JPL-Caltech / NASA Artist rendering of NASA's Voyager spacecraft.

Update: Voyager is officially “the first human-made object to venture into interstellar space,” according to NASA.

Even the spaciest of space nuts could be forgiven for being a little confused about what’s going on with Voyager 1. The first part of the story is straightforward enough: The probe was launched way back in 1977 along with its twin, Voyager 2, and two years later began a series of spectacular close encounters, first with Jupiter and its moons and then with Saturn, its moons and its magnificent system of rings. Then, while Voyager 2 went on to visit Uranus and Neptune, Voyager 1 veered off in the general direction of nowhere, with the goal of leaving the familiar territory of our Solar System and entering the unknown realms of interstellar space.

And now, says a new paper in Science, the remarkably resilient spacecraft has crossed that epochal line at last. What makes it all a bit confusing, however, is that it feels like Voyager 1 has flirted with interstellar space already—not once, but several times, over the past decade. In 2003, for example, word came that Voyager had passed the edge of the Solar System. In 2010 the craft was was merely nearing the edge. In 2012, it was preparing to exit. In March of this year, one group of scientists said, in essence “OK, it really has left,” to which NASA shot back “no it hasn’t.” In June, the probe was reported to be on the Solar System’s exit ramp.

“It’s a complicated question,” admits Donald Gurnett, of the University of Iowa, lead author of the new paper. “But I believe we have resolved it clearly.” Voyager 1 has indeed entered interstellar space, he says, but it hasn’t left the Solar System—which is admittedly like saying you’ve just entered Canada but you’re still in the United States. What’s more, that ambiguous crossing happened in August, 2012, many months before NASA insisted it didn’t.

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At this point, you might be forgiven for being just a wee bit suspicious, but Gurnett, who helped design the spacecraft back in the 1970’s, makes a compelling case. The first thing to understand is that interstellar space is defined not as the place where the Solar System ends, but rather where the solar wind—the stream of charged particles speeding outward from the Sun—runs into the interstellar medium, a thin soup of particles drifting amongst the stars. To be part of the Solar System’s family, you simply have to orbit the Sun. “Voyager is currently at three times the distance of Pluto,” says Gurnett, “but there are Solar System objects further out than that.” Being part of the family, however, does not mean you live at home anymore—and Voyager, Gurnett has concluded, has moved to the interstellar medium.

The place where the Sun’s particles dominate is known as the heliosphere, but the boundary region between heliosphere and interstellar space—a region known as the heliopause—is a turbulent and complex place. As the hot solar wind plows into the cold interstellar medium at a million m.p.h. (1.6 million k/h) or so, it creates what’s known as the termination shock; the particles, aligned until now with the Sun’s magnetic field, are wrenched into a new alignment with the fields between the stars.

Exactly where the heliopause gives way to interstellar space, however is something astronomers have never known with any precision. “Fifty years ago,” says Gurnett, “when the idea of the heliopause was first proposed, some people though it was just beyond Jupiter.” By the time Voyager 1 reached Jupiter in the late 70’s, it was clear that the border was much further away than that; the acid test would be when the density of charged particles coming from the sun, also known as plasma, increased from its relative sparsity inside the heliosphere, giving way to the high-density interstellar plasma expected outside.

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Unfortunately, the instrument designed to measure plasma density broke down back in 1980—but Gurnett and his team figured out another way. When an extra blast of particles from a solar storm, which occurs from time to time, reaches the heliopause, it causes a shock that sets the plasma vibrating, and the frequency of the vibrations (or rather, of the radio waves they generate) can tell you how dense the plasma is. In 2012, Voyager detected an increase in cosmic rays and a change in the plasma’s magnetic fields, making it clear that the craft was indeed on the exit ramp, and maybe even over the edge into deep space. But that was too circumstantial for most scientist to agree the crossing had actually happened.

In the fall of 2012 and the spring of 2013, however, the Sun cooperated by belching out extra particles, giving the Voyager team a chance to look for shock-triggered radio waves. They found them—but only after they got a data dump from Voyager’s tape recorders, which are played back once every six months. By comparing the frequency of the plasma vibrations at two different times, they could see that the density had indeed shot up—and figured out that the change happened on or about August 25, 2012.

So Voyager 1 is truly in the realm of the stars—at least according to the current findings—and has been for more than a year. (Voyager 2, meanwhile, which dallied to look at Uranus and Neptune, is lagging significantly behind.) “It’s the first time a human object has ever made it into interstellar space,” says Gurnett, with the satisfaction of someone who’s been waiting for this day since Jimmy Carter was president. “I’m just glad I lived long enough to see it.”

Voyager 1: Snapshots From the Journey Science & Society Picture Library / Getty Images 1 of 9

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