Since the signing of the landmark agreement, rapid advances have been made - despite cultural and language differences and geographical separation.

A much less predictable obstacle was the outbreak of SARS (Severe Acute Respiratory Syndrome) in April, which meant that planned meetings had to be cancelled and replaced by videoconferences, remote data transfer, web cams and interactive messaging dialogues. Bodo Gramkow, ESA's payload project manager for Double Star, had to obtain special permission to travel to China while the global alert issued by the World Health Organisation was still in force, but through the generous assistance of the CNSA and CSSAR, those involved could continue the work and save the mission. Gramkow and three engineers from Astrium subsequently stayed in an empty hotel.

Despite such unforeseen problems, the programme remained on course for completion by the end of 2003, as a result of the hard work and dedication of all involved. After 18 months of intense interface definition work, the hardware test phase began in autumn 2002 with a successful compatibility test of European and Chinese equipment at Imperial College, London.

In parallel, the assembly of the spacecraft structural-thermal model in China culminated with the successful completion of the environmental test programme in February 2003. Despite the inevitable delays caused by the SARS outbreak, the payload and the CSSAR subsystems were delivered to Beijing by 10 July for integration into the TC-1 flight spacecraft (TC are the initials of Tan-Ce, which means Explorer). The installation of Chinese equipment at ESA's Villafranca ground station was also successfully completed.

The launch campaign for the first Double Star satellite began in mid-November, and lasted five weeks. As planned, the launch took place before the end of the year, at 19:06 UT on 29 December 2003 (Fig. 4). The 1st and 2nd stages of the Long March 2C/SM rocket fired for 7 minutes, then the solid fuel upper stage injected the spacecraft into its operational orbit.

The TC-1 spacecraft was placed in a 570 x 78,970 km orbit with a 28.5 degree inclination. Although the apogee is about 12,000 km higher than expected, due to over-performance of the upper stage engine, this should not affect the scientific objectives. Indeed, the higher orbit means that the spacecraft will now be able to observe the Earth's bow shock, which was not in the original science plan. Preliminary estimates indicate that the number of conjunctions with Cluster have decreased slightly, but this will be compensated by the longer time interval of the individual conjunctions.

Launch of TC-1 on Long March 2C/SM rocket

The second satellite (TC-2) is now being assembled prior to launch in July 2004, when it will join the Cluster flotilla in a polar orbit. Meanwhile, the operational lifetime of Cluster has been extended for 3 years until 2005, in order to enable the unique, six-spacecraft investigation to take place.

"We have made remarkable progress, thanks to all scientists and engineers who have worked with such dedication on the project," said Philippe Escoubet, ESA's project scientist for both Double Star and Cluster. "In less than three years, China has developed the two spacecraft and their instruments, while the refurbishment of the European instruments has been completed."

The check-out of the TC-1 spacecraft started early January 2004 with the successful deployment of the solid boom carrying the FGM magnetometer. However, a second boom that carries the search coil for the STAFF/DWP experiment failed to deploy. After an investigation by Chinese spacecraft engineers, a second attempt was made on 16 February, but this, too, was unsuccessful. Subsequent analysis showed that the stability of the spacecraft was unaffected and its spin axis is, as expected, close to the north ecliptic pole.

FGM started commissioning on 8 January and the instrument electronics were checked out successfully after 4 days. At the time of writing, data acquisition is taking place to calibrate the instrument. STAFF/DWP has also finished its initial commissioning and the instrument is functioning nominally. However, as a result of the undeployed boom, the STAFF/DWPsensor detects more interference and noise coming from the spacecraft. The principle investigator team responded to this challenge immediately and is adapting the data acquisition in order to reduce this interference.

The PEACE (electron detector) experiment started its first electronic tests on 20 January and is working nominally. During this phase, scientists enjoyed their first opportunity to conduct combined observations when increased activity of the Sun resulted in a M6.1 flare on 21 January, followed by the arrival of an interplanetary shock at the Earth around 01:35 UT the next day. The increased pressure of the solar wind (more than 5 times higher than normal) resulted in a large compression of the magnetosphere. The bow shock was pushed toward Earth and detected in the PEACE data as it passed the TC-1 satellite, located at an orbital altitude of 12.6 Earth radii (80,360 km). The Cluster spacecraft, which were in the solar wind at the time, also observed the interplanetary shock. The good conjunction of spacecraft orbits will provide excellent study opportunities for similar events in the future.

Commissioning of the ASPOC (ion emitter) experiment began on 24 January and concluded successfully after 5 days with all four emitters working perfectly. This instrument will keep the spacecraft "grounded" by compensating for its surface charging, thus enabling very good low energy particle detection.

HIA, the fourth European instrument on TC-1, started its first check-out on 4 February. Electronics checks lasted two days and verified that all components were in perfect shape. High voltages were raised slowly to around 1500 V and the first ions were detected on 9 February. In-flight calibrations continued up to 13 February, when the instrument was declared fully functional by the PI team.

In the coming weeks, the instruments will be further calibrated and the data distribution system will be commissioned. The commissioning review will take place on 10 March, after which the nominal mission operation phase is expected to start.