The Cluster satellite project is one of the core projects in the scientific program of the European Space Agency (ESA). Four identical satellites encircle the Earth in closely spaced trajectories. Simultaneous measurements at four points allow to distinguish between temporal and spatial effects and shed new light on our understanding of the processes in and around the Earth's magnetosphere.

Cluster has an elliptical polar orbit, with a perigee of 4 and apogee of 20 Earth radii. The distances between the the four spacecraft vary from several hundred to several thousand kilometers. They cross various interesting regions, such as the magnetosphere, where the Earth magnetic field dominates, the solar wind, which consists mainly of electrons and ions emitting from the Sun, and the interaction region in between.

The payloads of all four satellites are identical and consist of 11 instruments to measure magnetic and electric fields and charged particles over a wide range of energies. Austria contributes to 3 of those instruments:

• ASPOC controls the voltage potential of the spacecraft.
• FGM measures magnetic fields.
• EDI measures electric fields with a new technique.

In addition, IWF is Co-Investigator for PEACE and CIS, instruments, which measure the velocity distribution of electrons and ions, and operates the Austrian Cluster Data Center (ACDC), which is part of the Cluster Science Data System (CSDS). Naturally, Cluster data are also analysed and interpreted in Graz.

Further information on the Cluster mission is found at ESA.