NASA's New Horizons spacecraft has provided key data explaining a long-standing mystery: why the solar wind slows as it approaches the boundary with interstellar space. A new study led by the Southwest Research Institute (SwRI) attributes the deceleration to drag from interstellar atoms. The findings were published in The Astrophysical Journal.
The solar wind, a stream of charged particles from the Sun, travels at supersonic speeds before gradually losing momentum. Understanding this process is critical for mapping the heliosphere's shape and its protective role against cosmic radiation. Previous models could not fully account for the observed slowdown.
Data from New Horizons, which is now in the outer heliosphere beyond Pluto, allowed researchers to measure the wind's velocity at unprecedented distances. The study, titled "The Gradual Slowing of the Solar Wind in the Outer Heliosphere," identifies neutral atoms from interstellar space as the primary cause of the drag. These atoms collide with solar wind particles, siphoning away their energy.
The results refine models of how our solar system interacts with the galactic environment. This has implications for future interstellar missions and our understanding of how other star systems shed their own stellar winds. It also helps explain the location of the heliopause, the outermost boundary of the Sun's influence.
The study relied on a single spacecraft's measurements; future missions could provide more comprehensive data across different regions of the heliosphere.