Voyager 1 entered interstellar space in 2012, crossing the heliopause on August 25 of that year. The heliopause is the boundary where the outward-flowing solar wind gives way to the interstellar medium outside the Sun’s heliosphere.
Scientists identified the crossing through changes in the spacecraft’s measurements. Voyager 1 detected a sharp decline in particles originating inside the Solar System and a rise in high-energy particles arriving from beyond the heliosphere. It also recorded changes in the surrounding plasma environment. NASA announced the result in 2013 after additional analysis confirmed the interpretation.
This milestone did not mean Voyager 1 had left the Sun’s gravitational influence or reached another star. Interstellar space begins beyond the heliopause, but the probe will take tens of thousands of years to pass near another star. Voyager 2 crossed the heliopause later, in 2018, making Voyager 1 the first human-made object to enter interstellar space.