Cluster was designed to investigate the relationship between the Sun and Earth's magnetic environment. The Sun pours magnetic energy into space, carried in a hot thin gas called the solar wind. The gas is made of the various particles that go to make up atoms, but which can be separated at high temperatures. Such a gas is called plasma and can carry electrical and magnetic fields.
Together with the ESA-NASA Solar Heliospheric Observatory (SOHO), Cluster is investigating the entire chain of events from the solar wind’s generation at the Sun to its collision with Earth's magnetosphere.
Cluster’s four identical spacecraft fly in formation. The separation between the spacecraft can be varied from just a few tens of kilometres to 10 000 km. Together, they are making the most detailed investigation ever of the interaction between the solar wind and Earth’s magnetic field, and simultaneous measurements from all four make it possible to distinguish between the spatial and temporal variations of this interaction.
Cluster is determining which physical processes are involved in the interaction between the solar wind and the magnetosphere by repeatedly visiting key regions in near-Earth space. In combination, the four spacecraft are able to make three-dimensional maps of the plasma structures contained in these regions. This work is more than pure science, as many of the phenomena being studied can pose a danger to space-borne electronics, perturb GPS reception and damage electrical power systems on the ground.