Interstellar Probe: Impact of the Voyager and IBEX results on science and strategy

摘要

The ongoing Voyager Interstellar Mission (VIM) and recent observations from the Interstellar Boundary Explorer (IBEX) and Cassini missions are providing significant new information about the interaction of the heliosphere with the very local interstellar medium (VUSM). With new observations have come significant new puzzles for describing the interaction physics. Direct measurements of the shocked, solar-wind flow speed are now possible (from Voyager 2) and show the flow remains supersonic. This is one more piece of evidence supporting 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. Observations of energetic neutral atoms (ENAs) by IBEX (in Earth orbit) 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 to ~6 keV, roughly circular, but asymmetric in intensity, and centered on an ecliptic longitude ~221 degrees and ecliptic latitude of 39 degrees. The ribbon 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, indicating that the flow is not the primary driver of the system as had been thought previously. The neutrals from both the glow and ribbon are also characterized by non-thermal distribution functions. ENAs are observed at higher energies as well by the Ion and Neutral Camera (INCA) on Cassini (in orbit about Saturn). A "belt" of emission,broader than the ribbon but similar to it, is seen up to ~50 keV. These observations emphasize the need for in situ measurements to understand the global nature of our local galactic environment, which is much more complex than previously thought. Only an interstellar probe with modern instruments and measurement requirements better defined by these recent observations can provide the new information required. Even more importantly, the broader scale of the interaction as revealed in these measurements suggests much greater flexibility in scientifically allowable, asymptotic trajectories from the heliosphere for the probe. This is a significant relaxation in the trajectory requirements that open up the trade space for Jupiter gravity assists to increase the flyout speeds.