The Voyager 1 probe is getting very close to the edge of the Solar System.
Voyager 1 is the most distant human-made object
Launched in 1977, the craft is now some 14 billion km (8.7 billion miles) from the Sun and on the cusp of deep space.
American space agency (Nasa) scientists told a conference in New Orleans on Tuesday that Voyager was moving through a region known as the heliosheath.
This is a vast, turbulent expanse where the Sun's influence ends and particles blown off its surface crash into the thin gas that drifts between the stars.
Soon - researchers cannot be sure when - the probe will break into deep space.
"Voyager 1 has entered the final lap on its race to the edge of interstellar space," said Dr Edward Stone, Voyager project scientist at the California Institute of Technology in Pasadena, US.
Last November, scientists debated whether Voyager had reached the so-called termination shock region.
This is where the "wind" of electrically charged particles coming off the Sun is slowed by pressure from the sparse gas found between the stars.
At the termination shock, the solar wind slows abruptly from a speed that ranges from 1.1-2.4 million km/h (700,000 to 1.5 million mph) and becomes denser and hotter.
Some researchers thought the probe had arrived at the shock; others thought it still had some way to go.
Now, at the 2005 Joint Assembly meeting organised by the American Geophysical Union, space scientists say they are confident - and agreed - that Voyager has gone beyond the termination shock and is flirting with deep space.
Predicting the location of the termination shock was hard, the researchers say, because the precise conditions in interstellar space are unknown.
Also, changes in the speed and pressure of the solar wind cause the termination shock to expand, contract and ripple.
The most persuasive evidence that Voyager 1 has crossed the termination shock is its measurement of a sudden increase in the strength of the magnetic field carried by the solar wind, combined with an inferred decrease in its speed.
This happens whenever the solar wind slows down.
Voyager 1 was initially given a mission life of five years but has continued to perform spectacularly.
The craft is carrying a time capsule in the form of a golden gramophone record, complete with stylus, which contains a recording of greetings from Earth in different languages as well as samples of music ranging from Mozart to singer Blind Willie Johnson.
Its twin, Voyager 2, launched a couple of weeks before Voyager 1, is moving on a different trajectory and is some 10.4 billion km (6.5 billion miles) away.
Solar wind: Stream of charged particles blown off the Sun and travelling at supersonic speeds
Termination shock: Area where particles from the Sun begin to slow and clash with matter from deep space
Heliosheath: A vast, turbulent expanse where the solar wind piles up as it presses outward against interstellar matter
Heliopause: The boundary between the solar wind and the interstellar wind, where the pressure of both are in balance
Interstellar wind: The gas and dust between the stars
Bow shock: The shock wave caused by the edge our Solar System travelling through deep space