Voyager surfs Solar System's edge
"It could be any day, but it could also be several more years."
Ed Stone cannot say when the Voyager-1 spacecraft will leave the Solar System, but he believes the moment is close.
The latest data from this extraordinary probe, reported in this week's Science journal, suggests it is surfing right on the very edge of our Sun's domain.
The particles streaming away from our star have reduced to a trickle at its present location, 18.5 billion km from Earth.
Particles flying towards it from interstellar space, by contrast, have jumped markedly in the past year.
It all points to an imminent departure, which would make Voyager the first man-made object to cross into the space between the stars.
Voyager's epic journey
"It's hard to imagine there's another layer between the one we're in and the outside," Dr Stone told BBC News. "Topologically, it makes sense that this is the outermost layer. The only question is: how thick is it?"
Launched way back in 1977, the probe has now travelled so far from home that its constant chatter of data takes 17 hours to arrive at the US space agency's receiving network. And chatter, it does.
Voyager's instruments are busy sampling the far-flung environment. This has allowed Dr Stone and colleagues to map the shape and reach of the heliosphere - the giant bubble of charged particles blown off from our Sun.
In 2004, it reached a turbulent region referred to as the heliosheath, where particles bounced around in all directions.
It was expected this would be the final stage before the leap to interstellar space. But, as has been the case throughout this 35-year mission, Voyager threw up yet another surprise.
Last year, it detected what appears to be a discrete boundary layer that Ed Stone's team call the "heliosheath depletion region" in Friday's three Science papers.
It is a kind of magnetic highway where energetic particles on the inside can get out easily, and the galactic cosmic ray particles on the outside can zoom in.
"It is where the Sun's magnetic field has piled up, compressed up against itself. It has also doubled in strength. It's smoother than anything we've ever seen with Voyager," Dr Stone explained.
The team is now watching the direction of the field lines very carefully. Currently, they orientate east-west, wound into a spiral by the rotating Sun. But when Voyager finally breaks through into interstellar space, they are expected to shift dramatically, running north-south.
This is an acid test for Dr Stone. Although some might argue the particle data is evidence of Voyager being outside the Solar System, the project leader believes the probe cannot truly be said to be beyond the Sun's domain until it has also escaped our star's magnetic influence.
But do not expect an immediate, definitive announcement from Nasa that Voyager is in interstellar space when the magnetic signal does switch.
Instead, the instrument scientists will sit and listen to the probe's chatter, perhaps for several months. They will want to be absolutely sure Voyager has broken through the so-called heliopause.
Like the surfer who rides the front of a breaking wave, battling the foam, Voyager will take some time to move completely clear of everything behind.
"The edge may be somewhat turbulent. We just don't know," Dr Stone told BBC News. "This is exploration after all, and we will find out how Nature makes this interface. But it will be moving because the Sun does 'breathe' in and out."
Voyager 1 is on course to approach a star called AC +793888, but it will only get to within two light-years of it and take some 40,000 years to make the passage.
Voyager 2, which was launched a few weeks before Voyager 1, is on a slightly slower path to interstellar space and is probably a few years from seeing the heliosheath depletion region.
Both probes have sufficient power in their plutonium "batteries" to keep working into the next decade.