Authors: Pontus Brandt (Johns Hopkins APL, Laurel, MD), L. Spilker, S. A. Stern, R. L. McNutt, M. Opher, H. Elliott, M. E. Hill, P. Mostafavi, P. Kollmann, A. Poppe, P. Swaczyna, E. Provornikova, A. Galli, M. Gkioulidou, J. Richardson, D. J. McComas, K. N. Singer, J. Spencer, A. Verbiscer, J. Parker

Our solar system has evolved through accretion of dust and gas as the Sun and its protective magnetic bubble – “the heliosphere” – have plowed through interstellar space on its journey through the galaxy. Over the course of its evolution, the solar system has encountered dramatically different interstellar properties resulting in a severely compressed heliosphere with periods of full exposures of interstellar gas, plasma, dust and galactic cosmic rays (GCRs) that all have helped shaped the system we live in today. Today, the heliosphere is about to enter the new and unknown environment of the G-cloud – a galactic event taking place before our eyes. Because of the lack of direct measurements there continues to be a large gap in our understanding how the Sun upholds its vast heliosphere and its potentially game-changing role in the evolution of our galactic home. Very basic and fundamental questions still remain and include

1. How is the heliosphere upheld by the physical processes from the Sun to the VLISM, and how do those globally manifest themselves?
2. How does the Sun’s activity, the interstellar medium and its possible inhomogeneity influence the dynamics and evolution of the global heliosphere?
3. How do the current VLISM properties inform our understanding of the evolutionary path of the heliosphere?

Voyager 1 and 2 – or the Voyager Interstellar Mission (VIM) – are now in the Very Local Interstellar Medium (VLISM) at 160.4 au and 133.8 au from the Sun, respectively (1 July 2023), where they are expected to operate until the mid-2030’s. Despite having a relatively limited payload for space plasma measurements, VIM has made many remarkable discoveries and uncovered many more mysteries demonstrating that the boundary to the VLISM is a new regime of space physics.

Now at 56.4 au from the Sun (1 July 2023), New Horizons is currently the only spacecraft in the outer heliosphere and is projected to have sufficient power to operate well into the heliosheath and possibly even beyond the heliopause. It is uniquely positioned to investigate the evolution of the solar wind, energetic particles, GCRs, and, in particular interstellar Pick-Up Ions (PUIs) that Voyager was not equipped to measure. Once past the circums-solar dust of the Kuiper Belt, New Horizons’ dust measurements should begin to reveal an interstellar component that will provide the strongest constraint to date on how interstellar dust grains interact with the heliosphere. Now beyond the infrared and UV haze of the circumsolar dust and hydrogen gas, the Alice UV camera holds promise to search for signatures of the hydrogen wall and perhaps even signatures of our neighboring interstellar clouds.

The fleet of missions in the inner heliosphere includes in-situ solar wind measurements and remote IBEX (and soon IMAP) ENA observations for critical images of the global dynamics. With this fleet, New Horizons and VIM, the community is presented the largest space constellation in history to explore our protective heliosphere.

In the late 2030’s, the position of Jupiter offers the only viable high-speed trajectory in more than a decade through the noseward hemisphere of the heliosphere for a dedicated and unifying Interstellar Probe through the heliosphere and into VLISM. In this presentation we provide an overview of the outstanding questions in the context of existing mission observations and the urgent need for sending a pragmatic Interstellar Probe the farthest frontier of Solar & Space Physics.