PARIS, Nov 6: Voyager 1, the legendary US space probe launched more than 26 years ago, may have crossed a key, stormy boundary as it passes through the last fringes of our Solar System and ventures towards the interstellar void beyond, scientists say.

Bearing greetings in 55 languages and sounds and pictures of life on Earth, the most distant man-made object may have crossed a zone of turbulence known as the termination shock, they say.

For the last couple of years, Voyager 1 has been crossing the heliosphere, a region where the energised particles spewed by the distant 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 the heliosphere eventually ends and yields to the relative 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.

The theory is that the line is somewhere between 12.75 and 18 billion kilometres 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.

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.

According to NASA’s Voyager website, Voyager 1 is now 90 AU, or 13.5 billion kms from the Sun.

Voyager 1 was launched on Sept 5, 1977, 16 days after its companion, Voyager 2.

Between them, these remarkable craft explored all the giant planets of the outer Solar System: Jupiter, Saturn, Uranus and Neptune and 48 of their moons, and sent back astonishing data of the rings and magnetic fields surrounding these planets.

Afterwards, their mission was reconfigured to send them on an exploration of the heliosphere and interstellar space.—AFP