Cluster probes Earth's magnetic environment, known as the magnetosphere, and how it is affected by the perpetual stream of subatomic charged particles coming from the Sun, known as the ‘solar wind’. The beautiful polar auroras are an impressive consequence of this interaction, but there are other less-benign effects. A very energetic solar wind can trigger the disruption of electrical power systems and telecommunications, as well as damaging man-made satellites. By understanding the interaction between the solar wind and the magnetosphere, Cluster helps us to better forecast the effects of sudden bursts of solar activity.
 
This is the first mission in which four identical spacecraft fly in formation. ESA chose the name Cluster because of its way of flying. The current Cluster spacecraft are a replacement for the original set lost in the failed Ariane 5 maiden flight in June 1996. After the failure, ESA Member States recognised the mission's importance and decided to rebuild Cluster.

Cluster data add to those provided by two other ESA missions (SOHO and Ulysses) that have also studied the Sun and its impact on the Solar System, but from a completely different perspective.

Double Star, another international mission with cooperation between ESA and the Chinese National Space Administration (CNSA), worked in coordination with the Cluster flotilla until its mission ended in late 2009. Double Star comprised two satellites, designed, launched (December 2003 and July 2004) and operated by CNSA, while ESA contributed eight scientific instruments to the mission and one ground station. The two spacecraft orbited at lower altitudes than the Cluster satellites, one with an equatorial orbit, and the other with a polar orbit. Data collected by these six spacecraft allow scientists to investigate fundamental physical processes in key regions of the magnetosphere in more details than ever before.

Objectives

Cluster studies the Sun-Earth interaction. It looks at how the solar wind impacts Earth’s protective magnetic shield, the magnetosphere.

A key feature of the solar wind is its high variability. When the solar activity is high and solar storms are frequent, magnetic storms and substorms can occur in the magnetosphere, among other space weather phenomena. It is at these times that the aurorae are very bright and can be seen at latitudes as low as those of Spain and Florida. Cluster’s goal is to help find out where and how exactly these processes work.

Cluster’s unique ability to make simultaneous measurements, with identical instruments and at selected distances, is the key to perform the first detailed 3D study of the changes and processes taking place in near-Earth space.
 
Cost
 
The mission cost about €315 million. This included the spacecraft, launch and operations of the four current Cluster spacecraft, a three-year lifetime extension and 40% (€16.7 million) of the funding for the payload. The rest of the payload was paid for by the countries building it.
 
Launch
 
Two Russian Soyuz/Fregat rockets launched the four Cluster spacecraft (two on each rocket) on 16 July and 9 August 2000, from the Baikonur Cosmodrome, in Kazakhstan.
 
Orbit
 
For many years, the Cluster satellites followed polar orbits that took them between 19 000 and 119 000 kilometres from Earth, allowing these spacecraft to orbit in and out of Earth’s magnetosphere. Over the last few years, due to lunar-solar perturbations, the orbits of the satellites have drifted from their nominal polar orbit to a more equatorial one, allowing the spacecraft to cross magnetospheric regions of great scientific interest not originally targeted by the mission.

The spacecraft fly in a pyramid-like formation when crossing a region of interest. The distances between them have been varied to study physical processes at different spatial scales: from only 100 kilometres, for small-scale features, to 10 000 kilometres apart.
 
Planned mission lifetime
 
Cluster's lifetime has been extended till the end of 2012. The extension is due to the good results being obtained. The longer lifetime means more opportunities to collect data in regions barely crossed by the mission so far. Cluster scientists and engineers aim to continue operating Cluster into the rise and peak of solar cycle 24. Cycle 23 was unusually long and Cluster will continue observations of the magnetosphere to provide us with full coverage of this intriguing solar cycle and its implications. The entire duration of the mission will provide scientists with a wealth of data, all of which are made available at the Cluster Active Archive, the first depository of high-resolution space plasma physics data (http://caa.estec.esa.int).
 
Spacecraft
 
Design

Each spacecraft is shaped like a giant disc. In the centre, there is a cylinder containing the main engine, two high-pressure fuel tanks and other parts of the propulsion system. Around the central cylinder is the main equipment platform, to which most of the subsystems, such as the power and computer processing hardware, are attached. The science experiments are on top. Electrical power comes from six curved solar panels attached around the outside of the platform. In orbit, various rod-shaped booms with sensors and communication antennae open out.

Dimensions 2.9 metres in diameter; 1.3 metres in height. Each spacecraft also carries two antenna booms, two 5-metre radial booms and four 42-metre experiment wire booms.

Mass 1200 kilograms each spacecraft. 650 kilograms correspond to the fuel and 71 kilograms to the scientific instruments.

Industrial involvement Prime contractor for the original Cluster and replacement Cluster satellites was Dornier Satellitensysteme GmbH, Friedrichshafen, Germany. This company led an industrial consortium involving 35 major contractors from all the ESA member countries and the United States.
 
What’s on board?
 
Each spacecraft carries an identical set of 11 instruments to investigate charged particles, electrical and magnetic fields. The instruments were built by European and American instrument teams led by Principal Investigators. The Cluster scientific community includes more than 200 Co-Investigators from ESA Member States, the United States, Canada, China, the Czech Republic, Hungary, India, Israel, Japan and Russia.

Fluxgate Magnetometer (FGM) (Imperial College, United Kingdom). Measures the magnetic field along the orbit. FGM can take high-resolution measurements with up to 67 samples per second.

Electron Drift Instrument(EDI) (Max-Planck-Institut für extraterrestrische Physik, Germany). Determines the strength of the electric field in the space around the spacecraft. As a by-product, the magnetic field strength is also measured.

Active Spacecraft Potential Control (ASPOC) experiment (Space Research Institute, Austria). Electrical charging of a Cluster spacecraft in orbit can have a severe impact on the performance of the scientific instruments. ASPOC is designed to ‘earth’ or neutralise the spacecraft by preventing a build-up in positive electrical charge.

Spatio-Temporal Analysis of Field Fluctuation (STAFF) experiment (Centre d'Étude des Environnements Terrestre et Planétaires, France). A magnetometer that looks at waves (rapid variations in the magnetic field) with frequencies from 8 Hz up to 4000 Hz, and a spectrum analyser.

Electric Field and Wave (EFW) experiment (Swedish Institute of Space Physics, Sweden). Measures the electric field to study plasma convection and waves (space plasma is composed of electrically charged particles, mainly electrons and protons and some other ions). The waves can travel at thousands of kilometres per second and last from a few milliseconds to many minutes.

Digital Wave Processing (DWP) experiment (University of Sheffield, United Kingdom). The control and computing brain for the wave experiments. It provides precise timing that allows correlation studies between the four spacecraft.

Waves of High frequency and Sounder for Probing of Electron density by Relaxation (WHISPER) experiment (Laboratoire de Physique et Chimie de l'Environnement et de l'Espace, France). The density of charged particles around the Earth determines wave generation and energy transfers. WHISPER probes this fundamental quantity.

Wide Band Data (WBD) instrument (University of Iowa, United States). Provides high-resolution measurements of both electric and magnetic fields in selected frequency bands.

Plasma Electron And Current Experiment (PEACE) (Mullard Space Science Laboratory, United Kingdom). Looks at all electrons in the space plasma that have low to medium energies, counts them and measures their direction of travel and speed.

Cluster Ion Spectrometry (CIS) experiment (Centre d'Etude Spatiale des Rayonnements, France). Analyses the composition of ions that have low to medium energies, counts them and measures their direction of travel and speed.

Research with Adaptive Particle Imaging Detectors (RAPID) (Max Planck Institute for Solar System Research, Germany). An advanced particle detector that records energetic electrons and ions, neatly complementing the PEACE and CIS experiments.
 
Operations
 
Ground control

The European Space Operations Centre (ESOC) in Darmstadt, Germany, carries out day-to-day operation of the spacecraft. Spacecraft telemetry, command and tracking are handled by the ESA ground stations located for instance in Villafranca (near Madrid) and Maspalomas (Gran Canaria), Spain, with additional support from NASA’s Deep Space Network for the WBD experiment.

Science Operations

The Joint Science Operations Centre (JSOC) at Rutherford Appleton Laboratory in the United Kingdom coordinates the science operations.

The Cluster Science Data System (CSDS) handles the scientific information sent back by the four spacecraft. The CSDS comprises eight national data centres - six in Europe, one in the United States and one in China.

The Cluster Active Archive (CAA) is the depository of processed and validated high-resolution Cluster data. The aim of the CAA is to maximise the scientific return from the Cluster mission by making all high-resolution Cluster data available to the worldwide scientific community and to ensure that the unique Cluster observations are preserved in a stable, long-term archive for scientific analysis beyond the end of the mission. The CAA can be accessed at http://caa.estec.esa.int.

ESA Project Scientist: Matthew Taylor
ESA Mission Manager: Philippe Escoubet
 


Published by ESA Media Relations, Paris
tel: +33 1 5369 7299, fax: +33 1 5369 7690
email: media @ esa.int (Media inquiries only)