The Cluster Mission, for the first time, explores the magnetosphere
with a fleet of four spacecraft orbiting in a group. The four spacecraft
combination is itself a new kind of tool able to make measurements that
a single spacecraft cannot make. Cluster, together with the solar
observatory SOHO, launched in 1995, forms the First Cornerstone of
ESA's Horizon 2000 programme. The Cluster project has succeeded despite
the loss of the original spacecraft quartet in a launch accident. To
learn more, see below.