Voyager 1, the most distant spacecraft from Earth, has reached a new milestone in its quest to leave the solar system.
The unmanned NASA probe, now 10.8 billion miles from the sun, has detected a distinct change in the flow of particles that surround it.
These particles, which emanate from the Sun, are no longer travelling outwards but are moving sideways.
NASA scientists say it means Voyager must be very close to making the jump to interstellar space -- the space between the stars.
NASA said last night that recent readings showed the average outward speed of the solar wind had slowed to zero, meaning the spacecraft was nearing the solar system's boundary known as the heliopause.
"It's telling us the heliopause is not too far ahead," said project scientist Edward Stone of the NASA Jet Propulsion Laboratory.
Scientists estimate it will take another four years before Voyager 1 completely exits the solar system and enters interstellar space.
"We knew this was going to happen. The question was when," Mr Stone said.
Launched in 1977, the nuclear-powered Voyager 1 and its twin Voyager 2 toured Jupiter, Saturn, Uranus and Neptune, then kept going in different directions. Voyager 1 veered north while Voyager 2 headed south.
Voyager 1 is currently hurtling at 38,000mph and when it finally leaves the solar system, scientists expect to see a telltale change in the wind.
Interstellar wind is slower, colder and denser than solar wind.