The New Horizons spacecraft has helped refine the boundary where the Sun’s influence ends and interstellar space begins. An analysis of data gathered by the SWAP (Solar Wind Around Pluto) instrument at distances ranging from 21 to 58 astronomical units from the Sun—where one astronomical unit equals the average distance between Earth and the Sun—has revealed that the solar wind gradually decelerates as it travels toward the outer reaches of the Solar System. These findings align with existing models of how solar material interacts with the interstellar medium.
The solar wind is a continuous stream of charged particles that leaves the Sun at speeds of 500 to 800 km/s. As it moves farther away, however, it begins to collide with neutral atoms from interstellar gas that penetrate the heliosphere—the region of space shaped by the solar wind. After undergoing a charge exchange, these atoms become part of the flow, increasing its mass and gradually slowing it down.
Measurements indicate that at distances between 30 and 43 astronomical units, the solar wind’s speed is already 5 to 10 percent lower than it is near Earth. By the time it reaches 58 astronomical units, the deceleration has grown to 13 to 15 percent. These results confirm that interstellar material exerts an increasingly strong influence on the solar wind as it approaches the heliosphere’s outer edge.
The next major milestone will be the region where the solar wind first experiences a sharp drop in speed due to pressure from the interstellar medium. According to data from the Voyager 2 spacecraft, which crossed this boundary at roughly 84 astronomical units, the flow speed there plummeted by about 46 percent in an instant.
Scientists estimate that New Horizons could reach this region by 2029, allowing for the first direct comparison of measurements taken by two spacecraft nearly two decades apart.
As of July 6, 2026, the New Horizons spacecraft is located 65 astronomical units from the Sun.
Studying these processes is important not only for understanding the boundaries of the Solar System. Similar “cocoons” of stellar wind exist around other stars, so data from New Horizons helps us better understand the structure of such systems throughout the galaxy.
Furthermore, the shape and size of the heliosphere determine how much galactic cosmic radiation—high-energy particles from interstellar space—can penetrate into the Solar System.
In the future, data from New Horizons will be combined with observations from the Voyager, IBEX, and IMAP missions to build the most accurate picture of the heliosphere’s outer boundary. This will help not only clarify where the Sun’s influence ends but also more precisely assess the radiation environment that future crewed missions to the Moon, Mars, and beyond will encounter.
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