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resurrection of clusterThe Resurrection of the Cluster Scientific MissionJ. CredlandHead of Scientific Projects Department, ESA Directorate for Scientific Programmes, ESTEC, Noordwijk, The NetherlandsR. SchmidtCluster Project Scientist, Space Science Department, ESA Directorate for Scientific Programmes, ESTEC, Noordwijk, The NetherlandsIntroductionThe launch of Cluster, one of the Cornerstone missions of the ESA Scientific Programme, was scheduled for the early morning of 4 June 1996. After a short delay during the countdown due to bad weather over the launch pad, Ariane-5 rose flawlessly to an altitude of 3.5 km, at which point a sudden swivelling of both solid-booster nozzles caused the vehicle to tilt sharply. The resulting intense aerodynamic structural loads caused the launcher to begin to break up, prompting the onboard safety systems to initiate self-destruction of all launcher elements. The original four-spacecraft Cluster mission was lost in the ensuing explosion.The Cluster mission was first proposed to the Agency in late 1982 and was subsequently selected, together with SOHO, as the Solar Terrestrial Science Programme (STSP), the first Cornerstone of ESA's Horizon 2000 Programme. This article gives an overview of the complex chain of events that have taken place between the loss of the original mission with the Ariane-5 launch failure and the recent approval of the recovery mission known as Cluster-ll.Cluster-ll is a new four-spacecraft mission for which the go-ahead was given on 3 April 1997, ten months after the loss of the original spacecraft. Cluster-ll will be basically Identical to the previous mission except for changes introduced as a result of the non-availablllty of components. The launch date for the new mission has been set for mid-2000. The four spacecraft will be launched by two Russian Soyuz rockets, from Baikonur In Kazachstan.The original Cluster mission was planned to conduct an in-sltu Investigation of plasma processes in the Earth's magnetosphere using four identical spacecraft simultaneously. Itwould permit the accurate determination of three-dimensional and time-varying phenomena and make It possible to distinguish clearly between spatial and temporal variations for the first time. The four Cluster spacecraft were to be placed Into nearly identical, highly eccentric polar orbits, with a nominal apogee of 19.6 Re, and a perigee of 4 Re. Such an orbit is essentially Inertially fixed, so that in the course of the two-year mission a detailed examination could be made of all significant regions of the magnetosphere. If launched In summer, the plane of this orbit bisects the geomagnetic tall at apogee during the northern-hemisphere summer, and passes through the northern cusp region of the magnetosphere six months later.The Cluster spacecraft also carried their own propulsion stages In order to perform all necessary manoeuvres for the transfer from the Geostationary Transfer Orbit (GTO) to the final polar orbit. In addition, it was required to change the In-orbit constellation of the satellites periodically by modifying the distances between them to match the scale lengths of the plasma phenomena being investigated. The relative separations would therefore vary from a few hundred kilometres to a few Earth radii. In the original programme baseline, all four spacecraft had to be injected Into GTO on a single Ariane-5 launch vehicle and then transferred in pairs to their mission orbits via a complex series of orbital manoeuvres.This article describes the sequence of events that eventually led to the approval of Cluster-ll by ESA's Science Programme Committee (SPC). Numerous meetings took place in order to either prepare position papers or, more often, to re-act to new challenges raised by