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
Cluster satellite FM2
The first four Cluster spacecraft were lost in
the Ariane 5 flight failure
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
/ESA's joint Double Star mission
Cluster mission overview
The four identical Cluster satellites research the protective
magnetosphere of the Earth that shields us from the continual
. Cluster FM5 to FM8 (FM1 to
FM4 were lost in the 1996 failed
) measure three dimensional
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).
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
where 1 RE
= 6371 km) and an apogee
of 19.6 RE
. Each orbit takes
approximately 57 hours
Space Operations Centre (ESOC) acquires telemetry
and distributes the science data from the spacecraft
The Cluster mission was proposed to ESA in 1982 and approved in
1986, along with the Solar and Heliospheric
(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.
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.
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
and the auroral
- The bow shock (see
illustration to the right) is the region in space between the Earth
and the sun where the solar wind decelerates
from super- to sub-sonic before being deflected around the Earth.
In traversing this region, Cluster makes measurements which help
characterize processes occurring at the bow shock, such as the
origin of hot flow anomalies and
the transmission of electromagnetic waves through the
bow shock and the magnetosheath from
the solar wind.
- Behind the bow shock is the thin plasma layer separating the
Earth and solar wind magnetic fields known as the magnetopause. This boundary moves
continuously due to the constant variation in solar wind pressure.
Since the plasma and magnetic pressures within the solar wind and
the magnetosphere, respectively, should be in equilibrium, the
magnetosphere should be an impenetrable boundary. However, plasma
has been observed crossing the magnetopause into the magnetosphere
from the solar wind. Cluster's four-point measurements make it
possible to track the motion of the magnetopause as well as
elucidate the mechanism for plasma penetration from the solar
- In two regions, one in the northern hemisphere and the other in
the south, the magnetic field of the Earth is perpendicular rather
than tangential to the magnetopause. These polar
cusps allow solar wind particles, consisting of ions and
electrons, to flow into the magnetosphere. Cluster records the
particle distributions, which allow the turbulent regions at the
exterior cusps to be characterized.
- The regions of the Earth's magnetic field that are stretched by
the solar wind away from the sun are known collectively as the
magnetotail. Two lobes that reach past the Moon in
length form the outer magnetotail while the central plasma sheet
forms the inner magnetotail, which is highly active. Cluster
monitors particles from the ionosphere
and the solar wind as they pass through the magnetotail lobes. In
the central plasma sheet, Cluster determines the origins of ion
beams and disruptions to the magnetic field-aligned currents caused
- The precipitation of charged particles in the atmosphere
creates a ring of light emission around the magnetic pole known as
the auroral zone. Cluster measures the time
variations of transient particle flows in the region.
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
) and magnetic (B
magnitudes and directions and the densities and distributions of
particles (electrons and ions) in the plasma.
||Magnetic field B magnitude and direction
||B vector and event trigger to all instruments
||Electric Field and Wave experiment
||Electric field E magnitude and direction
||E vector, spacecraft potential, electron
density and temperature
||Spatio-Temporal Analysis of Field Fluctuation experiment
||Magnetic field B magnitude and direction of EM
fluctuations, cross-correlation of E and
||Properties of small-scale current structures, source of plasma
waves and turbulence
||Waves of High Frequency and Sounder for Probing of Density by
||In active mode, total electron density ρ; in passive mode,
neutral plasma waves
||Plasma density ρ measurements unaffected by fluctuations in
||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
||Digital Wave Processing instrument
||Control over and communication between instruments 2-5 to yield
||Electron Drift Instrument
||Electric field E magnitude and direction
||E vector, gradients in local magnetic field
||Active Spacecraft Potential Control experiment
||Regulation of spacecraft's electrostatic potential
||Control over and communication between instruments 2-5 and
||Cluster Ion Spectroscopy experiment
||Ion times-of-flight (TOFs) and energies from 0 to 40 keV
||Composition and 3D distribution of ions in plasma
||Plasma Electron and Current Experiment
||Electron energies from 0.0007 to 30 keV
||3D distribution of electrons in plasma
||Research with Adaptive Particle Imaging Detectors
||Electron energies from 30 to 1500 keV, ion energies from 20 to
||3D distributions of high-energy electrons and ions in
Double Star mission with China
In late 2003 and the middle of 2004 the China National Space
launched the Double Star
satellites that work
together with Cluster to make synchronous measurements of the
magnetosphere at much greater spacecraft separations.
Partial list of discoveries
- March 9 - discovery of vortices ranging in size from 40,000
down to around 100 kilometres in Earth's polar magnetic field
- April 20 - The first direct measurements of Earth's ring
- May 20 - Cluster observes 'reverse reconnection',
simultaneously with a bright proton aurora observed by IMAGE.
- April 5 - The first unambiguous measurements of the thickness
of the Earth's Bow Shock.
- December 12 - Cluster determines the spatial scale of highspeed
flows in the magnetotail.
- February 4 - Cluster observes 3D magnetic reconnection.
- December 5 - Cluster helps predict 'killer electrons'.
- January 11 - Cluster observes magnetic reconnection region larger
than 2.5 million km in the solar wind.
- July 18 - Cluster observes the 'magnetic heart' of reconnection
in the magnetotail.
- July 20 - Cluster and Double Star discover density holes in the