In orbital mechanics, the Hohmann transfer orbit is an orbital maneuver
    using two engine impulses which, under standard assumptions, move a
    spacecraft between two coplanar circular orbits. This maneuver was named
    after Walter Hohmann, the German scientist who published a description of
    it in 1925.

    from Wikimedia Commons

    about: http://commons.wikimedia.org/wiki/Image:Hohmann_transfer_orbit.svg
    source: http://commons.wikimedia.org/
    rights: GNU Free Documentation license,
            Creative Commons Attribution ShareAlike license