A US spacecraft launched in 1977 has reached the final frontier at the edge of the Solar System.
Scientists say Voyager 1 has entered a "new region" of the Solar System which could be its gateway to interstellar space.
The border region is described as a "magnetic highway" for outgoing charged particles from the sun and incoming particles from the stars.
"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 project scientist Dr Edward Stone, from the California Institute of Technology in 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 of years away.
"The new region isn't what we expected, but we've come to expect the unexpected from Voyager."
Scientists are still gathering data from two instruments aboard the ancient spacecraft that measure charged particles.
Voyager 1 and 2 were launched 16 days apart in 1977 on a "grand tour" of planets including Jupiter, Saturn, Uranus and Neptune.
Voyager 1 is now the most distant human-made object, 11 billion miles from the sun.
Signals from the probe take around 17 hours to reach Earth.
Voyager 2, lagging some way behind, is about nine billion miles from the sun.
Both craft carry pictures and messages for any intelligent aliens that might intercept them as they journey between the stars.
New results from Voyager 1 were presented at the annual meeting of the American Geophysical Union in San Francisco.