Mission planning for any future star probe will adjust not only for conditions in the interstellar medium but also the Solar System’s outer reaches. Let’s confine ourselves for now to conditions in the outer heliosphere. Currently we have precisely one spacecraft operating here. New Horizons has only reached 65 AU from the Sun, while Voyager 1 exited the heliopause in 2012 at 121 AU, and Voyager 2 crossed in 2018 at about 119 AU. New Horizons won’t have sufficient power to keep taking data as it makes its own crossing in the 2040s, but from its current position in the Kuiper Belt we can look back at what the spacecraft has reported so far about the solar wind and the local interstellar medium.

New Horizons’ Solar Wind Around Pluto (SWAP) instrument is the key here, examining how the solar wind slows as we leave the inner system behind. A new study from Southwest Research Institute (SwRI) points out what happens as this stream of hot ionized hydrogen and helium nuclei fills the heliosphere. The wind’s speed varies, some 300 to 500 kilometers per second from sources near the solar equator and up to 600-800 km/s from regions near the corona.

You would expect this ‘wind’ to cool as it begins to push against the interstellar medium, and indeed it does, forming the termination shock that both Voyagers have penetrated and crossed, and toward which New Horizons now moves. It’s at the termination shock that we see a sharp drop in the solar wind speed that indicates the outer boundary, the heliopause, is approaching. New Horizons should still be functional when it reaches the termination shock, conceivably as early as the end of this decade. Voyager 1 found it at 94 AU, Voyager 2 at 84 AU, reminding us how malleable the heliosphere is as its outer boundaries adjust to the onset of interstellar plasma.

Image: An SwRI-led study sheds light on the deceleration of the solar wind as it journeys away from the Sun and interacts with and picks up interstellar material. NASA’s New Horizons spacecraft measured the solar wind as it traveled from just beyond Uranus’ orbit into the outer Kuiper Belt (red shaded region), detailing the gradual slowdown caused by interactions with interstellar materials (red line). Credit: SwRI.

We can learn a great deal as we accumulate data on solar wind interactions in the outer heliosphere. SwRI’s Heather Elliott led the study. Says Elliott:

“Eventually, the solar wind reaches the outer boundaries of the heliosphere — the sphere of influence where the solar wind affects the space environment — where it interacts with incoming interstellar material. The shape and properties of these heliospheric boundaries control the amount of Galactic Cosmic Rays (GCRs) that can enter our solar system and reach Earth. Therefore, the data from New Horizons combined with observations from other missions, such as IBEX, IMAP and Voyager will enhance our understanding of the edge of the solar system.”

So far, the data have been useful as New Horizons keeps moving outward. Along the way, the solar wind begins to run into neutral gas particles that have entered the heliosphere from the outside interstellar medium. The interaction with the solar wind, in which these atoms become ionized, adds mass to the solar wind, Elliott adds. And that is the mechanism for slowing the wind down.

In previous years, we have learned that between 30 and 43 AU, the solar wind has slowed 5 to 10 percent in comparison to its value near Earth. This is from data not only from New Horizons but also Voyager 2. Assuming New Horizons is still operational when it hits the termination shock, we would expect to see a sharp drop in the speed of the solar wind. In fact, Voyager 2 found a 46 percent drop in speed at the termination shock at its distance of 84 AU.

And note this from the paper:

The drop in speed in the Voyager 2 TS [termination shock] measurements was dramatic. At the Voyager 2 TS crossing, the speed went from ∼320 down to ∼140 km s⁻¹ a few days after the crossing, corresponding to a ∼ 56% speed reduction across the TS (J. D. Richardson & E. C. Stone 2009). A sudden speed drop of 56% would be large and steep enough to readily confirm that NH crossed the TS. Unlike Voyager 2, the SWAP instrument on NH also measures interstellar hydrogen pickup ions, such that the modification of the TS by the interstellar pickup ions will be measured at the upcoming NH TS crossing.

As a sidenote, it’s worth remembering that there is no clear boundary here. Indeed, the shape of the entire heliosphere flexes and churns in response to ambient conditions and thus is partially dependent on the clouds of interstellar material the Sun is moving through at the time. At present, we are in the whimsically named ‘Local Fluff,’ part of the Local Interstellar Cloud, and near or perhaps already edging into a region called the G-Cloud, a prominent citizen of which is the system called Alpha Centauri. In any case, we’ve learned from the IBEX (Interstellar Boundary Explorer) satellite that the interactions on the heliosphere paint a picture of a dynamic, changing shape as opposed to the smooth ‘bubble’ that is often depicted in artist renderings of the heliosphere.

IBEX and its successor satellite IMAP (Interstellar Mapping and Acceleration Probe) carry an interesting message of their own: We can continue to learn without having an actual set of instruments on the scene. In sharp contrast to New Horizons, these two spacecraft work by remote sensing, detecting energetic neutral atoms (ENAs) produced in the interaction of the solar wind with neutral atoms at the heliopause. So we have one satellite in a highly elliptical Earth orbit (IBEX) and another at the L1 Lagrange point, both of them helping us to understand conditions at the termination shock and beyond.

As Elliott pointed out in that first quote above, conditions in the heliosphere’s boundary with the LISM matter if for nothing else because of the dangers posed by Galactic Cosmic Rays (GCRs), leading to issues of spacecraft design both for manned as well as unmanned missions. It’s good to know that New Horizons is on the case and will remain so, but for how long? What I’m hearing is that the spacecraft’s Radioisotope Thermoelectric Generator (RTG) should be able to keep observations and return of data robust through the end of this decade, but as with the Voyagers, we’re moving toward the end of active life.

What will replace our one source in the outer heliosphere? The need for resources in and beyond the Kuiper Belt should have us moving toward mission designs and propulsion options that go beyond chemical methods. Sail missions like the Solar Gravitational Lens mission now being developed at the Jet Propulsion Laboratory continue to intrigue me, particularly as we begin to explore assembly options enroute to deliver the largest possible payload. We will need precursor ‘sundiver’ missions as we test out these technologies.

The paper is Elliott, “The Gradual Slowing of the Solar Wind in the Outer Heliosphere,” The Astrophysical Journal, Vol. 1001, Number 1 (3 April 2026). Full text.