Double Star follows in the footsteps of ESA's ground-breaking Cluster mission by studying the effects of the Sun on the Earth's environment. Conducting joint studies with Cluster and Double Star should increase the overall scientific return from both missions.

A key aspect of Europe's participation in the Double Star project is the inclusion of eight instruments, seven of which are identical to those currently flying on the four Cluster spacecraft. A further eight experiments are provided by Chinese institutes.

"We hope it will be possible to make co-ordinated measurements with both Cluster and Double Star." says Cluster Project Scientist Philippe Escoubet. "For example, we would hope to carry out a joint exploration of the magnetotail, a region where storms of high energy particles are generated. When these particles reach Earth, they can cause power cuts, damage satellites, and disrupt communications."

Six of the 11 Cluster principal investigators agreed to provide flight spares or duplicates of the experiments that are currently revolutionising our understanding of near-Earth space. This reuse of Cluster instruments has many advantages for both European and Chinese scientists. By flying experiments identical to those on Cluster, costs and development time have been reduced, minimizing the risk and ensuring that the project could meet the spacecraft development schedule.

ESA agreed to contribute Euro 8 million to the Double Star programme. This funding was used for refurbishment and pre-integration of the European instruments, and is used for acquisition of data for 4 hours per day and coordination of scientific operations.

The first European experiments to be flown on Chinese satellites are:

• Active Spacecraft Control (ASPOC), one instrument (TC-1 only)
• Hot Ion Analyser (HIA), part of the CIS instrument on Cluster, one instrument (TC-1 only)
• Fluxgate Magnetometer (FGM), two instruments (TC-1 and TC-2)
• Plasma Electron and Current Experiment (PEACE), two instruments (TC-1 and TC-2)
• Spatio-Temporal Analysis of Field Fluctuations (STAFF), one instrument (TC-1 only).

Data is relayed to the ESA ground station at Villafranca, Spain, and the Chinese ground stations in Beijing and Shanghai, China.

The Double Star Mission

Double Star is the first mission launched by China to explore the Earth's magnetosphere - the magnetic bubble that surrounds our planet.

As its name suggests, Double Star involves two satellites - each designed, developed, launched, and operated by the Chinese National Space Administration (CNSA).

The first of the two spacecraft (TC-1) was launched 29 December 2003 at 19:06 UT. The second spacecraft (TC-2) was launched 25 July 2004 at 07:05 UT. This schedule enables Double Star to operate simultaneously with ESA's Cluster mission - a mini-flotilla of four identical spacecraft launched into polar, elliptical orbits around the Earth in the summer of 2000.

The 'equatorial' spacecraft (TC-1) was launched into an elliptical orbit of 500 x 66 970 kilometres, inclined at 28.5 degrees to the equator. This enables it to investigate the Earth's huge magnetic tail, the region where particles are accelerated towards the planet's magnetic poles by a process known as reconnection. TC-1's nominal period of operations is expected to exceed 18 months.

The 'polar' satellite (TC-2) has a 700 x 39 000 kilometre polar orbit. Its instruments will concentrate on the physical processes taking place over the magnetic poles and the development of auroras. It is expected to operate for at least one year.

It is planned to synchronise the two Double Star orbits with those of ESA's four Cluster satellites so that all six spacecraft are studying the same region of near-Earth space at the same time. This orbital configuration will enable scientists to obtain simultaneous data on the changing magnetic field and population of electrified particles in different regions of the magnetosphere.