A goalie's guide to the galaxy in outer space

Next month sees the launch of a new space mission to explore the mysterious force field protecting us from solar winds. But what has football got to do with it?

The countdown has begun to launch four spacecraft that will fly in formation through the invisible protective barrier that shields the Earth from the harsh solar wind. The Cluster-2 mission aims to shed light on the planet's mysterious magnetosphere, a force field that prevents the loss of our atmosphere and evaporation of our oceans into space by the constant stream of electrically-charged particles given off by the Sun.

The countdown has begun to launch four spacecraft that will fly in formation through the invisible protective barrier that shields the Earth from the harsh solar wind. The Cluster-2 mission aims to shed light on the planet's mysterious magnetosphere, a force field that prevents the loss of our atmosphere and evaporation of our oceans into space by the constant stream of electrically-charged particles given off by the Sun.

It will be the first time that a flotilla of four spacecraft will fly in unison and operate as part of a single, co-ordinated system. They will fly in the shape of a tetrahedron - a four-sided structure - so that scientists can understand the complex three-dimensional interactions between the solar wind and the Earth's magnetic field. Scientists hope the mission will help improve the protection of satellites and astronauts who have to work in the harsh environment of space.

It will be the second attempt to launch Cluster. The first ended in abject failure in June 1996 when the maiden launch of the Ariane-5 rocket exploded just after take-off. Unlike its predecessor, Cluster-2 will be launched in two stages, with the first two spacecraft taking off next month on a Soyuz rocket from the Baikonur Cosmodrome in Kazakhstan. The second pair of spacecraft will be launched in August. Each of the four probes will carry an identical array of instruments to study the interactions between the solar wind and the Earth's magnetosphere simultaneously from different points in space.

The magnetosphere is generated by the iron core of the planet and extends out thousands of miles beyond the upper atmosphere. The field is constantly buffeted by the millions of electrically-charged particles streaming out from the Sun. "It's like a never-ending football game," says Philippe Escoubet, a Cluster scientist at the European Space Agency. "The Sun is kicking particles like balls. The Earth is the goal and its magnetic field is the goalkeeper. It is always trying to push the balls away, but some get past."

There are two weak points in the magnetosphere, called polar cusps, over the North and South Poles. Here, the charged particles of the solar wind can - and do - frequently penetrate the Earth's upper atmosphere, causing the ionisation of gases and the generation of the beautiful visual effects of the aurorae - the northern (and the southern) lights. Sometimes, the magnetosphere fails more completely during intense solar activity, enabling the electrical storms to interfere with apparatus on the ground as well as any satellites in orbit.

"The magnetosphere is not a perfect barrier," says Andrew Fazakerley, a Cluster scientist at the Mullard Space Laboratory in Surrey. In addition to the polar cusps, there are at least three processes taking place within the protective magnetic barrier that enable the solar wind to penetrate, with potentially serious consequences for communications satellites. "And we don't understand them," Dr Fazakerley adds.

Although life on Earth would not exist without the Sun, living things need the protection against the Sun afforded by the magnetosphere. Mars lacks an iron core and has no magnetosphere - and consequently is thought to have lost its precious water as the solar wind evaporated the Martian oceans into space.

The solar wind is a constant stream of charged particles flying out from the Sun at about 1,800 times the speed of Concorde. At certain moments, the Sun ejects clouds of very high-energy particles which can cross the 150 million kilometres separating Earth from its nearest star in just a few days. The most energetic particles of all, generated by solar flares, can reach Earth in just 30 minutes.

The medical and technical risks associated with solar activity are well known. Radio, navigation and radar can be affected, as can electricity transmission, microchips in satellites, telephone and television transmissions. Radiation hazards for astronauts can be very real, according to Paul Murdin of the Particle Physics and Astronomy Research Council. "Space is a radiation-rich environment. Astronauts who went to the Moon could not work in the nuclear industry because they had received their lifetime's dose of radiation," he explains. Passengers and crews aboard high-flying aircraft, such as Concorde, are more vulnerable to solar activity than those on low-flying planes, he adds.

About half the payload weight of each of the four Cluster-2 probes is taken up by fuel which will be used to keep the spacecraft positioned accurately. Identical instruments on board each probe will take readings of magnetic fields, solar particles and electromagnetic waves. A key feature of the the four-probe system is that the measurements will provide simultaneous readings from different regions within the magnetosphere, to reveal any fluctuation in the interactions with the solar wind.

"As we rely more and more on near-Earth space, we need to understand the processes going on there," says Manual Grande, a member of the Cluster-2 team at the Rutherford Appleton Laboratory in Oxfordshire, from where the mission will be controlled after launch. Hugo Alleyne, a space scientist at the University of Sheffield, agrees: "Cluster will explore the hazardous region of space where the world's communication, navigation and weather-monitoring satellites fly. The more we know about the conditions of this environment, the better equipped these satellites can be to withstand the onslaught of magnetic and solar storms, which can and indeed do, destroy them."

Next month's launch from the barren landscape of Kazakhstan promises to begin this process of discovery. When all four Cluster-2 spacecraft are in orbit it will take until the end of the year before they are in a position to begin sending data back to Earth. The mission, when complete in two years, should finally tell us more about one of the Earth's most precious protective shields.