Satellites track border clashes

The violent borders between the earth's and the sun's magnetic fields are full of mysteries that could be solved by a fleet of identical spacecraft to be launched next Thursday.

The Cluster satellites should help explain the secrets of aurorae Q coloured lights seen high in the earth's atmosphere mainly in the polar regions Q and of electrical storms that can disrupt power lines and send satellites out of control. Clashes between sun and earth may even cause changes in climate because they generate such large thermal and dynamic effects.

The Pounds 500 million satellites will swoop over the poles in an eccentric orbit, zooming fairly close to earth (26,000km away) and as far away as 140,000km.

The 3m-long cylindrical satellites will be launched by the European Space Agency's new Ariane V rocket in Kourou, French Guiana. Once in position, they will fly in formation, sometimes as little as 200km apart. Each of the four will be identically equipped with 11 instruments, measuring phenomena such as waves in the magnetic field, electrons and ionised atoms.

On the sunward side, the earth's magnetosphere stretches 60,000km out. At its edges it endures attack from the sun's magnetic field, in the form of charged particles from the solar wind, which can hit it at speeds of up to 3 million km per hour. The earth's magnetosphere deflects most of these particles but some get in through holes and cause intense electric currents, magnetic storms and particle accelerations. Aurorae are caused by solar wind particles trapped in the magnetosphere, which then spiral along the earth's magnetic field lines towards the poles.

Knowing more about what goes on could also help in developing fusion as an energy source. "It is a natural laboratory out there for plasma physics," says Eric Dunford, director of space science at the Rutherford Appleton Laboratory.

The expedition will be the first Cornerstone project of the ESA's Horizon 2000 science programme. It is financed 70 per cent by ESA and 30 per cent by Nasa. British scientists are principal investigators for four of the 11 instruments.