ESA’s Cluster mission consists of four ESA’s Cluster mission consists of four spacecrafts that were launched in pairs, in August 2000, using two Soyuz-Fregat rockets. Actually, Cluster should be called Cluster II, because the initial Cluster mission rocket failed on the 4th of June 1996. These four spacecrafts (SALSA, SAMBA, RUMBA and TANGO), aim to study the interactions of the Sun-Earth system through the flow of plasma incoming from the Sun and that interacting with the magnetosphere of the Earth. The mission has been developed by the European Space Agency, with the participation of NASA and a cooperation with the Chinese Space Agency (CNSA), and has been extended until the end of 2016.
Magnetosphere as shield against the solar wind – www.nasa.gov
Maybe you are wondering why four satellites? Let us answer this. Cluster has the same purpose of study as the creation of Aurora Borealis, the four spacecrafts of Cluster have 11 experiments on board. These measure parameters like magnetic and electric fields or plasma densities in a few different ways, based on different phenomena of the interaction particle-field.
There are four spacecrafts because a volume is needed to apply the Maxwell equations in order to get the currents. The simplest volume possible is the tetrahedron, made of four elements. The calculations for the orbits were done to keep the four spacecrafts in this constellation, distances between them can vary from 200km up to 2000km. This tetrahedral constellation would give the ideal measurements if it were a perfect tetrahedron, unfortunately it is not that simple. Instead, we need a quality factor for the geometry of the constellations in order to get good measurements.
Why is all this important? Well, it’s very important for our survival, and moreover, our technological survival, the knowledge of this coupling between Sun and Earth. The possibility of a solar storm is no joke, it could cause a global blackout leaving us without electronics for a while. For instance, there are a lot of researchers working on the Cluster database (in fact, it has more mentions and publications than ExoMars mission), to develop a good model for the magnetosphere, in a field of Physics called Magnetohydrodynamics, which will allow us to predict its behavior. And of course, to learn more about the foundations of the beautiful phenomena of Auroras.
-  http://www.cosmos.esa.int/web/csa/the-mission