The twin Voyager 1 and 2 spacecraft were launched in 1977 to explore the outer solar system. More than 25 years later they are approaching the solar system boundary region -- the heliopause -- where the Sun's dominance of the environment ends and interstellar space begins.
The primary mission was the exploration of Jupiter and Saturn. After making a string of discoveries there -- such as active volcanoes on Jupiter's moon Io and intricacies of Saturn's rings -- the mission was extended. Voyager 2 went on to explore Uranus and Neptune, and is still the only spacecraft to have visited those outer planets. The adventurers' current mission, the Voyager Interstellar Mission (VIM), will explore the outermost edge of the Sun's domain.