When did Voyager 1 reach interstellar space?

NASA launched Voyager 1 on 5 September 1977 to fly past Jupiter and Saturn. After completing that mission it kept heading outward, and on 25 August 2012 it crossed the heliopause, the boundary where the Sun’s stream of charged particles gives way to interstellar space. At that point it was about 121 times farther from the Sun than Earth is. Scientists confirmed the crossing in 2013 after studying changes in the plasma around the probe. Voyager 1 is the most distant human-made object, now more than 24 billion km away, so its radio signals take nearly a day to reach Earth.