The four Cluster spacecraft will fly in a tetrahedron formation. Image: ESA.

The 57 hour elliptical orbit has been selected so that the Cluster II spacecraft will travel over the planet's polar regions, allowing them to investigate in unprecedented detail all of the key plasma regions within the magnetosphere - the invisible bubble generated by Earth's magnetic field. For most of the mission, the satellites will fly in tetrahedral (lopsided pyramid) formation around the Earth, adjusting their relative distances by firing their onboard thrusters. Their separation distances will depend on the characteristics of the particular region of near-Earth space being studied and the spatial resolution required.

Since the orientation of their orbits is fixed, the Cluster II quartet will be able to investigate different regions of near-Earth space during the planet's annual circuit of the Sun. In the northern hemisphere summer, they will remain inside the magnetosphere and spend most of their time travelling through the extended magnetotail that stretches far into space on the leeward side of the Earth. With no tightly confined regions to study, the satellites may be as much as 18,000 km apart.

Six months later, their orbits will carry them between the magnetosphere and interplanetary space, where they will frequently be exposed to the supersonic solar wind. During this winter campaign, while carrying out detailed studies of the cusp regions over the magnetic poles and the boundary regions of the magnetosphere, the satellites may be only 200 km apart.

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