NASA thinks it's finally settled when Voyager 1 left our solar system: on or around August 25th, 2012. The first Voyager probe launched in 1977, but while it's indubitably passed into previously unexplored parts of space, it's been hard to say whether it's actually crossed over into interstellar space. Now, a paper published in Science lays out a convincing case that it did so over a year ago. University of Iowa physics professor Donald Gurnett and his co-authors, as well as NASA itself, believe that the density of plasma around Voyager 1 shows it's no longer within the boundaries of the heliosphere, the bubble of solar energy that surrounds our system. And by working backwards, it's possible to pinpoint (relatively speaking) the moment it left.
The real question isn't whether a case can be made that Voyager is in interstellar space; it's whether we can be sure about it. In March of 2013, astronomers suggested that the probe had left the solar system in August, but NASA's Voyager team disputed the assertion, saying that it hadn't observed one critical indicator: a change in the direction of the magnetic field around the probe. Last month, other scientists disagreed that this was the key sign, saying it was plausible that the interstellar magnetic field simply flowed in the same direction as that of the heliosphere. But the Voyager team hasn't taken a firm stand on the question until now.
"People are going to be looking to see if there are other interpretations. But this is the most straightforward one."
Ed Stone, who has worked on the Voyager project since the 1970s, thinks the evidence from this paper is strong. The theory that Voyager 1 is outside the heliosphere is "the general agreement now," he tells The Verge. "We all know that nature has been full of surprises for us, so people are going to be looking to see if there are other interpretations. But this is the most straightforward one." His conclusion, as well as that of the authors, is based on a decades-old paper (also co-authored by Gurnett) that estimated the distance to the edge of the heliosphere. In 1983 and 1993, two particularly strong solar storms sent shock waves through our system. When they hit the edge, they excited the plasma there and made it "ring" or oscillate, bouncing back a radio frequency that was caught by the Voyager probes.
This frequency gave Gurnett and his co-authors the density of the plasma at the heliopause (the very outer rim of the heliosphere) as well as a distance far beyond most other estimates. "People were always hoping it wouldn't be too far away — that it wouldn't take 20 years to get there," he says. But over time, the number started looking better and better.
A plasma sensor could have helped pinpoint the location — but Voyager's had been broken since 1980
In theory, that meant you could tell when Voyager 1 had left the solar system simply by periodically checking the plasma density. The heliopause is relatively hot and low-density, while interstellar space is cold and high-density. Unfortunately, the plasma instrument that would require has been largely broken since 1980. Instead, Gurnett's team relied on a radio wave receiver at the base of one of Voyager's large antennas, and a huge stroke of luck.
In 2013, another solar storm reached the edge of the heliosphere. It excited the plasma around Voyager, making the normally undetectable substance ring once again. And when Gurnett and his team got the data, collected between April and May, the frequency indicated that Voyager was in the dense plasma of interstellar space. When they received more detailed records stored on Voyager itself, they found that another, smaller wave had hit in October and November of 2012 — also apparently from outside the solar system. It was perfect timing for a rare event: "We had gone about nine years without ever seeing a plasma oscillation" from Voyager, says Gurnett.
If you're hungry for another milestone, there's always Voyager 2
By extrapolating how quickly density ramped up outside the edge of the solar system, Gurnett and his team could use those two Voyager data points to estimate when it actually crossed the boundary. The result was August 25th, the precise date that the last team of researchers had found a steep drop in radiation levels. As soon as Gurnett's team saw the multiple, matching data points, "we said: 'we're in interstellar. We're in interstellar plasma.'"
Barring future interpretations or new studies, we may know once and for all when mankind sent its first probe beyond the solar system. Now, the question becomes what Voyager 1 will discover in interstellar space. So far, all our observations have come from inside the solar bubble, and having a view from the void should give us much more detailed information. And for those hungry for another milestone, there's always Voyager 2, still far behind its sister probe at the outer layer of the solar system.
Update: NASA officially announced the findings and held a discussion in the wake of the paper's publication; that discussion has now wrapped up.