Both Voyager spacecraft have observed intense radio emissions in the 2-3 kHz range from the direction of the nose of the heliosphere. The timing of these events relative to large interplanetary shock events suggests that the emission results when the shocks strike the denser interstellar plasma beyond the heliopause. Using the observed time delay as a time-of-flight for the event to propagate from the Sun to the radio source region, it has been estimated that distance to the heliopause is between 110 and 160 AU. This range is consistent with estimates of the distance to the termiation shock (84+-6 AU) derived from the interplanetary intensity gradient of anomalous cosmic rays. These boundaries are not static, and as the solar wind pressure varies over the solar cycle the termination shock and heliopause are expected to move in and out by 20 to 30 AU. Interstellar Probe will explore these boundaries with a comprehensive suite of in situ and remote sensing instruments designed to reveal the detailed structure and dynamics of the heliosphere.

Plasma and neutral particle densities computed by a hydrodynamic model are plotted as a function of distance from the Sun. The solar wind density decreases with the inverse square of the distance until it reaches the termination shock, where it is expected to suddenly jump in density by as much as a factor of 4. Although interstellar plasma is diverted around the heliopause, interstellar neutral atoms can enter the inner heliosphere. Some of these become ionized by solar UV or by charge exchange with the solar wind and become singly charged "pick up ions". Once created, the pickup ions are carried out by the solar wind. Some fraction of these is accelerated to high energies at the solar wind termination shock to become "anomalous cosmic rays".