On October 1, NASA plans to switch off the two instruments on board that can measure it, to save $2 million a year. The spacecraft is healthy. The instruments work.
SWAP and PEPSSI are scheduled to switch off on October 1, 2026.
Solar Wind Around Pluto
Measures the slow, steady stream of charged particles the Sun blows outward, and the pickup ions riding in it: atoms from interstellar space that drift into the solar system, get stripped of an electron, and are swept up by the wind.
Built at Southwest Research Institute. 3.3 kg, 2.3 watts.
Pluto Energetic Particle Spectrometer Science Investigation
Measures the faster, higher-energy ions and electrons, from 25 to 1,000 keV. Together with SWAP it sees the whole range of particles, from the gentle wind to the energetic tail.
Built at Johns Hopkins APL. 1.5 kg, about 2.5 watts.
Combined, the two draw under 5 watts, about what a small phone charger uses.
Every dot is one day of real SWAP measurements from NASA’s public archive, 239,252 readings in all, placed at the distance where New Horizons took them. The right side of the chart is where the spacecraft is going. After October 1, it would go there without these instruments listening.
The public record stops before today. NASA’s archived solar wind product ends on January 27, 2020, at 46.4 AU; the pickup-ion product ends on March 31, 2017, at 38.1 AU. SWAP has kept measuring since then. The team has published results out to about 60 AU, and newer calibrated counts reached the archive in 2026, but not yet as speed, density and temperature. The hatched zone marks that gap. This page does not try to derive those values itself.
“Live” means the distance, not the particles. The spacecraft’s position is recomputed every second from JPL’s trajectory, and NASA’s CDAWeb service is queried when you open the page. Particle data is not streamed to the public in real time: New Horizons sends data home in scheduled sessions over NASA’s Deep Space Network, and its signal takes about nine hours to arrive.
Two NASA sources disagree slightly on where the spacecraft is. JPL Horizons and the CDAWeb trajectory file differ by about 0.02 AU, roughly 3 million km. This page uses JPL Horizons, JPL’s current solution, and shows the CDAWeb figure beside it.
Distance and time are tangled. The spacecraft moved outward while the Sun went through its 11-year cycle, so every trend here mixes “farther out” with “later in the cycle.” That is why this page draws no expected-density line.
Daily medians hide short events. Each dot is the median of that day’s readings, typically one every 15 minutes. The bold line is a 27-day running median, one solar rotation. Shocks and hours-long events are smoothed away.
The pickup-ion values were filtered by the science team. Days when the wind speed changed by more than 1% were removed, about 13% of samples.
The shock zone is a forecast. The termination shock moves with solar activity. Voyager 1 crossed it at 94 AU and Voyager 2 at 84 AU. The newest model puts New Horizons’ crossing between 2029 and 2040, as close as 73 AU. Gasser et al., 2026
PEPSSI is not plotted. Its public product reports averaged energetic-particle fluxes, which do not share an axis with these values. It is available in the archive.
Data: McComas, Elliott & Mukherjee, New Horizons Primary and Extended Missions: Solar Wind Parameters, NASA Planetary Data System. Download the daily values used here (CSV). Method in how it works.
Only one working spacecraft is headed for the edge of the heliosphere with modern particle instruments. The two that went before are running out of power and have been shutting instruments off one by one.
New Horizons has the power and fuel to keep working until about 2050, when it will be around 137 AU from the Sun.
NASA’s planetary science division pays about $10 million a year to run New Horizons. Heliophysics agreed to add $2 million a year for the solar wind and particle work, then did not pay it in fiscal 2025 or 2026.
Figures from NASA’s FY2026 spending plan: total $24.40 billion, science $7.29 billion, heliophysics $874.8 million, IMAP $39.5 million. Arithmetic is mine.
“The big dollars were already invested in New Horizons, so generally supportive of generating high-quality scientific returns for low-dollar spend.”
“We just can’t afford to keep the lights on on the heliospheric instruments.”
Congress has already funded NASA through December 11. Keeping these instruments on is a decision NASA can make on its own, and it needs to happen before October 1.
Started by James L. Green, NASA’s former Chief Scientist, and addressed to the leaders of NASA’s science, heliophysics and planetary divisions.
Sign on Change.orgA phone call to a local office is the fastest way to get a member of Congress to contact NASA. Offices take calls from anyone who lives in the district. Find yours: House, Senate.
Hi, my name is [your name] and I live in [town and zip code]. I’m calling to ask the [Representative / Senator] to contact NASA this week about the New Horizons spacecraft. On October 1, NASA plans to switch off two working science instruments, SWAP and PEPSSI, over a $2 million a year gap in the heliophysics budget. The spacecraft is healthy and is the only one left that can measure the solar wind at the edge of our solar system, just before it gets there. Please ask NASA to keep them on. Thank you.
Administrator Isaacman has said NASA is looking into this. A short, specific public note helps.
@NASAAdmin $2M a year keeps New Horizons’ SWAP and PEPSSI instruments on. The spacecraft is healthy and it’s the only one left that can measure the solar wind at the edge of the heliosphere. Please decide before Oct 1. #KeepSWAPandPEPSSIOn
Want to do more? The Planetary Society’s Save NASA Science hub covers the wider budget fight. Background: Kirby Runyon for the Planetary Society.
Status last checked September 25, 2026: NASA has not announced a change to the October 1 plan.