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 for the refurbishment of the spare Cluster spacecraft and its preparation for flight. This new spacecraft, considered to be the first of a new fleet, is called Phoenix. In the meantime various options for repeating 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 programme and the theory and modelling efforts all remain unchanged.