In our present view of the large scale structure of the heliosphere, the solar wind flows radially outward to a "termination shock" surrounded at somewhat greater distance by a contact surface called the heliopause, which is the boundary between solar wind and interstellar plasma. A bubble of solar wind therefore shields the inner heliosphere from the plasma, energetic particles, and fields of the interstellar medium; to observe these directly, one must get outside the heliopause. Although the size of the heliosphere is not certain, several recent estimates place the distance to the termination shock at ~80 to 90 AU, with the heliopause somewhat further beyond.
The Interstellar Probe Mission would be designed to cross the solar wind termination shock and heliopause and make a significant penetration into nearby interstellar space. The principal scientific objectives of this mission would be to (1) explore the structure of the heliosphere and its interaction with the interstellar medium; (2) explore the nature of the interstellar medium, and its implications for the origin and evolution of matter in the galaxy, and (3) investigate fundamental astrophysical processes occurring in the heliosphere and interstellar medium.
To achieve these objectives the strawman scientific payload includes an advanced set of miniaturized, low-power instruments specifically designed to make comprehensive, in situ studies of the plasma, energetic particles, fields, and dust in the outer heliosphere and nearby interstellar medium. Included would be in situ studies of particle acceleration at the termination shock, known to reach GeV energies, studies of the structure and motion of the shock, which is a model for astrophysical shocks throughout the cosmos, and studies of the penetration of gas and dust into the heliosphere. Note that the boundaries of our heliosphere provide a model for similar interactions occurring around stellar systems throughout the Galaxy. Once beyond the heliopause an Interstellar Probe would be able to make direct measurements of interstellar gas and dust, the interstellar magnetic field, and of low energy cosmic rays unaffected by processes of "solar modulation" that shield the inner heliosphere.
To accomplish its objectives an Interstellar Probe should acquire data out to a heliocentric distance of ~200 AU requires spacecraft velocities of ~10 AU/year (roughly three times that of Voyager-1) to achieve this within ~25 years or less. This is possible with standard chemical propulsion using a powered solar-flyby, but might also be achieved with more advanced propulsion techniques.