New Horizons data confirms solar wind slows in the outer heliosphere

Data transmitted by the New Horizons spacecraft has confirmed a measurable, gradual decline in the speed of the solar wind as it pushes into the furthest reaches of the solar system. This discovery, detailed in a study published this month in The Astrophysical Journal, provides new clarity on how the Sun’s influence interacts with the interstellar medium at the edges of the heliosphere.

New Horizons is currently roughly 66 AU from the Sun. Researchers led by the Southwest Research Institute utilized the Solar Wind Around Pluto (SWAP) instrument to analyze shifts in particle velocity between 21 and 58 AU. The findings indicate that the solar wind is currently 13 to 15% slower at 58 AU than it is at 1 AU near Earth. This represents a distinct increase in deceleration compared to earlier mission measurements taken between 30 and 43 AU, where the observed slowdown ranged only between 5 and 10%.

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The Mechanism of Deceleration

The solar wind originates from the Sun and travels at supersonic speeds — approximately 1 million miles per hour. As it expands, it encounters neutral interstellar gas atoms entering the heliosphere. Through a process known as charge exchange, these neutral atoms are ionized by solar wind ions. This interaction effectively increases the mass of the solar wind, causing it to lose velocity over vast distances. Dr. Heather Elliott, who led the research, explained the phenomenon:

"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. 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."

Dr. Heather Elliott, Lead Researcher, via SwRI

This gradual decrease in speed is distinct from the abrupt changes found at the Termination Shock. According to data from the Voyager 2 mission, that boundary — where solar wind velocity drops sharply as it nears the speed of sound, is marked by a 46% reduction in speed. Voyager 2 recorded this encounter at 84 AU.

Implications for Deep-Space Exploration

The research is not merely an exercise in planetary science; it has consequences for future human missions. The heliosphere serves as a protective bubble, regulating the entry of Galactic Cosmic Rays into the solar system. These rays pose risks to both electronic systems and the health of astronauts on long-term missions, including travel to the Moon or Mars. By refining models of the heliosphere’s outer boundaries, researchers hope to better predict radiation exposure for future explorers.

Beyond our own solar system, these findings offer a window into the nature of astrospheres, similar bubbles generated by other stars. Understanding the Sun's interaction with the interstellar medium allows scientists to infer how other stellar systems might behave in their respective galactic neighborhoods.

"Studying the heliosphere is like solving a cosmic puzzle. 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."

Dr. Heather Elliott, Lead Researcher, via SwRI

As Swri Associate Vice President Dr. Alan Stern explained, the New Horizons mission continues to build upon the legacy of the Voyager probes by providing high-quality, long-term datasets from a region where no other spacecraft currently operates.

"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. 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."

Dr. Alan Stern, Principal Investigator, via SwRI