Charge-exchange collisions in the interstellar medium lead to a weak coupling between the neutral and ionized hydrogen causing a pile-up of neutral hydrogen at the heliosphere nose, referred to as the "hydrogen wall." Interstellar Probe will explore the structure of this wall with both in situ (neutral particle) and remote sensing (UV) observations. Astronomical observations have shown that other stars also have hydrogen walls and thus presumably astrospheres analogous to our heliosphere. Knowledge of the structure of our heliosphere will be critical to the interpretation of observations of similar structures and winds observed in neighboring star systems. For example, understanding how the size of the hydrogen wall depends on the density the interstellar medium will make it possible to estimate the density of the interstellar material around other stars. In general, the study of the structure and dynamics of our heliosphere will serve as an example of how a star interacts with its environment.

• Does the heliosphere create a bow shock in the interstellar medium?

  
  

• What is the relation of the hydrogen wall outside the heliopause to similar structures and winds observed in neighboring stellar systems?

  
  

• How do the Sun and heliosphere influence the temperature, ionization state, and energetic particle environment and other properties of the local interstellar medium? How far does the influence extend?

  
  

• How are interstellar plasma and dust flows deflected as they encounter the heliosphere?

  
  

• How does particle acceleration occur at the termination shock and at other astrophysical shocks?