Thirty-five years ago today, NASA's Voyager 1 spacecraft set off on its journey to interstellar space - and it's now teetering on the edge.
It's not easy to pinpoint the exact moment it will pass through the heliopause, or boundary of the solar system, but measurements have been indicating for some months that it's very close.
"Even 35 years on, our rugged Voyager spacecraft are poised to make new discoveries as we eagerly await the signs that we've entered interstellar space," says Ed Stone, Voyager project scientist at the California Institute of Technology in Pasadena.
"Voyager results turned Jupiter and Saturn into full, tumultuous worlds, their moons from faint dots into distinctive places, and gave us our first glimpses of Uranus and Neptune up-close. We can't wait for Voyager to turn our models of the space beyond our sun into the first observations from interstellar space."
The team's recently seen changes in two of the three observations that are expected to alter once the probe hits interstellar space.
The prevalence of high-energy particles streaming in from outside our solar system has jumped, and the prevalence of lower-energy particles originating from inside our solar system has briefly dipped, indicating an increasing pace of change in Voyager 1’s environment.
The team's now analyzing data on the direction of the magnetic field, which could also mark the transition.
Although it was launched after its companion, Voyager 1 reached Jupiter and Saturn before Voyager 2, and was first to see the volcanoes of Jupiter's moon Io, the twisted nature of Saturn's outermost main ring, and the deep, hazy atmosphere of Saturn's moon Titan.
Voyager 1 also took the mission’s last image: the famous solar system family portrait that showed our Earth as a pale blue dot.
The probe is now about 11 billion miles away from the sun, heading north. Like its companion, it's been exploring the outer layer of the heliosphere, the giant bubble of charged particles the sun blows around itself.
"We continue to listen to Voyager 1 and 2 nearly every day," said Suzanne Dodd, Voyager project manager at NASA's Jet Propulsion Laboratory. "The two spacecraft are in great shape for having flown through Jupiter’s dangerous radiation environment and having to endure the chill of being so far away from our sun."
Dodd and her team are being as frugal as possible with the continually diminishing energy sources on the two spacecraft. They reckon there's enough power for the two spacecraft to continue collecting data and communicating it back to Earth through 2020 - possibly, even, until 2025.