (Wave Experiment Consortium).

Figure 11: Illustration of WHISPER (image credit: ESA)

WBD (Wide Band Data):

PI: D. A. Gurnett, University of Iowa, Iowa City, IA, USA. Objectives: Study of E-field wave form in the Earth's magnetosphere. A wideband receiver system measures electric and magnetic fields over a frequency range from 25 Hz to 577 kHz. The Cluster wideband receiver is similar to the instruments flown on ISEE-1 (International Sun Earth Explorer-1) and DE-1. WBD makes use of the EFW sensors (two electric dipole antennas, two search coil magnetometers); conversion frequencies: 0, 125 kHz, 250 kHz, 500 kHz; frequency resolution: limited by FFT (75 Hz typical); time resolution: 10-20 ms (per FFT spectrum); The WBD measurements complement those of the other WEC instruments and also provide a unique new capability to perform very-long baseline interferometry (VLBI) measurements. - The wideband technique involves transmitting band-limited waveform data to a ground station using a high-rate data link (262 kbit/s R/T or 131 kbit/s in burst mode). ²⁷⁾

Table 3: Specification of the WBD instrument

Figure 12: Illustration of the WBD instrument (image credit: University of Iowa)

DWP (Digital Wave Processor):

PI: H. Alleyne, University of Sheffield, UK. Objectives: Correlation of wave/particle phenomena and wave/particle interactions. DWP is the on-board control system (multiprocessor unit based on the use of transputers with parallel processing and re-allocatable tasks to provide a high-reliability system.) of all WEC instruments performing data compaction and compression, event selection, particle/wave correlation, and control of WHISPER.

Figure 13: Illustration of the DWP instrument (image credit: University of Sheffield)

EDI (Electron Drift Instrument):

PI: G. Paschmann, MPE, Garching, Germany). EDI is a collaborative effort between MPE, UNH (University of New Hampshire), UIA (University of Iowa), IWF (Institut für Weltraumforschung), Graz, Austria, and UCSD (University of California, San Diego). Objectives: accurate and highly sensitive measurements of the electric field. EDI measures the drift of a weak beam of test electrons which for certain emission directions return to the S/C after one gyration (the drift is related to the electric field and to the gradient in the magnetic field). EDI consists of two emitter/detector assemblies, each with a full view (FOV of 360º). Emission and tracking of two electron beams. ²⁸⁾

EDI fires two weak beams of electrons 10 km or more into the space around each spacecraft. When properly aimed, the electrons eventually return to receivers on the opposite side of the spacecraft. The returning beams are affected by the strength of the electric field in space and by the gradient in the ambient magnetic field. From beam firing directions and electron travel times, scientists can determine the strength of the electric field. As a by-product, the magnetic field strength is also measured.

Figure 14: Illustration of the EDI instrument (image credit: ESA)

CIS (Cluster Ion Spectrometry) experiment:

PI: H. Rème, CESR (Centre d'Etudes Spatial des Rayonnement), Toulouse, France. The CIS experiment is a collaborative effort of many international institutions, under the principal responsibility of CESR (Toulouse, France). Objectives: Study of the dynamics of magnetized plasma structures in the vicinity of the Earth's magnetosphere (physics of the bow shock, the magnetopause boundary, the polar cusp, the geomagnetic tail, and the plasma sheet). CIS is a comprehensive ionic plasma spectrometry package on-board the four Cluster spacecraft capable of obtaining full 3-D ion distributions with good time resolution (one spacecraft spin) with mass per charge composition determination.

CIS consists of two instruments, CODIF and HIA to measure both the cold and hot ions from the solar wind, the magnetosheath, and the magnetosphere (including the ionosphere) with sufficient angular, energy, and mass resolution. ^{29) 30)}

• CODIF (Composition and Distribution Functions) analyzer; energy range = 0.02 - 40 keV/q to measure the 3-D distribution of major ion species (H⁺, He²⁺, He⁺ , and O⁺). The instrument is a symmetrical hemispherical analyzer with RPA (Retarding Potential Analyzer) and TOF (Time-of-Flight) electronics, FOV = 360º x 8º; split geometric factor.

• HIA (Hot Ion Analyzer) for high time resolution of the solar wind (energy range = 3 eV/q - 40 keV/q). The instrument is a symmetric quadri-spherical analyzer; FOV = 360º x 8º with high resolution. (=>2.8º).

Figure 15: The CODIF (left) and HIA instruments of CIS (image credit: MPE)

PEACE (Plasma Electron and Current Analyzer):

PI: A Fazakerley, MSSL (Mullard Space Science Laboratory), University College, London, UK. Objectives: Study of the 3-D velocity distribution of electrons in the vicinity of its host spacecraft. PEACE consists of two electron sensor devices. LEFA = Low Energy Electron Analyzer (energy range = 0 - 100 eV). The instrument is a spherical electrostatic analyzer with a FOV of 180º x 3.8º. HEFA (High Energy Electron Analyzer), energy range = 0.1 - 30 keV. The instrument is a troidal electrostatic analyzer with a FOV of 360º x 4.6º. ³¹⁾

Figure 16: The PEACE instrument (image credit: ESA)

RAPID (Research with Adaptive Particle Imaging Detectors):

PI: P. Daly, MPS (Max Planck Institute for Solar System Research), Lindau/Harz, Germany, formerly known as MPAe (Max Planck Institut für Aeronomie). Objectives: Study of suprathermal plasma distributions in the energy range from 20 - 400 keV and 2 keV/n - 1500 keV for electrons and ions, respectively. ³²⁾

RAPID provides complete coverage of the unit sphere in phase space. It is made up of two instruments, the IIMS and IES.

• IIMS (Imaging Ion Mass Spectrometer) for ion distribution. IIMS is a position-sensitive instrument with solid state detectors and TOF (Time-of-Flight) electronics. FOV= 3º x 180º

• IES (Imaging Electron Spectrometer) for measuring energetic electrons. IES has a position-sensitive solid state detector. FOV = 17.5º x 180º.

Figure 17: Illustration of the RAPID instrument (image credit: MPE)

ASPOC (Active Spacecraft Potential Control):

PI: K. Torkar, IWF (Institut für Weltraumforschung), Graz, Austria. Objective: Control of the spacecraft charge potential to insure effective, complete measurement of the ambient plasma distribution functions. The basic approach involves the active emission of positive charges from the S/C sufficient to balance the excess of charge accumulating on the satellite by the environment (photo emission of electrons drives the S/C potential positive relative to the plasma potential).

The method employed is field ionization with a Liquid Metal Ion Source (LMIS) of the “solid needle” type. LMIS uses Indium as charged material. The instrument consists of an electronics box and two cylindrical emitter modules with five needles each. The released ions cancel out the electrical charge that the satellite acquires. The ion current is adjusted by using measurements made by other onboard instruments, which record the spacecraft's electrical potential (EFW and PEACE). ³³⁾

Figure 18: Illustration of ASPOC (image credit: ESA)

The ASPOC instrument consists of two cylindrical modules that both contain four ion emitters, as well as an electronics box with the microprocessor unit and current supply unit including the high voltage and heating supply for the ion emitters. The ion emitters are of the liquid metal type, and use Indium as charge carriers. A small needle gets wetted with Indium and heated over 157ºC, such that the Indium liquefies. The Indium at the tip of the accreting liquidity cone gets ionized when a potential of several kilovolts between the needle and an external electrode is applied and creates an ion beam.

The instrument uses a total of eight emitters, distributed over two modules with separate high voltage supplies. Only one emitter is operated at a time. The others serve as backup units and to extend the operability, as the life cycle of an emitter is limited by the Indium content of its reservoir. In total the Indium content suffices for an operational span of 32,000 hours at 10 µA ion current. The mass of the instrument is 1.9 kg, the power consumption is up to 2.7 Watt.

2) J. Credland, G. Mecke, J. Ellwood, F. Drigani, P. Ferri, et al., Special Section of the Cluster mission, spacecraft, payload, data, and mission operations, ESA Bulletin, No. 84, Nov. 1995, pp. 113-150, URL: http://www.esa.int/esapub/bulletin/bullet84/credl84.htm

3) “The Cluster-II Mission - Rising from the Ashes,” The Cluster-II Project Team, ESA Bulletin, 102, May 2000, pp.47-53, URL: http://www.esa.int/esapub/bulletin/bullet102/Cluster102.pdf

4) C. Ph. Escoubet, “Cluster-II: Scientific Objectives and Data Dissemination,” ESA Bulletin, 102, May 2000, pp. 54-60, URL: http://www.esa.int/esapub/bulletin/bullet102/Warhaut102.pdf

5) M. Warhaut, S. Matussi, P. Ferri, “Cluster-II: Evolution of the Operations Concept,” ESA Bulletin, 102, May, pp. 61-67, URL: http://www.esa.int/esapub/bulletin/bullet102/Escoubet102.pdf

6) J. Credland, R. Schmidt, “The Resurrection of the Cluster Scientific Mission,” ESA Bulletin, No. 91, August 1997, pp. 5-10, URL: http://www.esa.int/esapub/bulletin/bullet91/b91cred.htm

7) http://sci.esa.int/cluster/

8) The Cluster mission is covered in considerable detail in Space Science Reviews, Vol. 79, Jan. 1997, pp. 11-658.

9) http://clusterlaunch.esa.int/science-e/www/area/index.cfm?fareaid=8

10) Feb. 10, 2010, URL: http://sci.esa.int/science-e/www/area/index.cfm?fareaid=8

11) Information extracted from ESA Bulletin Nr. 141, Feb. 2010, p. 64 -- under the heading: “Programs in Progress: Status at end December 2009,”

12) “ESA extends missions studying Mars, Venus and Earth’s magnetosphere,” Feb. 10, 2009, URL: http://www.esa.int/esaCP/SEMACI05VQF_index_0.html

13) “Cluster's insight into space turbulence,” ESA, March 25, 2009, URL: http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=44480

14) Philippe Escoubet, “Watching solar activity muddle Earth’s magnetic field,” April 29, 2009, ESA, URL: http://www.esa.int/esaCP/SEMF75BNJTF_index_0.html

15) http://clusterlaunch.esa.int/science-e/www/object/index.cfm?fobjectid=42505

16) http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=40465

17) H. Laakso, P. Escoubet, H. Opgenoorth, J. Volpp, S. Pallaschke, M. Hapgood, “Cluster - A microscope and a telescope for studying space plasmas,” ESA Bulletin, No 121, Feb. 2005, pp. 10-17, URL: http://www.esa.int/esapub/bulletin/bulletin121/bul121b_laakso.pdf

18) ESA Bulletin, No 128, November 2006, p. 80,

19) http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=40465

20) J. Volpp, J. Godfrey, S. Foley, S. Sangiorgi, P. Appel, M. Pietras, P. Escoubet, H. Fiebrich, M. Schautz, B. Lehmann, “Sleeping Satellites - Nursing Cluster through Critical Eclipses,” ESA Bulletin No 129, Feb. 2007, pp. 26-32, URL: http://www.esa.int/esapub/bulletin/bulletin129/bul129c_volpp.pdf

21) C. P. Escoubet, C. T. Russell, R. Schmidt \[editors\], “The Cluster and Phoenix Missions,” published in a special Cluster issue of Space Science Reviews, Vol. 79, No. 1-2, 1997, Kluver

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33) K. Torkara, Corresponding Author Contact Information, E-mail The Corresponding Author, M. Fehringerb, C. P. Escoubetb, M. Andréc, A. Pedersend, K. R. Svenese, P. M. E. Décréau, “Analysis of Cluster spacecraft potential during active control,” Advances in Space Research, Vol. 36, Issue 10, 2005, pp. 1922-1927