Cluster II (spacecraft)

Did You Know?

• The China National Space Administration/ESA Double Star mission operated alongside Cluster II from 2004 to 2007.
• TC-2, one of the Double Star satellites, contributed to magnetar science as well as magnetospheric physics.
• As of 31 March 2026, 3868 publications were related to the Cluster and Double Star missions, including 3371 refereed publications.

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 collected 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, spinning at 15 rotations per minute, and their solar cells initially provided 224 watts of power. Solar array power gradually declined due to damage by energetic charged particles, but remained sufficient for science operations. The four spacecraft maneuvered into various tetrahedral formations to study magnetospheric structure and boundaries, with inter-spacecraft distances varying from around 4 to 10,000 km. Propellant for transfer to the operational orbit and formation maneuvers made up approximately half of each spacecraft's launch weight. The highly elliptical orbits initially reached a perigee of around 4 Earth radii and an apogee of 19.6 Earth radii, with each orbit taking approximately 57 hours. The orbit evolved over time; the line of apsides rotated southwards, and gravitational effects imposed a long-term cycle that reduced perigees to a few hundred kilometers in 2011 before they began rising again. Orbit modifications by ESOC altered the orbital period to 54 hours. ESOC acquired telemetry and distributed science data to online data centers, while the Joint Science Operations Centre at Rutherford Appleton Laboratory coordinated scientific planning.

History

The Cluster mission was proposed to ESA in 1982 and approved in 1986, along with SOHO, as part of the Solar Terrestrial Physics cornerstone of ESA's Horizon 2000 programme. The original Cluster spacecraft were completed in 1995, but the explosion of the Ariane 5 rocket carrying them in 1996 delayed the mission by four years while new instruments and spacecraft were built. On July 16, 2000, a Soyuz-Fregat rocket launched two replacement Cluster II spacecraft, Salsa and Samba, into a parking orbit from where they maneuvered into a 19,000 by 119,000 kilometer orbit with a period of 57 hours. On August 9, 2000, another Soyuz-Fregat rocket lifted the remaining two spacecraft, Rumba and Tango, into similar orbits. Rumba was also known as the Phoenix spacecraft, built largely from spare parts left over after the original mission failure. After payload commissioning, the first scientific measurements were made on February 1, 2001. ESA ran a competition to name the satellites across member states; Ray Cotton from the United Kingdom won with the names Rumba, Tango, Salsa and Samba. Originally planned to last until the end of 2003, the mission was extended several times, ultimately until September 2024. The last two satellites, SAMBA and TANGO, are expected to re-enter on August 31 and September 1, 2026.

Scientific objectives

Previous single and two-spacecraft missions could not provide the data needed to accurately study the boundaries of the magnetosphere. Because the plasma comprising the magnetosphere cannot be viewed using remote sensing, satellites must measure it in-situ. Four spacecraft allowed scientists to make three-dimensional, time-resolved measurements to create a realistic picture of complex plasma interactions between regions of the magnetosphere and between the magnetosphere and the solar wind. Each satellite carried 11 instruments designed to study small-scale plasma structures in key regions: solar wind, bow shock, magnetopause, polar cusps, magnetotail, plasmapause boundary layer, polar caps, and auroral zones. At the bow shock, where the solar wind decelerates from super- to sub-sonic, measurements helped characterize processes such as the origin of hot flow anomalies and transmission of electromagnetic waves. Behind the bow shock lies the magnetopause, a thin plasma layer separating Earth's and solar wind magnetic fields that moves continuously due to solar wind pressure variations. Cluster's four-point measurements tracked magnetopause motion and elucidated mechanisms for plasma penetration from the solar wind. At the polar cusps, where Earth's magnetic field is perpendicular to the magnetopause, solar wind particles flow into the magnetosphere; Cluster recorded particle distributions to characterize turbulent regions at the exterior cusps.

Discoveries and mission milestones

On January 6, machine learning techniques applied to 20 years of measurements from Cluster, Double Star, THEMIS, and the Magnetospheric Multiscale Mission provided proof of how the most violent particle acceleration in the universe are created through magnetic reconnection. On November 1, Cluster and THEMIS simultaneously observed Kelvin–Helmholtz instability on both flanks of the magnetopause. On October 22, RUMBA (Cluster 1) re-entered over the Pacific Ocean at 20:59 CEST. On April 3, Cluster observed plasma in the high-latitude magnetotail associated with cusp-aligned arcs. On March 19, the Earth ring current was observed in-situ by Cluster, THEMIS, and MMS and compared. On February 22, a new dataset to localize Cluster satellites in the geospace environment was released. On January 21, an ion flow vortex was identified as a novel electron accelerator in space. On December 1, 14 years of Cluster data were used to predict proton intensities around Earth. On November 13, polar cap auroral arcs were linked to lobe reconnection. On September 8, SALSA (Cluster 2) re-entered, the first Cluster II satellite to do so. On September 4, strong effects of chorus waves on radiation belts were expected for future magnetic superstorms. On August 30, the pioneer Cluster mission prepared its legacy phase near re-entry. On July 6, Cluster data contributed to a collisionless shocks database acquired at Earth, Venus, and Mars. On May 24, Cluster released a list of plasma jets found in the magnetosheath.