Cluster II (spacecraft)


Cluster II 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 cycles. The mission was composed of four identical spacecraft flying in a tetrahedral formation. As a replacement for the original 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 rockets 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., its mission was extended until September 2024. The China National Space Administration/ESA Double Star mission operated alongside Cluster II from 2004 to 2007.
The first of the four Cluster II satellites to re-enter the atmosphere did so on 8 September 2024 and the second one on 22 October 2025. The remaining two are expected to follow in summer 2026. The scientific payload operations of all satellites ended as the first satellite re-entered the atmosphere.

Mission overview

The four identical Cluster II satellites studied 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 was able to collect three-dimensional information on how the solar wind interacts with the magnetosphere and affects near-Earth space and its atmosphere, including aurorae.
The spacecraft were cylindrical and were spinning at 15 rotations per minute. After launch, their solar cells provided 224 watts power for instruments and communications. Solar array power 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 maneuvered into various tetrahedral formations to study the magnetospheric structure and boundaries. The inter-spacecraft distances could be altered and varied from around 4 to 10,000 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 elliptical orbits of the spacecraft initially reached a perigee of around 4 RE and an apogee of 19.6 RE. Each orbit took approximately 57 hours to complete. The orbit evolved over time; the line of apsides 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 imposed a long term cycle of change in the perigee distance, which saw the perigees reduce to a few 100 km in 2011 before beginning to rise again. The orbit plane rotated away from 90 degrees inclination. Orbit modifications by ESOC altered the orbital period to 54 hours. All these changes 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 acquired telemetry and distributed to the online data centers the science data from the spacecraft. The Joint Science Operations Centre at Rutherford Appleton Laboratory in the UK coordinated scientific planning and in collaboration with the instrument teams provided merged instrument commanding requests to ESOC.
The 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.

History

The Cluster mission was proposed to ESA in 1982 and approved in 1986, along with the Solar and Heliospheric Observatory, 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, 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 into similar orbits. Spacecraft 1, Rumba, was 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 made on February 1, 2001.
The European Space Agency ran a competition to name the satellites across all of the ESA member states. Ray Cotton, from the United Kingdom, won the competition 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 winning entry, 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 was extended several times. The first extension took the mission from 2004 until 2005, and the second from 2005 to June 2009. The mission was ultimately extended until September 2024, when the scientific payload operations on the satellites ended. The ultimate end of the Cluster project will happen in 2026 as the last satellite enters the atmosphere and is destroyed.

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 be viewed using remote sensing techniques, satellites must be used to measure it in-situ. Four spacecraft allowed 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 carried 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 made measurements which helped 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 made 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 southern, 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 recorded the particle distributions, which allowed 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 monitored particles from the ionosphere and the solar wind as they passed through the magnetotail lobes. In the central plasma sheet, Cluster determined 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 measured the time variations of transient particle flows and electric and magnetic fields in the region.

    Instrumentation on each Cluster satellite

Double Star mission with China

In 2003 and 2004, the China National Space Administration launched the Double Star satellites, TC-1 and TC-2, that worked together with Cluster to make coordinated measurements mostly within the magnetosphere. TC-1 stopped operating on 14 October 2007. The last data from TC-2 was received in 2008. TC-2 made a contribution to magnetar science 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 and looked at neutral sheet oscillations.

Awards

Cluster team awards:
Individual awards:
  • 2023 Hermann Opgenoorth, former Cluster Ground Based Working Group lead, was awarded the 2023 EGU Julius Bartels Medal
  • 2020 Daniel Graham was awarded the COSPAR Zeldovich medal
  • 2019 Margaret Kivelson, Cluster FGM CoI, received RAS gold medal
  • 2018 Hermann Opgenoorth, former Cluster Ground Based Working Group lead, was awarded the 2018 Baron Marcel Nicolet Space Weather and Space Climate medal
  • 2016 Stephen Fuselier, Cluster CIS CoI, received EGU Hannes Alfvén Meda
  • 2016 Mike Hapgood, Cluster mission scientific operations expert was awarded the Baron Marcel Nicolet Medal for Space Weather and Space Climate
  • 2014 Rumi Nakamura, Cluster CIS/EDI/FGM CoI, received EGU Julius Bartels Medal
  • 2013 Mike Hapgood, Cluster JSOC project scientist received RAS service award
  • 2013 Göran Marklund, EFW Co-I, received the EGU Hannes Alfvén Medal 2013.
  • 2013 Steve Milan, Cluster Ground based representative of the Cluster mission received UK Royal Astronomical Society Chapman medal
  • 2012 Andrew Fazakerley, Cluster and Double Star PI, received the Royal Astronomical Society Chapman Medal
  • 2012 Zuyin Pu, RAPID/CIS/FGM CoI, received AGU International Award
  • 2012 Jolene Pickett, a Cluster WBD PI, received the State of Iowa Board of Regents Staff Excellence
  • 2012 Jonathan Eastwood, FGM Co-I, received COSPAR Yakov B. Zeldovich medal
  • 2008 Andre Balogh, Cluster FGM PI, received RAS Chapman medal
  • 2006 Steve Schwartz, Cluster UK data system scientist and PEACE co-I, received RAS Chapman medal