Artist's impression of the Cluster II satellites high above the Earth. Photo: ESA

The first pair of Cluster II satellites is poised to blast off on Saturday from the Baikonur Cosmodrome in Kazakhstan. Two more are scheduled to follow in early August, completing a space quartet that should revolutionise our understanding of the interaction between the Sun and Earth.

Yet four years ago, the Cluster programme seemed dead and buried. On 4 June 1996, the spectacular failure of the European Ariane 5 rocket during its maiden launch grabbed headlines around the world. As the media focused on the drama of the incandescent debris raining down on the swamps of French Guiana, a much more low key drama was being played out behind the scenes.

On board the huge Ariane booster had been four Cluster satellites, a unique group of identical spacecraft designed to carry out the most detailed investigation of the magnetosphere ever undertaken. However, instead of looking forward to a stream of data from the 44 instruments on board this small flotilla, scientists could only watch in disbelief as a decade of work disappeared in smoke before their eyes.

Disaster struck the first Cluster mission when its Ariane 5 booster exploded shortly after launch. Photo: ESA

Phoenix
Despite this tragic setback, a dedicated group of scientists was determined to resurrect the Cluster mission. They were able to persuade the European Space Agency (ESA) to agree to the launch of a fifth Cluster satellite. It was appropriately dubbed Phoenix, after the mythical Arabian bird that rose from the ashes.

Phoenix was to be based on the Cluster Structural Model and equipped with spares of the experiments and subsystems left over from the original mission. New equipment would only be manufactured when necessary. By taking advantage of the existing hardware, together with the knowledge and experience gained in the original programme, Phoenix was expected to be fully integrated and tested by mid-1997, opening the way for a launch later that year.

This rapid response to the launch failure soon gave way to a longer term strategy. Aware that the scientific objectives of the Cluster mission could not be met by a single spacecraft, ESA began to study proposals to rebuild three or four full-size Cluster spacecraft, or to launch three smaller satellites alongside Phoenix. Eventually, on April 3, 1997, the agency agreed that the potential science return from a full Cluster reflight was so important that three full-scale replica spacecraft would be built, in addition to Phoenix.

Fast work
In a remarkable display of teamwork, ESA, the scientific community and industry came together to recreate the Cluster quartet in less than four years. Known as flight models (FM) 5 to 8, (at least until they are given names at the time of the first launch from Baikonur) the rebuilt spacecraft are now undergoing final launch preparations at Baikonur Cosmodrome.

The four indentical Cluster II spacecraft seen here in the clean room at IABG near Munich. Each spacecraft weighs 1.2 tonnes fully fuelled and carries a payload of 11 scientific instruments. Photo: ESA

Paying tribute to everyone who has put so much effort into achieving the remarkable revival of the Cluster mission, Project Manager John Ellwood said, "In the past three and a half years, a tremendous amount of work has been completed by both scientists and industry. It has been a fantastic achievement to build four satellites in such a short time."

Anatomy of a Cluster II spacecraft >