Bearing greetings in 55 languages and sounds and pictures of life on Earth, Voyager 1 is now the most distant man-made object and the only one to have crossed a zone of turbulence known as the termination shock, scientists say.
For the last couple of years, the probe has been crossing a region where energised particles spewed by the Sun - a flow of plasma known as the solar wind - start to crash into the atomic and molecular debris that come from interstellar space.
But exactly where this region, the heliosphere, eventually ends and yields to the serenity of interstellar space void has never been determined.
It is impossible to plot the boundary, known as the heliopause, accurately from Earth and no man-made object, until now, has ever ventured so far.
Not exactly sure
The theory is that the line is somewhere between 12.75 and 18 billion kilometers from the Sun.
To astronomers, this is measured as between 85 and 120 astronomical units (AU) - one AU being the distance between the Earth and the Sun.
One key hurdle in the crossing of the heliosphere is a phenomenon called "termination shock", in which interstellar atoms crash at brutal speeds into the energy stream released from the distant Sun.
Analysing data sent back by Voyager when it was 85 AU away, physicists from Johns Hopkins University, Maryland, believe that it suddenly encountered a drop in the solar wind speed and a burst of anomalous cosmic rays.
This suggests that it has now crossed the Rubicon and is entering the heliosheath, the last vestiges of sun's magnetic field and solar wind before the heliopause boundary.
The university argues that the spacecraft left the supersonic solar wind and passed into the subsonic region (possibly beyond the termination shock) on about 1 August 2002 at a distance of roughly 85 AU.
"This little engine ... was not designed for this kind of lifetime. It's absolutely remarkable"
an expert involved with the Voyager programme since 1972
It then re-entered the supersonic solar wind about 200 days later at about 87 AU from the sun."
Another team, from the University of Maryland, looked at different Voyager measurements and argue differently. They believe that the heliosheath is very close but still to be broached.
Both studies are published on Thursday in Nature, the British weekly science journal.
According to NASA's Voyager website, Voyager 1 is now 90 AU, or 13.5 billion km from the sun.
Voyager 1 was launched on September 5, 1977, 16 days after its companion, Voyager 2.
Between them, the craft have explored all the giant planets of the outer Solar System: Jupiter, Saturn, Uranus and Neptune and 48 of their moons, and sent back data of the rings and magnetic fields surrounding these planets.
"This little engine ... was not designed for this kind of lifetime," said Louis Lanzerotti, a Bell Labs expert on solar wind who has been involved with the Voyager program since 1972. "It's absolutely remarkable."
Afterwards, their mission was reconfigured to send them on an exploration of the heliosphere and interstellar space.
The two craft have enough electrical power and thruster fuel to operate until 2020, sending back precious data to the listening ears of NASA's Deep Space Network.
After that, they will wander the Milky Way, perhaps for eternity, each carrying an 18-centimeter (12-inch) gold-plated phonograph record containing sounds and images selected to portray the diversity of life on Earth.