The first two of the Cluster II satellites have been launched with the second pair expected to be sky-bound in early August.

The four identical spacecraft will fly in a close, tetrahedral formation gathering information on how the sun interacts with near- Earth space. The satellites follow elongated polar orbits and investigate major boundaries within the earth's magnetic environment. The orbit will mean at times they will be within the earth's magnetic shield and at times they will be outside, fully exposed to the charged particles of the solar wind.

Cluster will investigate how the solar wind interacts with the Earth's magnetosphere and also collect data on violent solar events when near- Earth space is stuck by high energy particles from solar flares and plasma clouds, blasted free during coronal mass ejections.

With the sun spot cycle at its maximum at the moment, these eruptions are expected to be at their most frequent. Magnetic storms on the sun can cause power cuts, damage to TV broadcasting, weather and communications satellites and disruption to radio communications.

The fact that there are four satellites in a cluster means that scientists can develop a three-dimensional data map.