Cluster satellite FM2
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.
Cluster mission overview
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
[173994]) 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 R
E (Earth radii,
where 1 R
E = 6371 km) and an
apogee of 19.6 R
E. 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.
History
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.
Scientific objectives
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.
- 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
wind.
- 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
by substorms.
- 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.
Instrumentation
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.
| Number |
Acronym |
Instrument |
Measurement |
Purpose |
| 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 |
Notes
Double Star mission with China
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.
Partial list of discoveries
2002
- 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
current density
2003
- May 20 - Cluster observes 'reverse reconnection',
simultaneously with a bright proton aurora observed by IMAGE.
2004
- 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.
2005
- February 4 - Cluster observes 3D magnetic reconnection.
- December 5 - Cluster helps predict 'killer electrons'.
2006
- 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
solar wind.
2007
2008
2009
References
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