The Cluster mission is a European Space Agency (ESA) unmanned space mission to study the Earth's magnetosphere using four identical spacecraft flying in a tetrahedral formation. The first four Cluster spacecraft were lost in the Ariane 5 flight failure on 1996 June 4, leading to the construction of four new spacecraft and their successful launching in 2000 on Soyuz-Fregat rockets. Cluster operated alongside China National Space Administration/ESA's joint Double Star mission.
The four identical Cluster satellites research the protective magnetosphere of the Earth that shields us from the continual solar wind. Cluster FM5 to FM8 (FM1 to FM4 were lost in the 1996 failed launch) measure three dimensional data from the collision of the solar wind with the Earth's magnetic field, its changes over time and the effects on near-Earth space and its atmosphere, including aurorae.
The spacecraft are cylindrical (290 x 130 cm, see ) and are spin-stabilized at 15 rotations per minute. Their solar cells provide 224 watts power for instruments and communications. The four spacecraft maneuver into various tetrahedral formations in order to study the magnetospheric structure and boundaries. The inter-spacecraft distances can be varied from around 100 to 10000 kilometers (km). The propellant for the maneuvers makes up approximately half of the spacecraft's launch weight.
The highly elliptical orbits of the spacecraft reach a perigee of around 4 RE (Earth radii, where 1 RE = 6371 km) and an apogee of 19.6 RE. Each orbit takes approximately 57 hours to complete. The European Space Operations Centre (ESOC) acquires telemetry and distributes the science data from the spacecraft online.
The Cluster mission was proposed to ESA in 1982 and approved in 1986, along with the Solar and Heliospheric Observatory (SOHO). Though the original Cluster spacecraft were completed in 1995, the explosion of the rocket carrying the satellites in 1996 delayed the mission for another four years while the instruments were rebuilt.
On 16 July 2000, a Soyuz-Fregat rocket from the Baikonur Cosmodrome launched two of the Clusters (Salsa and Samba) into a parking orbit from where they maneuvered under their own power into a 19,000 by 119,000 kilometer orbit with a period of 57 hours. Three weeks later on 9 August 2000 another Soyuz-Fregat rocket lifted the remaining two Cluster spacecraft (Rumba and Tango) into similar orbits. Spacecraft 1, Rumba, is also known as the Phoenix spacecraft, since it is largely built from spare parts left over after the failure of the original mission. After commissioning of the payload, the first scientific measurements were officially made on 1 February 2001.
The ESA ran a competition to name the Cluster satellites, which attracted participants from many countries. Ray Cotton from the United Kingdom won with the names Rumba, Tango, Salsa and Samba. Ray's town of residence, Bristol, was awarded with scale models of the satellites in recognition of the naming and connection with the satellites.
Originally planned to last until the end of 2003, the mission has been extended several times. The first extension took the mission from 2004 until 2005 and the second from 2005 to June 2009. The mission has now been extended until end 2012.
Previous single and two-spacecraft missions were not capable of providing the data required to accurately study the boundaries of the magnetosphere. Because the plasma comprising the magnetosphere cannot presently be accessed using remote sensing techniques, satellites must be used to measure it in-situ. Four spacecraft allow scientists make the 3D, time-resolved measurements needed to create a realistic picture of the complex plasma interactions occurring between regions of the magnetosphere and between the magnetosphere and the solar wind.
Each satellite carries a scientific payload of 11 instruments designed to study the small-scale plasma structures in space and time in the key plasma regions: the solar wind and bow shock, magnetopause, polar cusps, magnetotail and the auroral zone.
Details of the 11 instruments aboard each of the four Cluster spacecraft are provided in the table below. Briefly, the instruments are dedicated to measuring the electric (E) and magnetic (B) field magnitudes and directions and the densities and distributions of particles (electrons and ions) in the plasma.
|1||FGM||Fluxgate Magnetometer||Magnetic field B magnitude and direction||B vector and event trigger to all instruments except ASPOC|
|2||EFW||Electric Field and Wave experiment||Electric field E magnitude and direction||E vector, spacecraft potential, electron density and temperature|
|3||STAFF||Spatio-Temporal Analysis of Field Fluctuation experiment||Magnetic field B magnitude and direction of EM fluctuations, cross-correlation of E and B||Properties of small-scale current structures, source of plasma waves and turbulence|
|4||WHISPER||Waves of High Frequency and Sounder for Probing of Density by Relaxation||In active mode, total electron density ρ; in passive mode, neutral plasma waves||Plasma density ρ measurements unaffected by fluctuations in spacecraft potential|
|5||WBD||Wide Band Data receiver||Electric field E waveforms and spectrograms of terrestrial plasma waves and radio emissions||Motion of terrestrial fluctuations, e.g. auroral kilometric radiation|
|6||DWP||Digital Wave Processing instrument||Data manipulation||Control over and communication between instruments 2-5 to yield particle correlations|
|7||EDI||Electron Drift Instrument||Electric field E magnitude and direction||E vector, gradients in local magnetic field B|
|8||ASPOC||Active Spacecraft Potential Control experiment||Regulation of spacecraft's electrostatic potential||Control over and communication between instruments 2-5 and 10|
|9||CIS||Cluster Ion Spectroscopy experiment||Ion times-of-flight (TOFs) and energies from 0 to 40 keV||Composition and 3D distribution of ions in plasma|
|10||PEACE||Plasma Electron and Current Experiment||Electron energies from 0.0007 to 30 keV||3D distribution of electrons in plasma|
|11||RAPID||Research with Adaptive Particle Imaging Detectors||Electron energies from 30 to 1500 keV, ion energies from 20 to 450 keV||3D distributions of high-energy electrons and ions in plasma|
In late 2003 and the middle of 2004 the China National Space Administration launched the Double Star satellites that work together with Cluster to make synchronous measurements of the magnetosphere at much greater spacecraft separations.