Tan Ce 2 is the second spacecraft to be built for the Double Star programme, a unique collaboration between Chinese and European scientists. Its predecessor, Tan Ce 1 (Explorer 1), was successfully launched on a similar rocket from a launch site in Xichang on 29 December 2003 and is now returning a rich stream of data.

Eagerly awaited by UK scientists, who have played a major role in the Double Star missions, Tan Ce 2 will complete a six spacecraft Sino-European constellation designed to solve a 30 year-old space mystery: what happens when magnetic storms are generated above the Earth?

Mysterious Storms And Earth's Magnetic Shield
Double Star is the first Chinese programme dedicated to space science, and the first international space mission involving China. It has been designed to operate alongside the European Space Agency's groundbreaking Cluster mission, in which a mini-flotilla of spacecraft flies in formation around the Earth.

This joint enterprise involving East and West will revolutionise our knowledge of the Earth's magnetosphere - the huge, tadpole-shaped region of space dominated by our planet's magnetic field - and its struggle to protect our planet from the supersonic particles that stream towards us from the Sun.

The four Cluster spacecraft have been orbiting together since 2000, flying over Earth's magnetic poles and revealing for the first time processes which were only hinted at by earlier missions involving single spacecraft.

Together with Cluster, the two Tan Ce spacecraft will be used to study the link between the solar wind and geomagnetic activity, and to provide the most detailed, multi-dimensional view of the complex magnetosphere ever obtained.

The most exciting investigation is the search for the cause of explosive magnetospheric substorms, which has been eluding space scientists since their discovery, more than 30 years ago. These storms can cause havoc with compasses and power systems on the ground.

During August and September, the four Cluster spacecraft will be spending much of their time travelling on the nightside of the Earth, in a region known as the magnetotail. Careful orbit design has arranged that the two Double Star spacecraft will periodically join them in this region. Flying closer to the Earth, they will complete the chain of six spacecraft located at different distances from our planet.

Scientists hope that this arrangement will enable them to tie down the location of the onset of a substorm, when large amounts of energy are explosively released from the Earth's magnetic field by a process known as magnetic reconnection.

This energy accelerates charged particles in the magnetotail and drives powerful electrical currents down into the ionosphere, a region of the upper atmosphere. The most obvious manifestations of these particle streams are spectacular auroral displays and disruptions to the magnetic field.

As yet, no one knows exactly where the energy release process begins, so it is not possible to choose between several theories of the origin of these substorms. Mission scientists hope that accurate measurements by the Cluster/ Double Star sextet will answer this problem, and in doing so beat a dedicated five spacecraft mission being developed by NASA (named THEMIS, due for launch in autumn 2006) to solving the mystery!

"On its own, Double Star is scientifically important because it provides new measurements in key regions of magnetosphere," said Andrew Fazakerley (MSSL-UCL), one of the Principal Scientific Investigators (PIs) from the UK.

"For example, it will provide important new information on the Earth's ever-changing radiation belts."

"However, the really exciting part is that the orbits of the two spacecraft are explicitly designed for co-ordinated measurements with Cluster," he added.

"So, when Cluster is in the distant magnetic tail and Double Star is in the near tail, we shall be able to see simultaneously for the first time what happens in both of these key regions when the huge amounts of energy that drive the substorms are released."

UK Experiments
UK teams play major roles in both Double Star and Cluster, both through provision of instruments and involvement in science operations.

Seven of the eight European instruments on the pair of Double Star spacecraft (including five led by the UK) are copies of instruments on Cluster.

The Plasma Electron and Current Experiment (PEACE) on TC-1 and TC-2 was provided by the Cluster team at Mullard Space Science Laboratory, led by Andrew Fazakerley. This measures the speed, direction and population of electrons around the spacecraft.

Principal Investigator for the Fluxgate Magnetometer (FGM) experiments on TC-1 and lead Co-Investigator for the TC-2 FGM is Chris Carr from the Cluster team at Imperial College London. These instruments can measure a magnetic field in space 1,000 times weaker than the field at the Earth's surface.

An experiment on TC-1 that measures waves (rapid variations in the magnetic field) includes the Digital Wave Processor (DWP) instrument, developed by the Cluster team at the University of Sheffield, under the leadership of Hugo Alleyne.

In addition, Double Star will draw on science operations expertise at the Rutherford Appleton Laboratory (RAL). RAL has been running the Cluster Joint Science Operations Centre (JSOC) since the beginning of 2001 and has adapted this to provide a similar service for Double Star.

This European Payload Operations Service (EPOS) works with the European instrument teams on Double Star to co-ordinate the commanding of their instruments and delivers the finalised commanding to the Double Star Science Application System in Beijing.

RAL is also providing the Double Star Data Management System that will exchange key data products generated by the instrument teams between national data centres in Austria, France and the UK, and enable scientists and the general public to browse and retrieve those products.

Mike Hapgood, lead scientist for both the Cluster JSOC and Double Star EPOS, says, "This is a great opportunity to advance our understanding of the large-scale behaviour of the Earth's magnetosphere."

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