Cluster facts

Spin rate: 15 rpm Spacecraft diameter: 2.9 m Spacecraft height: 1.3 m Dry mass: 550 kg Propellant mass: 650 kg Solar array power: 224 W Downlink rate: 2 to 262 kbit/s

The magnetosphere protects life on Earth from the stream of charged solar particles - electrons and protons - that are blasted continuously from the Sun.

Cluster's role will be to look at how the magnetosphere interacts with this solar wind, and the high-energy particles from more violent solar events such as flares and so-called coronal mass ejections (CMEs).

There is currently a great deal of interest in these space weather phenomena. The charged particles, which move at hundreds of kilometres a second, can knock out electronics onboard satellites and in extreme cases interfere with power grids on the Earth's surface.

Weak points

Armed with data from Cluster II, scientists hope that they will be able to predict such threats to satellites with more certainty.

Scientists have spent four years rebuilding the Cluster programme

Early in the mission, the spacecraft will spend most of their time flying on the side of the Earth that faces away from the Sun.

After six months, they will move in front of the planet to investigate the polar cusps.

These are weak points in the Earth's magnetic shield, where charged particles penetrate the upper atmosphere and generate the spectacular Northern and Southern Lights.

From the end of December, data will be coming down from the four satellites at a rate of one gigabyte (two compact disks) every day.