Solar Wind Faces a Hidden Interstellar Drag at the Solar System’s Edge

New Horizons observations indicate interstellar material slows the solar wind as it moves outward, adding a gradual drag beyond the termination shock. Simulations align with Voyager 2 data, showing the wind is about 5–10% slower between 30–43 AU and 13–15% slower at ~58 AU, due to ionized interstellar matter shaping the heliosphere’s boundaries and affecting cosmic-ray exposure—insights that could inform future interstellar exploration.