Cluster II on launch pad

Cluster II is one of Europe's cornerstone missions

By BBC News Online's Jonathan Amos

Europe's next major space mission which will investigate the weather in space is on the launch pad at Baikonur in Kazakhstan and ready for blast-off.

The Cluster II spacecraft are designed to gather information on how the stream of charged particles coming from Sun interact with the Earth's protective magnetic shield.

The mission actually comprises four identical satellites that will fly in a close, tetrahedral (triangular pyramid) formation - a first for a group of spacecraft.

The first Cluster pair to fly are ready to go from Baikonur

European Space Agency officials hope the weekend's launch will be a case of second time lucky: Cluster I was destroyed on the ill-fated maiden flight of the Ariane-5 heavy-launch rocket back in 1996.

"I watched in disbelief when it blew up - it was like a death," project scientist Professor Steve Schwartz, of Queen Mary & Westfield College, London, told BBC News Online.

"But there were no scientific doubts that the mission should be rebuilt. Obviously there were some politicians who needed to be convinced but the scientific rationale behind Cluster remained. And this time it will work."

Cluster II will now go up in pairs on separate Soyuz rockets. This weekend's blast-off will be followed by an identical procedure currently scheduled for 9 August.

The Russians have developed a special booster stage for their rocket programme called Fregat that can manoeuvre satellites into just the right position for deployment.

Satellite damage

Following highly elongated, polar orbits which take them between 19,000 and 119,000 km from the Earth, the Cluster quartet will investigate most of the major boundaries and regions of interest within the Earth's magnetic environment - the magnetosphere.

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 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 study the physics of Earth's near-space environment, giving scientists better equations with which to forecast the violent space weather that can threaten satellites," said Professor Andre Balogh, from Imperial College London and one of the lead scientists on the Cluster project. "They are not space weather satellites as such, but the information they give us will help us to improve our forecasting of space weather in the future."

Magnetic shield

Cluster II follows the design of Cluster I exactly - almost. "Some of the original components were simply not available for the rebuild and we had to secure new ones, but essentially it is exactly the same design," said Professor Balogh, whose team have been responsible for the FGM, or Fluxgate Magnetometer, that will take precise measurements of the magnetic fields encountered by Cluster.

Each satellite is shaped like a giant disc, 1.3 m high and 2.9 m 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 Fregat booster was developed for precision deployment

About half the launch weight of each disc will be fuel. Manoeuvrability is the key for Cluster - the craft are designed to take measurements in different locations.

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, weak points in 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 quartet at a rate of one gigabyte (two compact disks) every day.

Professor Schwartz has been working to co-ordinate the handling of the data, which will be fed to eight centres around the world. These will process the raw data from a specific set of instruments and then make the information available to the other data centres to complete the data set.

"There will be a lot of data but the whole thing should be manageable," Professor Schwartz said. "We're used to dealing with a stream from one spacecraft but here we have to keep remembering to multiply everything by four which makes a difference not only in terms of volume but in complexity as well."

The Earth is bombarded by solar particles