Impact of Recent Voyager, IBEX, and Cassini Results on Science and Strategy for an Interstellar Probe Mission

The ongoing Voyager Interstellar Mission (VIM) and recent observations from the Interstellar Boundary Explorer (IBEX) and Cassini missions have revealed the interaction of the heliosphere with the very local interstellar medium (VLISM) to be much more complex than heretofore assumed by our present day concepts. These discoveries call for a major revision of the strategy for the Interstellar Probe mission. With new observations have come significant new puzzles for describing the interaction physics. Direct measurements of the shocked, solar-wind flow speed obtained from Voyager 2 show the flow remains supersonic. Other in situ instruments on Voyager 1 and Voyager 2 continue to reveal significant fluxes of energetic particles in the heliosheath while pointing to a more remote location for the modulation region and source of the anomalous cosmic rays (ACRs). This evidence supports the idea that the bulk of the energy density in the plasma resides in a non-thermal component that extends to very high energies. There are both quantitative and qualitative implications for the overall heliospheric structure. Remote observations by IBEX and the Ion and Neutral Camera (INCA) on Cassini of energetic neutral atoms (ENAs) originating from the interaction region(s) of the solar wind and the VLISM show unexpected structure on a variety of scales. In addition to the general “glow” of the sky in ENAs, IBEX data show a relatively narrow “ribbon” of atomic hydrogen emission from ~200 eV to ~6 keV, roughly circular, but asymmetric in intensity, which may be ordered by the interstellar magnetic field. It passes through, rather than being centered on, the “nose” from which the local, neutral interstellar wind enters the heliosphere, suggesting that the flow is not the primary driver of the system as has been thought. The neutrals from both the glow and ribbon are also characterized by non-thermal distribution functions. INCA on Cassini sees a “belt” of emission in ENAs, broader than the ribbon and tilted significantly away from it, at higher energies (10s of keV). These in situ and remote observations emphasize the need for a new generation of measurements to understand the global nature of our Sun’s interaction with the local galactic environment. Only an interstellar probe with modern instruments and measurement requirements better defined by these recent observations can provide the new information required. To provide a sufficiently fast flyout speed from the heliosphere, careful trades must be done taking into account instrument masses, measurement capabilities, and mission scientific requirements.