SAN ANTONIO — June 29, 2026 —A new Southwest Research Institute (SwRI) study based on data from NASA's New Horizons spacecraft has uncovered fascinating insights into why the solar wind gradually slows as it moves toward the edge of the solar system and the boundary with interstellar space.
New Horizons is currently roughly 66 AU from the Sun. One AU, the distance from the Earth to the Sun, is roughly 93 million miles. The researchers, led by SwRI's Dr. Heather Elliott, studied how the solar wind's speed, measured by the Solar Wind Around Pluto (SWAP) instrument on New Horizons, changed between 21 and 58 AU compared to measurements taken nearer the Sun.
"As the solar wind travels away from the Sun at supersonic speeds, roughly 1 million miles per hour, eventually it encounters incoming interstellar neutral gas particles entering the heliosphere," Elliott said. "These neutral interstellar atoms become ionized via charge exchange with solar wind ions, adding mass to the solar wind by picking up interstellar material that slows the solar wind down."
Previously, New Horizons and Voyager 2 measurements between 30 and 43 AU indicated the solar wind was 5 to 10% slower than at 1 AU near Earth. Now, New Horizons researchers found at 58 AU that the solar wind is 13 to 15% slower than the wind at 1 AU. This gradual slowdown aligns with previous models of how interstellar material enters the heliosphere and affects the solar wind. It also demonstrates how the Sun's influence decreases over long distances.
This gradual decrease in speed pales in comparison with the sharp drop in speed expected when New Horizons eventually reaches the heliosphere's Termination Shock (TS), which is where solar particles rapidly drop in speed to less than the local plasma (solar wind and interstellar pickup ions combined) speed of sound. The termination shock is a stark indication that the incoming interstellar material heavily affects the properties of the solar wind as it nears the outer boundary of the heliosphere. Voyager 2 measured a sharp 46% drop in speed at the termination shock at a distance of 84 AU.
"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," Elliott said. "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."
This is important for astronauts working outside of Earth's protective atmosphere on the Moon, and eventually on humanity's journey to Mars, because GCRs pose one of the most severe risks to long-term space travel. They can increase cancer risk in travelers and negatively impact technology.
"This new data could be highly beneficial in predicting the outer boundaries of the heliosphere and solar system and ultimately the amount of GCR radiation exposure of astronauts, satellites and spacecraft to harmful cosmic radiation, especially as we look toward more ambitious deeper-space exploration," Elliott said.
These findings also provide insights into astrospheres, the protective heliosphere-like bubbles generated by other stars in the galaxy. Astrospheres share many similarities with the Sun's heliosphere and could help us understand how other stars interact with the interstellar material surrounding their systems.
"Studying the heliosphere is like solving a cosmic puzzle," said Elliott. "Not only do we learn more about how the Sun's influence ends, but we also gain a deeper understanding of the boundary between our solar system and interstellar space — a critical step toward planning future interstellar travel."
"NASA's New Horizons continues to be the only spacecraft in the Sun's outer heliosphere and yielding important new insights to build on what the venerable Voyager probes discovered," said SwRI Associate Vice President Dr. Alan Stern, the Principal Investigator of the New Horizons mission. "Our studies of the heliosphere, like this one, are virtually continuous and provide crucial new datasets to better understand the Sun's outer heliosphere and its termination region far beyond Pluto's orbit."
The Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, designed, built and operates the New Horizons spacecraft and leads the mission for NASA's Science Mission Directorate. The Marshall Space Flight Center (MSFC) Planetary Management Office in Huntsville, Alabama provides NASA oversight for New Horizons. Southwest Research Institute, based in San Antonio, directs the mission via Principal Investigator Dr. Alan Stern, who leads the science team, payload operations and science planning. New Horizons is part of the New Frontiers Program managed by NASA's MSFC.
SwRI built, calibrated and operates the New Horizons solar wind instrument Solar Wind Around Pluto (SWAP). This research is part of the NASA Solar wind with Hydrogen Ion charge Exchange and Large-Scale Dynamics Heliophysics Drive Center (SHIELD) led by Boston University, which models the outer boundaries of the heliosphere to understand how Galactic Cosmic Rays enter our heliosphere.
The study "The Gradual Slowing of the Solar Wind in the Outer Heliosphere," was published this month in The Astrophysical Journal. DOI: https://doi.org/10.3847/1538-4357/ae39c6 .
