The Voyager 1 probe, launched by Nasa in 1977, is on the cusp of entering the inter-stellar void. In doing so, the spacecraft, which is currently 100 times further from the sun than the earth is, will be the first man-made object to leave the solar system.
The machine, which has already travelled 11 billion miles since its launch, will soon enter a region that astronomers call inter-stellar space, where the high-speed solar winds diminish and the magnetic winds of deep space intensify.
It is this change in the magnetic field direction and the type of wind - interstellar wind is slower, colder and denser than solar wind - that will confirm that Voyager 1 has finally crossed the boundary.
The probe, which travels at around 11 miles per second, is expected to leave the solar system in the next few months, although it might take years to pass completely into inter-stellar region
On its journey, Voyager 1 has explored Jupiter and Saturn, taking dramatic pictures of both. It also took the famous “Pale Blue Dot” picture, an image of the earth taken in 1990 from a distance 3.7 billion miles.
The picture, requested by Carl Sagan, required the Nasa team to turn its camera round and focus on earth, which appears as tiny spec set against the vastness of space.
Pale Blue Dot
Voyager’s sister probe, Voyager 2, was also launched in 1977. After following Voyager 1 to Saturn and Jupiter, it was re-tasked with exploring Neptune and Uranus. Despite their age, both craft have continued to send Nasa information via radio waves, picked up by earth's Deep Space Network (DSN), a global ring of antennas that supports interplanetary spacecraft missions.
Now the two spacecraft are to leave the solar system and move into the void that exists between solar systems in the Milky Way galaxy.
According to Ed Stone, the Voyager project scientist at the California Institute of Technology: "Voyager tells us now that we're in a stagnation region in the outermost layer of the bubble around our solar system. Voyager is showing that what is outside is pushing back. We shouldn't have long to wait to find out what the space between stars is really like."
Both spacecraft have a copper phonograph record on board, with more than 100 photographs of the earth, a sample of languages and various sounds. Although scientists agree that the probability of an alien civilisation finding either probe, Sagan noted that "the launching of this 'bottle' into the cosmic 'ocean' says something very hopeful about life on this planet."
Both should to continue transmitting until around 2020, after which they are expected to run out of power.
Pictures taken by the Voyager spacecrafts: