SWAP, PEPSSI, and the secrets of the Solar System

NASA's New Horizons is poised to make discoveries about the farthest reaches of our Solar System, but two of its key instruments are in danger of being shut down prematurely. The science-journalism coverage adds useful context, while the strongest evidential footing still comes from the underlying data, papers or institutional documentation.

It matters because astronomy does not advance on single detections. The field builds confidence by accumulating independent observations across different wavelengths, instruments and epochs until isolated signals become defensible conclusions. What looks convincing in one dataset can dissolve when a second instrument looks at the same target, and what looks marginal can solidify when follow-up campaigns confirm the original reading. The current standard requires that a result survive this triangulation before the community treats it as settled. Written by Kirby Runyon Planetary geologist and research scientist at the Planetary Science Institute September 15, 2026 “Why have we not gone back. Of course, we’ve heard it since 1972 regarding humans on the Moon.

But NASA has also not sent a mission back to Venus since the Magellan mission ended in 1994, and at that same time, it had been 20 years since NASA had been to Mars with the twin. NASA’s New Horizons spacecraft made its historic flyby of Pluto in 2015, 11 years ago this past summer.

If I were a betting person, I would bet that another ten years from now, by 2036, people will start asking, “Why have we not gone back?” about Pluto and the broader Kuiper belt. 1 of this year, two instruments on New Horizons, the SWAP (Solar Wind Around Pluto) and PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation) instruments, will be.

1 AU is the average Earth-Sun distance of about 150 million kilometers or 93 million miles) from the Sun, putting the spacecraft squarely outside the Sun’s heliosphere and. Numerical computer models of the magnetohydrodynamics (I love big science words like that!) of the heliosphere show that when it encounters a cold, dense interstellar cloud of.

What gives the story weight is not just the object itself, but the way the measurement trims the range of plausible physical explanations. Astronomy has accumulated enough cases to know that the most interesting results are rarely the ones that confirm expectations cleanly; they are the ones that confirm some expectations while complicating others, or that open a parameter space that previous instruments could not reach. The scientific community evaluates these contributions by asking whether the new data constrain a model in a way that older data could not, and whether those constraints survive systematic review.

For the lifetime of most humans alive today, New Horizons is our only chance to measure the link between interstellar space, the heliosphere, habitable ocean worlds, and our home. This conceptual mission would send a spacecraft speeding more than double the speed of Voyager or New Horizons on a 50-year mission into the VLISM, roughly 500 AU away.

Because this item comes through The Planetary Society as science journalism, it should be treated as contextual reporting rather than primary evidence. Good science reporting can identify why a result matters, connect it to the wider literature and make technical work readable, but the decisive evidence remains in the original paper, dataset, mission release or technical record. That distinction is especially important when a story is later repeated by aggregators, because repetition increases visibility, not evidential strength.

The next step is to see whether other instruments and other wavelengths tell the same story. Campaigns with JWST, the VLT, the forthcoming Extremely Large Telescopes and radio arrays will provide the spectral coverage and spatial resolution needed to move from detection to physical characterization. The timeline for that kind of confirmation is typically measured in years, not months, which is worth keeping in mind when reading the current result.

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