The heliosphere is a large and complicated structure whose dimensions are not definitively known. Recent measurements suggest that the Voyager 1 spacecraft may have crossed the “termination shock” of the solar wind and passed intermittently into the interstellar medium (Figure B.1). Although the interpretations of the observations are controversial, it is quite certain that this region is unlike anything ever sampled previously. Continued tracking of the two Voyager spacecraft should provide the size of our heliospheric cavity within the next several years.¹

The solar wind continually rams into the local interstellar medium, and through complex interactions forms the large-scale outer boundaries of our solar system. The latter has three distinct components:

• The termination shock where the solar wind is abruptly slowed and heated prior to being deflected by the interstellar medium;

• The heliopause that separates the solar wind from the interstellar medium; and

• The bow shock or bow wave where the interstellar flow is slowed, heated, and deflected by the solar wind.

Between these boundaries that separate layers of solar wind from the interstellar medium, this “interstellar interaction” generates an approximate factor-of-two enhancement of neutral hydrogen (the “hydrogen wall”) in the upstream direction—the direction from which interstellar material is moving toward the Sun at ~25 km/s. Understanding the structure and dynamics of the interstellar interaction and our solar system’s outer boundaries is a primary goal of heliospheric science, and it is also relevant to understanding how other stars interact with the interstellar medium.

A variety of indirect measurements have provided only limited information on the nature of the interstellar interaction. These indirect techniques include measurements in the inner heliosphere of interstellar neutral atoms