Interstellar Probe: The Next Step

An “Interstellar Probe” to the nearby interstellar medium has been discussed in the scientific community for almost 60 years. The key concept has always been to depart from the Sun outward “as fast as possible.” Scientific goals have principally focused on heliospheric topics throughout multiple studies, with potential “bonus science” in both astrophysics and planetary science. The passages of Voyagers 1 and 2 into that medium have only raised multiple new questions, rather than “solving” the outstanding question of the interaction of the solar wind with the nearby interstellar medium. In particular, solar activity apparently continues to have an effect on nearby interstellar space, magnetic field changes in crossing from the heliosheath into the local medium are only in magnitude and not direction, and the three-dimensional structure of the energetic neutral atom (ENA) “ribbon” remains unknown. The power levels on the Voyagers continue to decrease toward the operational floor which is likely to be reached within the next five years, limiting the extent of our exploration, and ending heliophysics deep-space measurements beyond the asteroid belt for the indefinite future. The salient question for a dedicated mission is “What can the Interstellar Probe do that no other mission can do?” The answer requires an in-depth look at current capabilities for such a mission, e.g., solar system escape speed, data downlink bandwidth, and mission lifetime with science topics, technological readiness of mission and instrument concepts, and realistic mission costs. To provide technical input to the upcoming Solar and Space Physics Decadal Survey, NASA has contracted with the Johns Hopkins University Applied Physics Laboratory (APL) to execute a “First Pragmatic Interstellar Probe Mission Study.” The effort focuses on near-term engineering readiness (ready for launch by 2030) but also includes input regarding compelling science and associated required measurements and instrumentation, assuming that such a mission would commence during the next Decadal time period. This is not a Science Definition Team (SDT) exercise, but rather an assessment of possibilities. In that spirit, we continue to seek input from across the international space science community regarding potential science goals, measurements, instruments, and their implementation readiness in order to help inform the engineering team in support of a concept mission. We provide a status report on this ongoing effort.