Cluster was one of the two missions - the other being the Solar and Heliospheric Observatory (SOHO) - constituting the Solar Terrestrial Science Programme (STSP), the first `cornerstone' of ESA's Horizon 2000 Programme. After the catastrophic Ariane-5 accident on 4 June 1996 which destroyed the four Cluster spacecraft, the European Space Agency Science Programme Committee gave approval to refurbish the spare Cluster spacecraft and make it ready for flight. This new spacecraft, considered to be the first of a new fleet, is called Phoenix. In the meantime various options to repeat the Cluster four-point measurements are being studied.

Since Phoenix, as the fifth Cluster spacecraft, will be equipped with the spare Cluster experiments, the instrumentation articles in this book are still appropriate to the new mission. Furthermore, the objectives of the recovery mission, the ground systems, the ground observation program and the theory and modelling efforts all remain unchanged. Thus this series of articles will continue to be essential to the Cluster community and to the general scientific community as the recovery mission is implemented.