Towards the "Terra Incognita" (2)

It is appropriate to consider the VIM as three distinct phases: the termination shock, heliosheath exploration, and interstellar exploration phases. The two Voyager spacecraft began the VIM operating, and are still operating, 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 to be encountered by a spacecraft as a result of this interstellar wind/solar wind interaction will be the termination shock where the solar wind slows from supersonic to subsonic speed and large changes in plasma flow direction and magnetic field orientation occur.

Passage through the Termination Shock ends the Termination Shock Phase (TSP) and begins the Heliosheath Exploration Phase (HEP). Voyager 1 in 2004 completed the TSP of the mission when the spacecraft was 94 AU from the Sun. After passage through the Termination Shock, the spacecraft will be operating in the Heliosheath environment which is still dominated by the Sun's Magnetic Field and particles contained in the Solar Wind. The HEP ends with passage through the Heliopause which is the outer extent of the Sun's Magnetic Field and Solar Wind. The thickness of the Heliosheath is uncertain and could be tens of AU thick taking several years to traverse. Passage through the Heliopause begins the Interstellar Exploration Phase (IEP) with the spacecraft operating in an interstellar wind dominated environment. This interstellar exploration is the ultimate goal of the Voyager Interstellar Mission.