The two Voyager space probes were launched in the 1970s, and in their tour of our Solar System they greatly increased our knowledge of the outer planets (as well as taking some great photos).
They’re still going over 30 years later, and are now at the very edge of the Solar System.
The heliosphere is a giant bubble formed by the solar wind, charged particles emitted by the sun, as it blows against the interstellar medium, made of other particles that flow between the stars.
Voyager 1 is currently believed to be in the heliosheath, the turbulent area outside the bubble of the heliosphere. NASA rocket scientists expect that in about 4 years the spacecraft will experience a sudden drop in temperature as it finally leaves the sun’s influence and drifts into interstellar space.
For more about the Voyagers and their outward journey, see the NASA Jet Propulsion Laboratory website.