The Limits of Our Solar System

The heliosphere is the bubble the Sun carves out of the interstellar medium. The solar wind moves outward at supersonic speeds and carries with it the Sun’s magnetic field. The interstellar medium is also moving at supersonic speeds and carries with it the interstellar magnetic field. The boundary between the solar wind and the interstellar medium is called the heliopause. Before these plasmas interact at the heliopause, they both go through shocks that cause the flows to become subsonic and change direction. Voyager 1, which recently crossed the termination shock of the solar wind, is the first spacecraft to observe the region of shocked solar wind called the heliosheath. The interstellar neutrals are not affected by the magnetic fields and penetrate deep into the heliosphere, where they can be observed both directly and indirectly. This chapter discusses these outer boundaries of the solar system and the interactions between the solar wind and the interstellar medium, and describes the current state of knowledge and future missions that may answer the many questions that remain.