Voyager 1 left Earth in 1977 on a journey that took it to Jupiter and Saturn before taking a turn toward eternity: interstellar space. After 35 years, new data coming from the spacecraft indicates that it may finally be leaving the Solar System.
The Voyager 1 probe is the farthest man-made object from Earth--something that isn’t expected to change for the foreseeable future. It’s about 120 times further from us than the Sun--11.1 billion miles away. This is such a massive distance that it takes a radio signal 16 and a half hours to reach the spacecraft from earth.
Part of Voyager’s interstellar mission is to explore the edge of the solar system--the heliosheath. Voyager 1 crossed the termination shock, the space where solar wind from the Sun slows dramatically because of interactions with the interstellar medium. The general consensus in the scientific community is that Voyager 1 crossed the termination shock in 2004 or 2005.
That puts the probe in the heliosheath. The heliosheath is theorized to be a thin area where the Sun and the interstellar medium interact. In the heliopause, the solar wind is deflected or stopped by the opposing force of the stellar winds outside the Solar System. The far edge of the heliosheath is the heliopause, which Voyager 1 seems to be approaching now.
The heliopause isn’t a physical barrier, it’s the spot where the stellar wind, which comes from the Sun, is no longer strong enough to push back against the stellar winds of the surrounding stars. Recent readings from Voyager 1 show a dramatic increase in the amount of galactic cosmic rays that the probe is encountering. It will likely take several weeks to analyze the data coming from Voyager to confirm that the probe has entered interstellar space.
If you want to know more give a listen to this excellent episode of Radiolab, which discussed the Voyager probe and the complexities involved with figuring out whether it's actually left the solar system.