Interstellar Probe: Pushing the Frontier of Space Science

An Interstellar Probe beyond our heliosphere in to the largely unexplored interstellar medium (ISM) would be the furthest and boldest step in robotic space exploration ever taken. A dedicated payload of in-situ and remote sensing instruments would uncover the new regime of physics at work in the heliospheric boundary region and offer the first external view of the global heliosphere that is currently missing in the family portrait of all other types of astrospheres observed. Beyond about 400 AU the Probe would reach the ISM and for the first time begin its sampling of the properties of the local interstellar cloud (LIC) that our Sun and neighboring star systems are immersed in.

An Interstellar Probe has been discussed since around 1960 in several NASA and international studies. The compelling science objectives have remained almost unchanged and are focused on understanding the plasma physics in the interaction region between the heliosphere and the ISM. Their importance have been amplified by the recent unexpected findings by the Voyager 1 and 2 spacecraft that are nearing their end of life at less than 150 AU from the Sun. Remote observations in Energetic Neutral Atoms (ENAs) by the NASA IBEX and Cassini missions have made the remarkable discoveries of ENA emission morphologies that have come as a complete surprise and still lack a satisfactory explanation. Hubble Space Telescope observations have now also made it clearer that the Sun is about to exit the LIC and perhaps already has, which is a unique event of astronomical scales that an Interstellar Probe could explore in-situ for the first time. In addition to these top-priority objectives, contributions of unprecedented science value to planetary sciences and astrophysics are possible including flybys of at least one Kuiper Belt Object, in-situ and remote observations of the dust debris disk, and the extra-galactic background light.

Here we review the outstanding questions and current state of understanding of the global heliosphere, the ISM and what planetary and astrophysics augmentations can offer. We summarize the compelling science case for an Interstellar Probe, including a range of possible science payloads and the associated operation scenarios. The results stem from the study of a Pragmatic Interstellar Probe currently underway, funded by NASA, and led by The Johns Hopkins University Applied Physics Laboratory with active participation from a large, international team of scientists and engineers. The study focuses on finding realistic mission architectures among a trade space of propulsion options, trajectories, risks and reliability challenges. The study considers operation out to 1000 AU, a survival probability of 85% over 50 years and electrical power of no less than 400 W at the beginning of mission. Over twice the speed of Voyager 1 (the fastest spacecraft currently) has already been achieved in the design using conventional propulsion, with a direct inject to Jupiter followed by a Jupiter Gravity Assist. In order to provide input requirements to the mission study, several possible payloads with different mass allocations and associated mission requirements, trade-offs and risks have been identified.