Over 50% of a satellite's initial weight is fuel

The European Space Agency's Cluster II mission will deploy a squadron of four satellites into the Earth's near-space environment.

These have now been loaded aboard their rockets and will be launched in pairs from the Baikonur Space Centre in Kazakhstan. The dates are currently set for 12 July and 9 August.

The first will almost certainly change because the same launch date has also been given to the next module of the International Space Station.

After rendezvous and commissioning, the team of satellites will start streaming back data at the end of December. This, combined with information from other spacecraft, will help scientists make better predictions of space weather in the future.

Phoenix rises

Cluster II is a "recovery" mission. The first attempt was destroyed at launch when the Ariane 5 rocket blew up 40 seconds into its flight, blasting $450m of work into pieces.

But Esa decided to rebuild the mission.

"The scientific questions still needed answering," said Dr Paul Murdin of the UK's Particle Physics and Astronomy Research Council, which provided $42m of the $345m rebuild cost.

The Ariane 5 launch was provided free, but this time ESA will pay for the two Soyuz rockets: "We learned there is no such thing as a free launch," said Dr Murdin.

The expense of rebuilding Cluster has set the Esa science programme back by one year, he added.

Synchronised satellites

At a briefing in London on Tuesday, Dr Alan Fazakerley explained why a squadron of satellites flying in formation was needed.

Each satellite measures the number of particles and the strength of the electromagnetic field in its immediate vicinity, he said.

Cluster facts

Dimensions 2.9m diameter 1.3m high 50m antennae Weight Spacecraft 480 kg Fuel 650 kg Instruments 70 kg Altitude Minimum 25,500 km Maximum 127,400 km

"But because they are in four different places, we can get an idea of the overall picture. For example, we can see whether the particles are moving in a sheet, a channel or even a whirlpool."

There are identical sets of 11 scientific instruments on board each Cluster satellite. These will reveal how gas eruptions on the Sun interact with the Earth's magnetic field.

The squadron will be in close tetrahedral formation on the Sun side of the Earth, 600 km apart. On the night side of the Earth, where the magnetic field is less concentrated, they will use their onboard rockets to move 6,000 km apart.

From the end of December, the data is expected to flood in at the rate of one gigabyte (two compact disks) every day. It will be immediately available to scientists.

Radiation danger

Cluster scientists cite a number of reasons why a better understanding of space weather is needed in order to improve forecasts.

Solar storms, when directed at Earth, are most dangerous for satellites. The charged particles can knock out onboard electronics.

But this radiation is also of concern to the crew of high-altitude aeroplanes and frequent flyers.

And the electromagnetic disturbances which can accompany solar storms do in severe cases bring down power grids on Earth, as happened in Canada during the last solar maximum.

Finally, astronauts who are spacewalking have neither the Earth's atmosphere or their spacecraft's aluminium walls to shield them and are at serious risk from solar storms.