At 12 billion miles away, it is earth's most-travelled emissary. But has Voyager 1 stepped out of our solar system? Doubts remain.
In 2012, the Voyager mission team announced that the Voyager 1 spacecraft had passed into interstellar space, travelling further from Earth than any other manmade object.
But, since that historic announcement, there has been some uncertainty.
Two researchers working with Voyager 1 have drawn up a test to show whether the spacecraft is inside or outside of the heliosphere — the space dominated by solar particles and magnetic fields put out by the sun.
They predict that Voyager 1 will cross the current sheet — a huge surface within the heliosphere — at some point within the next one to two years.
When that happens, Voyager team members should see a reversal in the magnetic field surrounding the probe, proving that it is still within the heliosphere.
If this change doesn't occur in the next two years or so, then Voyager is almost certainly already in interstellar space, the researchers said.
The new test is outlined in a study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union.
Voyager 1 is one of two probes launched in 1977 to explore the outer planets in our solar system. Along with its twin, Voyager 2, it made some surprising discoveries. Both spacecraft have been travelling along different flight paths and at different speeds.
Taking advantage of a special alignment of the outer planets that happens every 176 years, the spacecraft took slingshots from one planet to the next, assisted by the first planet's gravity.
Voyager 2 flew past Jupiter, Saturn, Uranus and Neptune, while Voyager 1 focused on Jupiter and Saturn.
After its close approach to Jupiter when it sent back tantalising pictures back home, Voyager 1 found two new moons of Saturn as well – Thebe and Metis. Additionally, it sent back detailed pictures of the Galilean moons (Io, Europa, Ganymede and Callisto) as well as Amalthea.
In 1998, Voyager 1 sped past Pioneer 10 to become the farthest machine humans have ever sent from Earth. It is speeding away at about 11 miles per second (17km per second).
Nasa placed two records on board the spacecraft. Sounds ranging from whale calls to the music of Chuck Berry were placed on board, as well as spoken greetings in 55 languages.
The 12-inch, gold-plated copper disks included pictorials showing how to operate it, besides giving the position of the sun among nearby pulsars, in case it fell into curious alien hands.
The craft is powered by a plutonium-based thermoelectric generator which will support most of its operations till 2025.