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COSPAR-IDs of FM6 and FM7 were switched. See https://discosweb.esoc.esa.int/objects?filter=%5B%5B%22cosparId%22%2C%222000-041B%22%5D%5D, https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2000-041B
 
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{{Short description|European Space Agency space mission}}
{{External links|date=January 2021}}
{{Infobox spaceflight
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| operator = [[ESA]] with [[NASA]] collaboration
| website = http://sci.esa.int/cluster
| COSPAR_ID = '''FM6''' (SALSA): 2000-041A041B<br />'''FM7''' (SAMBA): 2000-041B041A<br />'''FM5''' (RUMBA): 2000-045A<br />'''FM8''' (TANGO): 2000-045B
| SATCAT = '''FM6''' (SALSA): 26411<br />'''FM7''' (SAMBA): 26410<br />'''FM5''' (RUMBA): 26463<br />'''FM8''' (TANGO): 26464
| mission_duration = Planned: 5 years <br/>Final: {{time interval|16 July 2000|22 August 2024}}
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}}
 
'''Cluster II'''<ref>{{cite web |url=http://www.esa.int/esaMI/Operations/SEMYSMQJNVE_0.html |title=Cluster II operations|publisher=European Space Agency |access-date=29 November 2011 }}</ref> was a space mission of the [[European Space Agency]], with [[NASA]] participation, to study the [[Earth]]'s [[magnetosphere]] over the course of nearly two [[solar cycle]]s. The mission was composed of four identical spacecraft flying in a [[Tetrahedron|tetrahedral]] formation. As a replacement for the original [[Cluster (spacecraft)|Cluster]] spacecraft which were lost in a launch failure in 1996, the four Cluster II spacecraft were successfully launched in pairs in July and August 2000 onboard two [[Soyuz-Fregat]] [[rocket]]s from [[Baikonur]], [[Kazakhstan]]. In February 2011, Cluster II celebrated 10 years of successful scientific operations in space. In February 2021, Cluster II celebrated 20 years of successful scientific operations in space. {{As of|March 2023}}, its mission has beenwas extended until September 2024.<ref name="esa-20230307">{{cite web |url=https://sci.esa.int/web/director-desk/-/extended-life-for-esa-s-science-missions |title=Extended life for ESA's science missions |work=[[ESA]] |date=7 March 2023 |access-date=20 March 2023}}</ref> The [[China National Space Administration]]/ESA [[Double Star mission]] operated alongside Cluster II from 2004 to 2007.
 
The first of the satellites of Cluster II to re-enter the atmosphere did so on 8 September 2024. The remaining three are expected to follow in 2025 and 2026.<ref name="salsareenter"/> The scientific payload operations of all satellites ended as the first satellite re-entered the atmosphere (other flight operations are still being performed with the remaining flying satellites until the satellites have all re-entered).<ref name="endofpayloadoperations">{{cite web |url=https://www.mps.mpg.de/planetary-science/cluster-mission |title=Cluster II: Mission to the Earth's Magnetosphere |work=[[Max Planck Institute]] |date=2024 |access-date=9 September 2024}}</ref>
 
== Mission overview ==
The four identical Cluster II satellites studystudied the impact of the Sun's activity on the Earth's space environment by flying in formation around Earth. For the first time in space history, this mission iswas able to collect three-dimensional information on how the [[solar wind]] interacts with the [[magnetosphere]] and affects near-Earth space and its [[atmosphere]], including [[aurora (astronomy)|aurorae]].
 
The spacecraft arewere cylindrical (2.9 x 1.3&nbsp;m, see [http://clusterlaunch.esa.int/science-e/www/object/index.cfm?fobjectid=31381 online 3D model]) and arewere spinning at 15 rotations per [[minute]]. After launch, their [[solar cell]]s provided 224 [[watt]]s power for instruments and communications. Solar array power has gradually declined as the mission progressed, due to damage by energetic charged particles, but this was planned for and the power level remains sufficient for science operations. The four spacecraft maneuvermaneuvered into various tetrahedral formations to study the magnetospheric structure and boundaries. The inter-spacecraft distances cancould be altered and has varied from around 4 to 10,000&nbsp;km. The [[propellant]] for the transfer to the operational orbit, and the maneuvers to vary inter-spacecraft separation distances made up approximately half of the spacecraft's launch weight.
 
The highly [[Ellipse|elliptical]] [[orbit]]s of the spacecraft initially reached a [[perigee]] of around 4 R<sub>E</sub> (Earth radii, where 1 R<sub>E</sub> = 6371&nbsp;km) and an [[apogee]] of 19.6 R<sub>E</sub>. Each orbit took approximately 57 [[hour]]s to complete. The orbit has evolved over time; the line of apsides has rotated southwards so that the distance at which the orbit crossed the magnetotail current sheet progressively reduced, and a wide range of dayside magnetopause crossing latitudes were sampled. Gravitational effects imposeimposed a long term cycle of change in the perigee (and apogee) distance, which saw the perigees reduce to a few 100&nbsp;km in 2011 before beginning to rise again. The orbit plane has rotated away from 90 degrees inclination. Orbit modifications by ESOC have altered the orbital period to 54 hours. All these changes have allowed Cluster to visit a much wider set of important magnetospheric regions than was possible for the initial 2-year mission, improving the scientific breadth of the mission.
 
The [[European Space Operations Centre]] (ESOC) acquiresacquired [[telemetry]] and distributesdistributed to the online data centers the science data from the spacecraft. The Joint Science Operations Centre (JSOC) at [[Rutherford Appleton Laboratory]] in the UK coordinatescoordinated scientific planning and in collaboration with the instrument teams providesprovided merged instrument commanding requests to ESOC.
 
The [https://csa.esac.esa.int/ Cluster Science Archive] is the [[ESA]] long term archive of the Cluster and Double Star science missions. Since 1 November 2014, it is the sole public access point to the Cluster mission scientific data and supporting datasets. The Double Star data are publicly available via this archive. The Cluster Science Archive is located alongside all the other [[ESA]] science archives at the [[European Space Astronomy Center]], located near Madrid, Spain. From February 2006 to October 2014, the Cluster data could be accessed via the [http://caa.esac.esa.int/caa/ Cluster Active Archive].
 
==History==
The [[Cluster (spacecraft)|Cluster]] mission was proposed to ESA in 1982 and approved in 1986, along with the [[Solar and Heliospheric Observatory]] (SOHO), and together these two missions constituted the Solar Terrestrial Physics "cornerstone" of ESA's Horizon 2000 missions programme. Though the original Cluster spacecraft were completed in 1995, the explosion of the [[Ariane 5]] rocket carrying the satellites in 1996 delayed the mission by four years while new instruments and spacecraft were built.
 
On July 16, 2000, a Soyuz-Fregat rocket from the [[Baikonur Cosmodrome]] launched two of the replacement Cluster II spacecraft, (Salsa and Samba) into a parking orbit from where they maneuvered under their own power into a 19,000 by 119,000 kilometre [[orbit]] with a period of 57 hours. Three weeks later on August 9, 2000, another Soyuz-Fregat rocket lifted the remaining two spacecraft (Rumba and Tango) into similar orbits. Spacecraft 1, Rumba, iswas also known as the [[Phoenix (mythology)|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 made on February 1, 2001.
 
The [[European Space Agency]] ran a competition to name the satellites across all of the [[European Space Agency|ESA]] member states.<ref>{{cite journal|url=http://www.uv.es/~maicasj/cluster-contest.pdf|title=European Space Agency Announces Contest to Name the Cluster Quartet|journal=XMM-Newton Press Release|pages=4|publisher=European Space Agency|bibcode=2000xmm..pres....4.|year=2000}}</ref> Ray Cotton, from the [[United Kingdom]], won the competition with the names [[Rumba]], [[Tango]], [[Salsa (dance)|Salsa]] and [[Samba]].<ref>{{cite web|url=http://sci.esa.int/cluster/27816-bristol-and-cluster-the-link/|title=Bristol and Cluster – the link|publisher=European Space Agency|access-date=2 September 2013}}</ref> Ray's town of residence, [[Bristol]], was awarded with scale models of the satellites in recognition of the winning entry,<ref>{{cite news|url=http://www.spaceref.com/news/viewpr.html?pid=5446|archive-url=https://archive.today/20130903050003/http://www.spaceref.com/news/viewpr.html?pid=5446|url-status=dead|archive-date=September 3, 2013|title=Cluster II – Scientific Update and Presentation of Model to the City of Bristol|newspaper=Spaceref|date=9 July 2001|publisher=SpaceRef Interactive Inc.}}</ref><ref>{{cite web|url=http://sci.esa.int/cluster/27719-cluster-presentation-of-model-to-the-city-of-bristol-and-science-results-overview/|title=Cluster – Presentation of model to the city of Bristol and science results overview|publisher=European Space Agency}}</ref> as well as the city's connection with the satellites. However, after many years of being stored away, they were finally given a home at the [[Rutherford Appleton Laboratory]].
 
Originally planned to last until the end of 2003, the mission has beenwas extended several times. The first extension took the mission from 2004 until 2005, and the second from 2005 to June 2009. The mission haswas now beenultimately extended until September 2024, when the scientific payload operations on the satellites ended.<ref name="esa-20230307" /> The ultimate end of the Cluster project (especially the Cluster II satellites) will happen in 2026 as the last satellite enters the atmosphere and is destroyed.<ref name="endofpayloadoperations" />
 
==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 (physics)|plasma]] comprising the magnetosphere cannot be viewed using remote sensing techniques, satellites must be used to measure it in-situ. Four spacecraft allowallowed 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 carriescarried a scientific payload of 11 instruments designed to study the small-scale plasma structures in space and time in the key plasma regions: solar wind, [[bow shock]], [[magnetopause]], polar cusps, [[magnetotail]], [[plasmapause]] boundary layer and over the polar caps and the auroral zones.
 
* The '''[[bow shock]]''' 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, the spacecraft makemade measurements which helphelped characterize processes occurring at the bow shock, such as the origin of hot flow anomalies and the transmission of [[Electromagnetic radiation|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 makemade 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 southsouthern, 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 recordsrecorded the particle distributions, which allowallowed 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 monitorsmonitored particles from the [[ionosphere]] and the solar wind as they passpassed through the magnetotail lobes. In the central plasma sheet, Cluster determinesdetermined the origins of ion beams and disruptions to the magnetic field-aligned currents caused by [[Magnetospheric convection and magnetic storms#Substorms|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 measuresmeasured the time variations of transient particle flows and electric and magnetic fields in the region.
 
==Instrumentation on each Cluster satellite==
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| Active Spacecraft Potential Control experiment
| Regulation of spacecraft's electrostatic potential
| EnablesEnabled the measuremeasurement by PEACE of cold electrons (a few eV temperature), otherwise hidden by spacecraft photoelectrons
|-
| 2
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| DWP
| Digital Wave Processing instrument
| Coordinates the operations of the EFW, STAFF, WBD and WHISPER instruments.
| At the lowest level, DWP providesprovided electrical signals to synchronise instrument sampling. At the highest level, DWP enablesenabled more complex operational modes by means of macros.
|-
| 4
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| WBD
| Wide Band Data receiver
| High time resolution measurements of both electric and magnetic fields in selected frequency bands from 25&nbsp;Hz to 577&nbsp;kHz. It providesprovided a unique new capability to perform [[Very-long-baseline interferometry]] (VLBI) measurements.
| Properties of natural plasma waves (e.g. [[auroral kilometric radiation]]) in the Earth magnetosphere and its vicinity including: source location and size and propagation.
|-
| 11
| WHISPER
| Waves of High Frequency and Sounder for Probing of Density by Relaxation
| Electric field '''E''' spectrograms of terrestrial plasma waves and radio emissions in the 2–80&nbsp;kHz range; triggering of plasma resonances by an active sounder.
| Source location of waves by triangulation; electron density within the range 0.2–80&nbsp;cm<sup>−3</sup>
|}
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|display-authors=etal}}</ref><ref>{{Cite web|date=September 21, 2005|title=ESA Science & Technology - Double Star and Cluster observe first evidence of crustal cracking|url=https://sci.esa.int/web/cluster/-/37944-double-star-and-cluster-reveal-crustal-cracking-on-a-magnetar|url-status=live|access-date=2021-07-14|website=sci.esa.int|language=en-US|archive-url=https://web.archive.org/web/20200201144704/https://sci.esa.int/web/cluster/-/37944-double-star-and-cluster-reveal-crustal-cracking-on-a-magnetar |archive-date=2020-02-01 }}</ref> as well as to magnetospheric physics. The TC-1 examined density holes near the Earth's [[bow shock]] that can play a role in bow shock formation<ref>{{Cite web|date=June 20, 2006|title=ESA Science & Technology - Cluster and Double Star discover density holes in the solar wind|url=https://sci.esa.int/web/double-star/-/39559-cluster-and-double-star-discover-density-holes-in-the-solar-wind|url-status=live|access-date=2021-07-14|website=sci.esa.int|archive-url=https://web.archive.org/web/20210829204211/https://sci.esa.int/web/double-star/-/39559-cluster-and-double-star-discover-density-holes-in-the-solar-wind |archive-date=2021-08-29 }}</ref><ref>{{Cite web|last=Britt|first=Robert Roy|date=June 20, 2006|title=CNN.com - Earth surrounded by giant fizzy bubbles - Jun 20, 2006|url=http://www.cnn.com/2006/TECH/space/06/20/space.bubbles/index.html|url-status=live|access-date=2021-07-14|website=www.cnn.com|archive-url=https://web.archive.org/web/20060622145225/http://www.cnn.com:80/2006/TECH/space/06/20/space.bubbles/index.html |archive-date=2006-06-22 }}</ref> and looked at neutral sheet oscillations.<ref>{{Cite web|date=March 30, 2006|title=ESA Science & Technology - Cluster and Double Star reveal the extent of neutral sheet oscillations|url=https://sci.esa.int/web/double-star/-/38999-cluster-and-double-star-reveal-the-extent-of-neutral-sheet-oscillations|url-status=live|access-date=2021-07-14|website=sci.esa.int|archive-url=https://web.archive.org/web/20210418194124/https://sci.esa.int/web/double-star/-/38999-cluster-and-double-star-reveal-the-extent-of-neutral-sheet-oscillations |archive-date=2021-04-18 }}</ref>
 
== Awards ==
 
Cluster team awards:
 
*2019 Royal Astronomical Society Group Achievement Award<ref>{{cite web |title=Citation for the 2019 RAS Group Achievement Award (G): The Cluster Science and Operations teams |url=https://ras.ac.uk/sites/default/files/2019-01/awards/Group%20Achievement%20Award%20-%20Cluster.pdf |archive-url=https://web.archive.org/web/20231017093637/https://ras.ac.uk/sites/default/files/2019-01/awards/Group%20Achievement%20Award%20-%20Cluster.pdf |archive-date=17 October 2023 |url-status=live }}</ref>
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*2000 ESA Cluster launch award
 
Individual awards:
 
*2023 Hermann Opgenoorth (Univ. of Umea, Sweden), former Cluster Ground Based Working Group lead, was awarded the 2023 EGU Julius Bartels Medal<ref name="egu-20221130">{{cite web |title=EGU announces its 2023 awards and medals! |url=https://www.egu.eu/news/937/egu-announces-its-2023-awards-and-medals/ |publisher=[[European Geosciences Union]] |archive-url=https://web.archive.org/web/20230307162827/https://www.egu.eu/news/937/egu-announces-its-2023-awards-and-medals/ |archive-date=7 March 2023 |date=30 November 2022 |url-status=live}}</ref>
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== Discoveries and mission milestones==
=== 2024 ===
* September 8 - Re-entry of ''SALSA'' (Cluster 2) satellite, the first of the Cluster II satellites to re-enter the atmosphere.<ref name="salsareenter">{{cite news |last1=Foust |first1=Jeff |title=ESA performs targeted reentry of Cluster satellite |url=https://spacenews.com/esa-performs-targeted-reentry-of-cluster-satellite/ |publisher=SpaceNews |date=September 9, 2024 |access-date=September 9, 2024 }}</ref>
 
=== 2023 ===
* April 28 - Magnetic reconnection at high and low latitudes during the passage of an ICME<ref>{{cite journal
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