The Voyager spacecraft have been on a trajectory towards the edge of the solar system for 35 years, since they completed their first mission to tour the outer planets. Voyager 1 is currently the most distant man-made object from Earth and is close to becoming the first to enter interstellar space, the region outside of the sun’s sphere of influence.
Voyager 1, a spacecraft weighing 722 kg and powered by plutonium that was launched in September 1977, has entered a previously unknown region of space known as the magnetic highway (due to magnetic field lines allowing low-energy particles to move out of the heliosphere freely and cosmic rays to move in from interstellar space) and is close to becoming the first manmade object to enter interstellar space. It is on the edge of the heliosphere (the region of space under the influence of the Earth’s Sun, a ‘bubble’ of charged particles blown into the space around the Solar System by the solar wind) in the heliosheath, which is its outermost layer.
They are fuelled by the radioactive decay of plutonium-238; heat from this decay is converted into electricity, which powers them. Their lifetime is considered to be compatible with the length of the mission as they appear to be close to crossing into interstellar space, and scientists are certain that the spacecraft will last long enough to do so. Voyager 1 and 2 both receive and send data to the Deep Space Network (DSN) about their surroundings, helping scientists map the unexplored regions of space.
The Voyager 1:
It is impossible to predict exactly when the Voyager might cross into interstellar space, but it could take up to around 2 years. Voyager 2 is also traveling towards interstellar space, but will likely reach it several years after Voyager 1.
Read more at: http://www.astronomynow.com/news/n1212/05voyager/