The spacecraft formation is an equilateral tetrahedron in the Northern Hemisphere Cusp and while crossing the magnetopause in the Southern Hemisphere. At the magnetopause, one plane is to be aligned with the expected orientation of the surface. The spacecraft separation while the formation is an equilateral tetrahedron will be about 600 km. Note that the spacecraft formation evolves around the orbit - a natural consequence of the spacecraft following slightly different orbits. The spacecraft will follow these orbits for about six months, until Phase 2 begins.

The equliateral tetrahedron is the best formation for deriving vector quanitites using the gradients of quantities measured at each spacecraft. Other formations may be more useful for different purposes.

above: Cluster Phase 1 orbit (click to enlarge)

        left: Graph showing Cluster Phase 1 orbits.
        (click to enlarge)

Image source : Escoubet, C.P., R. Schmidt and M.L. Goldstein, Cluster- science and mission overview, in The Cluster and Phoenix Missions, Kluwer Academic Publishers, 1997, reprinted from Space Science Reviews vol 79, v-vii, 1997.)

By Andrew Fazakerley
Last  updated on26/06/00  by S.Szita