By the start of VIM, "Voyager 1" was at a distance of 40 AU from the Earth while "Voyager 2" was at 31 AU. VIM is in three phases: termination shock, heliosheath exploration, interstellar exploration phase. The spacecraft began VIM in an environment controlled by the Sun's magnetic field with the plasma particles being dominated by those contained in the expanding supersonic solar wind. This is the characteristic environment of the termination shock phase. At some distance from the Sun, the supersonic solar wind will be held back from further expansion by the interstellar wind. The first feature encountered by a spacecraft as a result of this interstellar wind–solar wind interaction was the termination shock where the solar wind slows to subsonic speed and large changes in plasma flow direction and magnetic field orientation occur.