The Cluster mission is comprised of four identical spacecraft launched
into large, elliptical polar orbits around the Earth. After transfer to
their operational orbits they will fly in formation to allow scientists to
measure subtle changes in the interaction between the Earth and the Sun.
The four spacecraft will look at how particles from the Sun interact with
the Earth's magnetic field. Cluster will observe the magnetic and
electrical interactions between the Earth and the Sun, by making direct
measurements of the fields and particles trapped in the Earth's magnetic
field. For the first time, small-scale fluctuations in interplanetary
space will be measured between each spacecraft as they orbit the Earth.
For more information see the Cluster