“…The GeoSail mission concept is motivated by the desire to achieve long residence times in the Earth's magnetotail, enabling high resolution statistical characterisation of the plasma in a region subject to a variety of external solar wind conditions (Alexander et al, 2002;Leipold et al, 2010a;Macdonald et al, 2000Macdonald et al, , 2003Macdonald et al, , 2007aM c Innes et al, 2001). This is accomplished by the novel application of a solar sail propulsion system to precess an elliptical Earth-centred orbit, interior to the lunar orbit, at a rate designed to match the rotation of the geomagnetic tail, the orientation of which is governed by the Sun-Earth line.…”