NASA’s New Horizons Reveals a Hidden Interstellar Slowdown at the Solar System’s Edge

For obvious reasons, the Sun’s influence weakens as you get closer to the edge of the solar system. But things might be a lot more complicated than we previously thought, according to a new study.

Specifically, interstellar atoms bleeding into our solar system’s peripheral regions place an additional “break” on supersonic solar particles. In a recent study published in The Astrophysical Journal, scientists examined data from NASA’s New Horizons spacecraft to get a better understanding of how such events shape the environment at the edge of the heliosphere. According to the paper, the gradual decrease in solar wind speeds prior to termination shock is caused by the steady accumulation of ionized interstellar material as it travels through the outer heliosphere.

“Studying the heliosphere is like solving a cosmic puzzle,” Heather Elliott, the study’s first author and an astrophysicist at Southwest Research Institute, said in a statement. “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.”

New Horizons, on the other hand, launched in 2006 to study Pluto up close. Currently, the spacecraft is halfway to the edge of the solar system, journeying outwards at about 66 AU from the Sun. New Horizons is still among the farthest-traveling spacecraft out there, prompting scientists to use its data to make some meaningful comparisons to previous observations from Voyager 2.

To study how distance and interstellar material each influence solar wind speed, the team devised a simulation based on measurements to work out the technical details. The simulations indicated that measurements by New Horizons were generally consistent with previous observations by Voyager 2 at similar distances. For instance, both spacecraft showed that solar wind was 5 to 10% slower between 30 and 43 AU as opposed to solar wind at 1 AU, or the distance between the Sun and Earth. The most recent New Horizons data indicated that solar wind was 13 to 15% slower at 58 AU.

Importantly, this wasn’t just because the spacecraft was simply farther away from the Sun. Without accounting for interstellar material, the simulated speed was “well above” the actual speeds measured by New Horizons, according to the paper. In other words, there is a gradual speed drop in solar winds at farther distances from the Sun. However, the “shape and properties of heliospheric boundaries” that determine what galactic cosmic rays enter our solar s