NASA's Voyager 2 probe that was launched in 1977 has detected an increase in cosmic rays that originate outside our solar system, indicating that it is near the interstellar space.
Once Voyager 2 exits the outermost layer of the heliosphere -- the vast bubble around the Sun and the planets dominated by solar material and magnetic fields -- it will become the second human-made object, after Voyager 1, to enter interstellar space, NASA said in a statement on Friday.
Voyager 2 is little less than 17.7 billion km away from Earth -- or more than 118 times the distance from Earth to the Sun.
It is the only spacecraft to visit all four giant outer planet -- Jupiter (1979), Saturn (1981), Uranus (198 ) and Neptune (1989).
Voyager scientists have been watching for the spacecraft to reach the outer boundary of the heliosphere, known as the heliopause.
"We're seeing a change in the environment around Voyager 2, there's no doubt about that," said Voyager Project Scientist Ed Stone from NASA's Jet Propulsion Laboratory in Pasadena, California.
Since late August, the "Cosmic Ray Subsystem" instrument on Voyager 2 has measured about a five percent increase in the rate of cosmic rays hitting the spacecraft compared to early August.
The probe's Low-Energy Charged Particle instrument has detected a similar increase in higher-energy cosmic rays.
"We're going to learn a lot in the coming months, but we still don't know when we'll reach the heliopause. We're not there yet -- that's one thing I can say with confidence," Stone added.
Three other spacecraft are on interstellar trajectories. New Horizons is closing in on a Kuiper Belt Object more about a billion miles beyond Pluto on January 1, 2019.
Pioneer 10 and Pioneer 11 probes are no longer functioning, but will eventually travel into interstellar space.
In May 2012, Voyager 1 experienced an increase in the rate of cosmic rays similar to what Voyager 2 is now detecting.
"That was about three months before Voyager 1 crossed the heliopause and entered interstellar space," said NASA.