Artist's impression of the Cluster spacecraft in orbit. Photo: ESA

The Cluster quartet, Salsa, Samba, Rumba and Tango, now flying in their final orbit around the Earth in tetrahedral formation, are ready to enter the scientific commissioning and operation phase. Thus Cluster is at last joining the SOHO spacecraft and making the first cornerstone of ESA's "Horizons 2000" programme a reality.

The final activity ushering in the new phase of the existence of Cluster was performed last week, with the successful deployment of the antenna and experiment booms, three altogether on each spacecraft. This was done through careful manoeuvring, under the control of the ESOC operations team in Darmstadt.

Over the next few months all scientific instruments onboard the spacecraft will be gradually brought to life. The same instrument on each spacecraft will be switched on, one after another. Over a period of three months the instruments will undergo a series of health and calibration checks. By early December all 44 instruments on the four spacecraft will be operational and ready to start the scientific mission.

The four Cluster spacecraft were launched in two pairs from the Baikonur cosmodrome in Kazakhstan -- Salsa and Samba on 16 July and Rumba and Tango on August 9. Their current highly elliptical orbits vary from 17,200 km at perigee to 120,600 km at apogee.

By making simultaneous measurements in a tetrahedral formation, the Cluster quartet will be able to make the most detailed three-dimensional study yet of the Sun-Earth connection and of the changes and processes taking place in near-Earth space.

For the first time ever in space history, four identical spacecraft are operated simultaneously in orbit, opening new horizons for future multi-spacecraft missions.

The Cluster project involves more than 70 laboratories and over 250 scientists from many countries, including Europe, the United States, Russia, China, Japan, India, Israel, Hungary, Poland and the Czech Republic.

Cluster 2 is part of an international programme to investigate how the Sun and Earth interact. The four satellites will join an armada of spacecraft from many countries (including ESA's SOHO satellite) which are already studying the Sun and the high-speed wind of charged particles - mainly electrons and protons - which it continually blasts into space.