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

Following highly elongated, polar orbits which take them between 19,000 and 119,000 km from the Earth, the Cluster quartet will investigate 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.

Cluster will give scientists better equations with which to predict space weather in the future.

"All the communication satellites above the Earth are affected by these massive bursts from the Sun," said John Credland. "If we could get pre-warning of these bursts, we could stop knockouts of spacecraft and their circuits. We could also stop power line surges on Earth which have also been responsible for some quite spectacular blackouts in the past."

'Back on track'

Rumba and Tango will follow almost identical manoeuvres to their sister ships to make a rendezvous set for 15 August.

Scientists have spent four years rebuilding the Cluster programme

Wednesday's launch was an emotional occasion for Professor Roger Bonnet, Esa's Director of Science.

Cluster II replaces the original mission destroyed in the Ariane-5 explosion in 1996. Speaking to reporters and Esa staff, he proudly announced that Cluster was now "back on track".

"After so many years of expectation, Esa has been able to stick to its promises to the scientists," he said. "We are very pleased to announce that in a few weeks the whole mission will be in operation.

"With Soho (Solar and Heliospheric Observatory) and Cluster, Europe is now fully participating in the investigation of the Sun-Earth relationship and space weather activities."

Giant discs

Each satellite is shaped like a giant disc, 1.3m high and 2.9m across. Most of the 11 instruments on each spacecraft will sit behind reinforced panelling inside the disc. Others have to be slung on booms to avoid interference from the spacecraft themselves.

The craft will fly in a pyramid formation