China launches Double Star twin

Tan Ce 2 was launched a day early because of fears of deteriorating weather

The second spacecraft built for the Chinese-European Double Star space mission has been successfully launched - a day earlier than planned.

Tan Ce 2 took off from a base in Taiyuan in Northern China, attached to a Long March 2C carrier rocket.

It was put into a highly elliptical polar orbit, where for a year it will study the planet's magnetic field.

Its predecessor Tan Ce 1- launched in December 2003 - has already proved fruitful in its investigations.

The two craft will work in conjunction with the European Space Agency's (Esa) four Cluster satellites that were deployed in 2000.

Understanding the local space environment is important as long as man has aspirations to explore space Chris Owen

Together, the spacecraft will study the Earth's magnetosphere. This is the region around the planet which is protected by the Earth's magnetic field from the radiation and matter ejected by the Sun.

This huge, tadpole-shaped region of space extends many tens of thousands of kilometres from the planet, and the orbits of the Cluster quartet and the Double Star duo are designed to clip its boundary.

Solar winds

Remarkable phenomena such as the aurorae (northern and southern lights) result when fast-moving charged particles (ions and electrons) breach the shield and are pulled in towards the poles and collide with air molecules.

Scientists hope the satellites' investigations will reveal how these phenomena are "switched on".

"We know that these events occur, but we don't know where the light switch is, or what the processes are that turn the lights on - how they connect with the energy in space," Dr Chris Owen, of the Mullard Space Laboratory at University College London, UK, told BBC News Online.

The "solar wind" particles travel at approximately 400km/h. They push on the Earth's magnetic field on its "day side", changing its shape.

Tan Ce 2 (or Explorer 2 as the Europeans refer to it) will observe this interaction from a polar orbit, sampling the cusps where the Sun's energy enters the magnetosphere. Tan Ce 1, in contrast, is engaged in an equatorial orbit.

'Exotic behaviour'

Dr Owen likens the process to a wind sock, whose open end is next to the Sun, and whose tip is in the Earth's magnetic field. The length of the wind sock consists of field lines propelling the ions away from the Sun and towards the Earth.

Tan Ce 1 and 2 are two metres in diameter

When the wind enters the Earth's magnetic field, it is responsible for all sorts of "exotic behaviour", he says.

Spacecraft malfunctions, telecommunication problems and electrical blackouts have all been attributed to the buffeting the Earth sometimes receives from huge solar ejections.

Even carrier pigeons are said to become disorientated and fly in the wrong direction during periods of turbulent "space weather".

The satellites will examine the physical processes involved, such as particle acceleration, diffusion, injection and the movement of ions.

High hopes

The Double Star mission is the first collaboration between Europe and China on a major space venture.

China provided the launch facilities, the chassis for each satellite and half the scientific instruments.

European institutes contributed the remaining eight instruments and part of the network of data systems on the ground.

The orbits of all six satellites will be synchronised to provide the most detailed and multi-dimensional view of the Earth's magnetosphere to date, and on a larger scale than that possible with Cluster alone.

"Understanding the local space environment is important as long as man has aspirations to explore space," said Dr Owen.

"On a larger scale, these studies may provide fundamental information about the nature of the Universe."