Voyager Interstellar Mission

The Voyager Interstellar Mission is exploring the interaction of the heliosphere with the local interstellar medium (LISM). Voyager 1 (V1) and Voyager 2 (V2) are both in the heliosheath, making the first in situ observations of the shocked solar wind beyond the termination shock (TS). The heliosheath extends to the heliopause; beyond the heliopause is the interstellar medium. The goal of the Voyager Interstellar Mission, as the name implies, is to make the first in situ observations of the region outside our heliosphere. As the Voyagers proceed outward toward that goal, they are making measurements of totally unexplored regions of the heliosphere. The Voyager crossings of the TS provided the first concrete information on the scale size and the shape of the heliosphere. Voyager 1, in the northern hemisphere of the heliosphere, crossed the TS at 94 Astronomical Units (AU) while V2, in the southern hemisphere, crossed it at 84 AU. Based on these TS distances and model predictions, the he-liopause (HP) and LISM are probably 30-50 AU further out. The asymmetry in the TS crossing distances verifies that the southern hemisphere of the heliosphere is pushed inward, probably by the interstellar magnetic field. Although the uncertainties in the HP position are large, the Voyager spacecraft have a good chance of reaching this boundary in their operational lifetimes. The observed asymmetry may allow V1 and V2 to cross the HP at roughly the same time and provide simultaneous observations of the LISM. The V1 and V2 TS and heliosheath observations provided many surprises that we are still working to understand. A major outstanding problem is the source of the anomalous cosmic rays (ACRs). Before the Voyager TS crossings, the ACRs were thought to be accelerated at the TS. The ACR source was not found at either Voyager TS crossing; new hypotheses place the source either in the flanks of the TS or nearer to the heliopause. Plasma flows in the heliosheath are also a puzzle, with radial flows at V2 a factor of two greater than at V1 and flows at V2 more tangential than meridional, both contrary to predictions. The heliosheath magnetic field is highly variable on time scales of tens of minutes to hours, showing that this region is very dynamic. None of the foregoing observations were anticipated. These and future revelations of the unexpected nature of the heliosheath, HP, and LISM will continue to drive theoretical modeling …