Both Voyagers carry a phonograph record as a greeting to any form of life, should it be encountered. Photo: AP
WHEN the Voyager 1 spacecraft launched from the fields of Cape Canaveral in late 1977 en route to Jupiter, Saturn and their moons, the probe's planetary tour was expected to last just five years.
Three decades and three months later the pioneering craft has reached the edge of the solar system, having completed its original mission as well as fly-bys of Uranus and Neptune.
Sometime in the coming months - no one is quite sure of the exact timing - Voyager 1 will cross the boundary where the sun's magnetic influence ends and the cooler regions of interstellar space begin, a place no man-made object has reached.
Scientists at the American Geophysical Union in San Francisco announced on Tuesday the probe had encountered a ''magnetic highway'', where particles inside the solar system mix with those outside.
''Although Voyager 1 still is inside the sun's environment, we now can taste what it's like on the outside because the particles are zipping in and out on this magnetic highway,'' said Edward Stone, a Voyager project scientist from the California Institute of Technology, Pasadena.
''We believe this is the last leg of our journey to interstellar space. Our best guess is it's likely just a few months to a couple years away. The new region isn't what we expected, but we've come to expect the unexpected from Voyager,'' he said.
While the craft hurtles to the rim of the solar system at 520 million kilometres a year - it is now about 18 billion kilometres from the sun - it will be another 700 centuries before it has travelled the distance to our nearest star.
The craft's sister probe, Voyager 2, launched 16 days before Voyager 1, is also flying towards the edge of our solar system.
The instruments of both craft will continue to measure ultraviolet sources among the stars, and were expected to return data to Earth for two or more decades until their plutonium power sources run out.